COMMERCIAL HVAC SYSTEMS Water Source Heat Pump Systems

Transcription

1 COMMERCIAL HVAC SYSTEMS Water Source Heat Pump Systems Technical Development Program

2 Technical Development Programs (TDP) are modules of technical training on HVAC theory, system design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications. Although TDP topics have been developed as stand-alone modules, there are logical groupings of topics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth of this offering allows for customization into a complete HVAC curriculum from a complete HVAC design course at an introductory-level or to an advancedlevel design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts. This TDP module will provide an understanding of the components in water source heat pump systems, configuration options, system benefits, and many applications associated with the overall system. WSHP systems have become a very popular choice for use in commercial buildings where individual zones of control are required to maintain comfort conditions. Building types that exhibit a simultaneous cooling and heating load are ideal candidates. WSHP systems have other desirable characteristics like zoning capability, ease of design, and reliability so that buildings where little or no reclaim will take place are often still considered for using a WSHP system Carrier Corporation. All rights reserved. The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing systems. Judgment is required for application of this information to specific installations and design applications. Carrier is not responsible for any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design. The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Carrier Corporation. Printed in Syracuse, NY CARRIER CORPORATION Carrier Parkway Syracuse, NY 13221, U.S.A.

3 Table of Contents Introduction... 1 Water Source Heat Pump Unit Operation... 2 Cooling Mode... 3 Heating Mode... 3 Mode Changeover... 4 WSHP System Operation... 4 Cooling Mode (Summer Operation)... 5 Heating Mode (Winter Operation)... 6 Intermediate Season (Spring and Fall Operation)... 7 System Advantages and Disadvantages... 7 System Advantages... 8 System Disadvantages Product Overview WSHP Components Compressor Reversing Valve Refrigerant-to-Water Heat Exchanger Expansion Metering Device Refrigerant-to-Air Heat Exchanger Fan Assembly Condensate Drain Filters Unit Controls Hydronic Accessories WSHP Unit Types Horizontal Units Vertical Units Console Units Stack Units Rooftop Units Water-to-Water Heat Pump Units System Overview Components Cooling Tower Boiler (Heat Adder) Air Distribution System Ventilation Air Distribution Units Water Pumps Air Separator and Expansion Tank Controls Example System Configurations Single-Story Buildings Two to Four-Story Buildings High-Rise Buildings... 36

4 System Variations...36 Variable Volume and Temperature (VVT ) System...36 VAV in the Interior Zone...37 Variable Flow Systems...37 Hot Water Pre-Heating...38 Sprinkler Piping Integrated with the WSHP Loop...38 System Without a Boiler...39 System With a Storage Tank...39 System Application Topics...41 WSHP Units With Reheat...41 Freeze Protection...41 Antifreeze Solutions...43 Water Conditioning...44 Ventilation System Energy Recovery...45 System IAQ Topics...48 Acoustics Considerations...52 Refrigerants...53 Airside Economizer...53 Water-Regulating Valves...54 Maintenance...54 Geothermal Systems...55 Closed-Loop Systems...56 Open-Loop Ground Water Systems...59 Geothermal System Advantages...60 Codes and Standards...61 Performance Related Codes and Standards...61 Safety Related Codes and Standards...64 UL/CSA and ETL...64 System Sizing and Layout Tips...64 WSHP Units...65 Cooling Tower Selection...66 Boiler (Heat Adder)...67 Ventilation System...68 Piping Systems...71 Pumps...73 Air Separator and Expansion Tank...74 Controls...74 WSHP Thermostats and Controllers...75 Water Sensors and Switches...77 Pump Control...77 Cooling Tower and Boiler...77 Ventilation System...78 System Safety and Alarms...78 Reducing Operating Cost...79 Overall System Control...79 Summary...81 Work Session...82 Work Session Answers...86 References...89 Appendix A Water Quality...90

5 Introduction Water source heat pump (WSHP) systems have become a popular choice for commercial buildings where multiple zones of control are desired. They can also be applied successfully in smaller installations like residences. This TDP module will discuss both applications, but will concentrate on the commercial applications. The usage of WSHP systems breaks down to roughly 60 percent new construction and 40 percent retrofit and replacement. In this TDP module, we will learn about the various types of water source heat pump units that can be used to comprise a HVAC system. The internal components in a typical WSHP unit and the function of each will be explained. This will allow the reader to understand how the WSHP units operate when connected to a system water loop. System components will be then discussed. These include boilers, towers, pumps, piping, and controls. See Figure 1. Figure 1 Typical WSHP System There are many overall variations for WSHP systems. They may be categorized into two main groups. The first system type is a standard closed-loop system where the loop piping runs inside the building. This system typically includes a boiler (also called a heat adder), a cooling tower (also called a heat rejecter), pumps, and controls as shown in Figure 1. The second type of WSHP system uses the Earth s resources as a heat sink. These systems are called geothermal systems. A lake, river, well, or the ground itself is used to add or remove heat to maintain an operable water temperature. Some of these systems are closed-loop and some are open loop. Open-loop means the water is used in a once-thru configuration. A separate section on geothermal systems is included in this TDP. Commercial WSHP systems are popular because they can supply simultaneous heating and cooling. This leads to comfortable conditions in zones that have different requirements. A WSHP system typically requires a dedicated outdoor air unit to maintain required ventilation levels as specified by ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) Standard This TDP module will examine the various ways to deliver ventilation air. Control of the individual units, the water loop temperature, and other components in the system are also covered in this TDP. Recommendations on proper application of units to minimize radiated sound along with the required maintenance considerations are included. 1

6 This TDP module is meant to complement the Carrier System Design Guide on Water Source Heat Pumps. In this TDP we will reference the Design Guide in several areas, the most notable being the design process and layout of an example building. The Design Guide can be used for a step-by-step approach to selecting and designing an entire WSHP system including an operating cost analysis comparison to other systems. After reading this TDP, the reader will understand how WSHP units and systems work, and why they are a popular, reliable, and versatile alternative to other air-conditioning systems. Water Source Heat Pump Unit Operation A water source heat pump is a mechanical reverse cycle device that is used to transfer heat from one medium to another. A water source heat pump extracts heat from the water when in the heating mode and rejects heat to the water when in the cooling mode. The water supply may be a recirculating closed loop or a once-thru system using water from a well, a pond, or a stream. Water for closed-loop heat pumps is usually circulated at 2.25 to 3.00 gpm per ton of unit cooling capacity. A once-thru groundwater heat pump can operate with a lower water flow, but the same range is still recommended. The WSHP refrigeration circuit consists of a refrigerant-to-water heat exchanger, compressor, refrigerant metering device, refrigerant-to-air heat exchanger (or refrigerant-to-air coil), fan, reversing valve, and controls. Heat is transferred from one medium to the other by a hermetic refrigerant circuit. The most common design configurations for packaged WSHPs are horizontal units, which are positioned above a dropped ceiling; vertical units, which are usually located in basements, utility closets, or equipment rooms; and console units, which are designed for under window mounting in the conditioned space. We will discuss all the types of units in detail later in the TDP. Reversing Functions Unlike a traditional airconditioning unit, in a heat pump, the function of the heat exchangers can be reversed. The feature that most distinguishes a heat pump from the typical refrigeration system is that it is reversible. This allows the unit to provide cooling in summer and heating in winter at a relatively efficient level. In a WSHP unit a reversing valve switches the compressor discharge from the refrigerant-to-water heat exchanger for cooling to the refrigerant-to-air exchanger for heating. Many of the other components required for a heat pump are the same as for a traditional airconditioning unit. However, in the heat pump, the function of the heat exchangers can be reversed, so that they each must function as an evaporator and condenser. 2

7 Cooling Mode In the cooling mode, the WSHP unit s refrigerant-to-water heat exchanger acts as a condenser and its refrigerant-to-air heat exchanger acts as an evaporator. The reversing valve is energized for cooling. During the cooling mode, heat is extracted from the air by evaporating the refrigerant in the refrigerant-to-air heat exchanger. This extracted heat plus the compressor heat is rejected into the water loop. This is called the heat of rejection. Hot gas from the compressor discharge is directed by the reversing valve to the refrigerant-to-water heat exchanger. Here the hot gas is condensed into liquid as the gas gives up heat to the colder water passed through the exchanger. Liquid refrigerant then passes through a metering device that causes a drop in its pressure and temperature. The cold liquid-vapor mixture then enters the refrigerantto-air heat exchanger where it evaporates. The indoor air is cooled to condition the space. Cool refrigerant vapor then is drawn into the compressor where its temperature and pressure are increased so the cycle can be Figure 2 repeated. See Figure 2. Cooling Mode Heating Mode In the heating mode, the WSHP unit s refrigerant-to-water heat exchanger acts as an evaporator and its refrigerant-to-air coil acts as a condenser. The reversing valve is deenergized for heating. The hot compressor discharge gas is directed by the reversing valve to the refrigerant-to-air heat exchanger, which will act as a condenser. Air is then heated as it passes over the refrigerantto-air heat exchanger as it condenses the refrigerant and heats the space. Liquid refrigerant flows through the metering device to the refrigerant-towater heat exchanger that acts as the evaporator. Heat is extracted from the water loop as it passes through the refrigerant-to-water heat exchanger and the cold liquid refrigerant evaporates. The cold vapor then is drawn into the compressor and the cycle repeats. See Figure 3. The heat that is extracted is known as the heat of absorption. Figure 3 Heating Mode 3

8 Mode Changeover The reversing valve in the unit s refrigeration circuit is located on the compressor discharge. The reversing valve causes the changeover from cooling to heating mode and from heating to cooling mode. In a reversible system, some of the traditional components must have special features to allow for reverse flow. For example, the metering device must be capable of metering flow in both directions. This type of metering device is usually called bi-directional. In addition to the reversing valve and metering device, there are other design considerations that make the heat pump different from the conventional air-conditioning unit. The compressor is specially designed to operate over the wide range of compression ratios encountered in the heating mode. Both the air and water heat exchangers are specially designed for WSHP use because they must both evaporate and condense refrigerant. For example, the refrigerant-to-air heat exchanger, when acting as a condenser in the heating cycle, must have adequate surface area to keep the condensing temperature and pressure at reasonable levels. WSHP System Operation A typical closed loop system as shown in Figure 4 consists of a number of WSHP units, a closed circuit cooling tower, a boiler, a common piping loop comprised of a supply and return line, and a water-circulating pump with a standby. Standard hydronic accessories are also required such as an expansion tank, air separator, and piping vents. Notice the arrangement of the components with the loop flow passing through the heat rejecter (tower), then through the heat adder (boiler), expansion tank, pump, and then out to the heat pump system. The piping is almost always a reverse return Figure 4 system as shown in Figure 4. In a Typical Closed-Loop WSHP System reverse return system, the combined supply and return length of water piping through each unit is essentially the same. This results in a system that is more self-balanced than a direct return system. For a complete discussion on reverse return piping, see TDP-502, Water Piping and Pumps. There are several variations of this standard closed loop configuration, which we will discuss later. Closed loop systems use recirculated water. Open loop ground source systems use a oncethru (non-recirculating) design. Water circulating in the loop acts as a heat source for WSHP units operating in the heating mode and as a heat rejection sink for those units operating in the cooling mode. Thus, heat may be transferred from building zones that need cooling to zones that need heat. This reclamation of heat is one of the major advantages of the WSHP system. The circulating loop temperature is maintained between 60 F and 90 F. When more zones require heating than cooling, the loop temperature drops (approaching 60 F) and the boiler is activated to make up the heat deficit. 4

9 When more zones need cooling than heating, the loop temperature rises (approaching 90 F), and the cooling tower is activated to reject unneeded heat. A separate ventilation system provides outdoor air to the zones. The ventilation system provides fresh air either directly to each WSHP unit or by ducting the fresh air to within a few feet or less of each unit if a ceiling plenum return air system is used. Additional discussion of the ventilation system can be found later in this manual. The loop flow is often variable. A VFD (variable frequency drive) controlled loop pump varies the total loop flow based on supplying water only to those WSHP units whose compressors are running. In this way, pump energy is saved. See the Variable Speed Pumping section of this TDP. Let s take a look at the system operating modes for various times of the year to learn the heat recovery capability of a WSHP system. Cooling Mode (Summer Operation) Loop Flow A VFD-controlled loop pump varies the total loop flow by supplying water only to those WSHP units whose compressors are running. In the summer season, the conditioned spaces will need cooling. Usually a WSHP system consists of multiple WSHP units, each with a dedicated thermostat. All units supply cool, filtered air to their zones. Each refrigerant-towater heat exchanger transfers the heat from the cooling load plus the heat of compression into the common water loop. This process raises the temperature of the loop. When the loop temperature approaches the upper limit of about 90 F, the heat rejecter (cooling tower) is staged to remove heat from the loop. The cooling tower maintains a maximum loop water temperature of 90 F. Individual units will cycle on and off Figure 5 to satisfy their zone thermostat. The WSHP fan runs continuously during Typical WSHP System in Cooling Mode the occupied cycle. See Figure 5. The upper loop temperature on WSHP systems is maintained between 90 and 95 F. For discussion throughout this TDP, we will use a temperature of 90 F. Block Load and Diversity The WSHP system is made up of many zones. These zones will each have a peak load. The peak load may occur at different times for different zones. Each WSHP unit in each zone must be sized to meet the zone peak load. The total capacity (tons) of all the WSHP units in the building is calculated from the zone peak loads. Loop Temperature The upper loop temperature on WSHP systems is maintained between 90 F and 95 F. For discussion throughout this TDP, we will use a temperature of 90 F. 5

10 Because zones on different sides of a building will have their peak load occur at different times, the total installed capacity of the WSHP units will usually be greater than the block load of the building. The block load is the single largest combination of actual zone loads at a particular time. The block-cooling load is seldom the sum of the zone peak loads and it is normally smaller than the sum of these peak loads. In northern latitudes the block load occurs typically in late afternoon in July or August. Diversity can be found by dividing the block load by the sum of the peak loads for all zones. Most large commercial buildings will exhibit some diversity ranging from 80 to 85 percent. Unlike WSHP systems, central chilled-water systems can usually take advantage of diversity in the sizing of the chiller. The chiller total installed tons need be only the size of the block load. The installed WSHP unit s capacity, however, will have a sum greater than the block load. Heating Mode (Winter Operation) In very cold weather, all the conditioned spaces may need heating. If the building is configured so that even on the coldest winter day, some spaces still do not have a heat loss, then those spaces have a year round cooling load and will not require heating. This condition will be discussed next. In heating mode, each unit will supply heated, filtered air to their zones. Each refrigerant-to-water heat exchanger acts as an evaporator and absorbs heat from the water loop. This lowers the temperature of the loop. When the loop temperature approaches the lower limit of about 60 F, the heat adder is staged to add heat to the loop. The heat adder maintains a minimum loop water temperature (leaving the heat adder) of 60 F. Individual units cycle on and off to satisfy their thermostat. The fan runs continuously during the occupied cycle. See Figure 6. Figure 6 Typical WSHP System in Heating Mode All WSHP units may be required to run together continuously on full heat during a period of warm-up after a night set-back mode. Consequently, the heat adder must be sized for the heat of absorption of all units on the loop as a worse case. The heat adder size may be reduced if some of the heat pumps will never start in the heating mode, such as units in computer room areas or units in internal zones that have a year round cooling load. If an optional storage tank is provided, the tank can be used to reduce the size of the heat adder. 6

11 Intermediate Season (Spring and Fall Operation) In intermediate seasons, some units may be in the cooling mode (adding heat to the common water loop) while others are in the heating mode (absorbing heat from the loop). During this condition, the loop may be in equilibrium and not require heat to be added or rejected. The loop water temperature is allowed to vary within the approximate 60 to 90 F range. See Figure 7. This operation is a true heat recovery scenario. Perimeter units in heating mode are being supported by interior units in the cooling mode. Some WSHP system configurations use large 10 to 15-ton sized vertical units in core areas that will operate in cooling mode year round. These units will reject a greater amount of heat to the loop and will support a greater number of smaller units operating in the heating mode. Figure 7 Typical WSHP System in Intermediate Mode Some WSHP designs use a variable flow in the water loop. A piping run is fitted with a bypass to allow a minimum water flow for the system loop pumps. We will discuss variable flow later in the TDP. System Advantages and Disadvantages The WSHP system is a good choice for office buildings, hotels, condominiums, libraries, and schools. Almost any type of commercial building that has several temperature control zones, some of which need to be heated while others need to be cooled, are prime candidates for a WSHP system. The WSHP system is an especially good choice where potential energy savings from heat reclaim are substantial, and where the owner is committed to energy conversation. However, many water source heat pump systems are applied to building types that do not have simultaneous heating and cooling requirements. The WSHP system has other attributes besides heat recovery that make it a desirable choice. Compared to many other building cooling and heating systems, the WSHP system offers distinct advantages for those involved in the construction and ownership of commercial buildings. 7

12 System Advantages Low Installed Costs A traditional strength of the WSHP system is its low installed cost compared to conventional central plant or floor-by-floor systems. Features are shown in Figure 8. Since WSHP units are small, the duct sizes are also relatively small. The same system provides heating and cooling. Water piping does not need to be insulated. Additionally, a large portion of the installation costs can be deferred to the time of tenant occupancy in a new construction Figure 8 project. Since the WSHP units themselves represent 50 percent of the Low Installed Cost Features cost of a typical WSHP project, the individual units do not have to be purchased until a tenant has been secured for each apartment in the building. This improves project cash flow and capital costs. Also, a central mechanical room is not required to house large equipment. Compared to four-pipe fan coil or unit ventilator systems, the WSHP system only requires a single circulating piping system composed of a supply and return pipe. This results in substantial lower first cost. Low Operating Costs Operating costs of WSHP systems compare favorably and often are lower than packaged, unitary, and air-cooled central systems. Units serving individual zones can operate close to fullload efficiency while the rest of the units in the system remain off. For example, a central system that utilizes an air-cooled water chiller under the same building loads could be operating at a less efficient part-load condition. Many WSHP systems offer the maximum diversity possible with only those units operating that are required based on individual space control. Standard WSHP units applied in closed-loop applications using a boiler and a cooling tower provide competitive full load efficiencies with cooling EERs (energy efficiency ratio) of about 11.0 and heating COPs (coefficient of performance) around 3.7. High-efficiency models are also available with cooling EERs above 15.5 and COPs of 4.5. At the higher efficiency levels the WSHP system can compare favorably to central water-cooled chiller plant systems that use centrifugal or screw chillers. The value for EER is found by dividing the net unit cooling capacity (in Btuh) by the total unit input Watts. The value for COP is found by dividing the total heat of output (in Watts) by the total input Watts. 8

13 Water source heat pump systems have a distinct advantage over other systems because they utilize heat recovery inherently. The circulating loop is common to all WSHP units in the building. Heat removed from zones requiring cooling is redistributed to zones needing heat in a most efficient manner that results in low operating costs. Therefore, WSHP systems also allow for a level electrical demand throughout the year. Utility companies endorse and often promote WSHP systems. Design Guide Refer to the Carrier Water Source Heat Pump Design Guide for an operating cost comparison to other commercial systems. Operating Flexibility During building occupancy, WSHP units heat or cool based on the needs of each individual zone and the central boiler or cooling tower is activated only as needed. A zone can be as small as a single room. Adjacent zones can be either heated or cooled. See Figure 9. After normal building occupancy hours, individual tenant zones can be conditioned without operating an entire central plant. The circulating loop may contain sufficient water volume to meet after hour tenant needs by simply activating the circulating loop pump. If there is not sufficient water volume, the boiler or tower can be activated to maintain loop temperature. Figure 9 Operating Flexibility of Adjacent Zones Reliability Since the early 1960s, WSHP systems have been in operation in the United States. According to the Electric Power Research Institute (EPRI), WSHP systems have an average reliability of 98.5 percent at least as well, if not better, than all similarly priced systems. According to EPRI studies, the compressors used in WSHP units have a long service life and very low replacement rates. Also, part of the reliability advantage of a WSHP system is the redundancy. If one heat pump unit fails, the rest of the system stays operational. See Figure 10. Figure 10 Redundancy Advantage 9

14 Design Flexibility Units are available in many sizes and configurations to meet space needs. Units can be installed under windows, in mechanical closets on each floor, in ceiling cavities, or stacked in vertical floor-to-floor arrangements as shown in Figure 11. Energy used by WSHP units can be individually metered and billing provided to tenants when required. Common system components like the boiler, cooling tower, and pumps must be subdivided among tenants. Future adjustments to the WSHP system layout can easily be made if Design Flexibility Advantage the original circulating loop is designed with capped stub-outs located at intervals along the main piping loop. Even without extra stub-outs, new branch takeoffs can be added with hot-taps during the unoccupied hours, on weekends or in the evening. When applications allow, such as in high-rise buildings, WSHP units can be arranged in a vertically stacked floor-to-floor arrangement. The stack unit manufacturer can provide units with built-in risers. It is a simple matter to add or relocate a zone. Each WSHP unit requires minimal discharge ductwork and requires only a two-pipe flexible connection to the circulating loop, power and control wiring, and condensate piping. Finally, the WSHP system easily integrates with energy management systems. Easy to Design The WSHP system utilizes packaged equipment with minimal ductwork. Piping and controls are simple and units are pre-engineered. Major WSHP manufacturers offer design guides that engineers can use to design whole systems. See Figure 12. Figure 11 Figure 12 System Design Guide 10

15 Minimal Building Space Usage Since WSHP units are small, they can be located in ceiling cavities or in closets and require less space when compared to alternative systems. And since water is used to transfer heat energy through the building instead of air, less building space is required for vertical shafts. Finally, a smaller mechanical room can be used since there is no need for a central chiller plant. See Figure 13. Ease of Maintenance and Repair Figure 13 Units Require Less Building Space The vast majority of installed WSHP units are from 1/2 to 5 tons in capacity and weigh from 100 to 300 lb each. Therefore, WSHP units are easily installed and removed. Should a WSHP unit malfunction, it can be removed from the system while the rest of the system continues to function properly. The malfunctioning unit can be serviced on-site, as shown in Figure 14, or be removed and replaced with a spare unit. In this way, repairs can be carried out in a remote shop or central maintenance area. Figure 14 Ease of Maintenance and Repair In-house maintenance technicians with a reasonable knowledge of small packaged air conditioning systems can perform normal maintenance routines. Should units need replacement, manufacturers maintain warehouse stock of popular models with as little as one-day delivery. Also, unlike some central systems employing large pieces of equipment, WSHP systems do not typically require a licensed engineer or operator to run the system. 11

16 System Disadvantages As with any system, there are some potential disadvantages with WSHP systems as shown in Figure 15. Here is a brief discussion of them. Acoustics For installation, WSHP units are located close to or in the conditioned space. Each heat pump contains a compressor and a fan that can generate radiated sound. However, there are various installation practices based on the unit type that, when followed, result in an acceptable WSHP operating sound level. Figure 15 Disadvantages of a WSHP Condensate Piping Each WSHP unit requires an individual condensate piping line to be run from the unit. The drain pan under the refrigerant-to-air heat exchanger may require chemical cleaning to keep it free of algae that may clog the condensate drains. Secondary Drain Pan A secondary drain pan is used as a precaution on many projects. This is subject to local requirements. A condensate pump may also be required on some WSHP units in order to remove condensate that cannot drain by gravity. Also, there is potential leakage of condensate from WSHP units into the finished ceiling if the drain pan were to overflow. Separate Ventilation System Many central system and rooftop packaged designs incorporate the introduction of outdoor air into the unit. All WSHP systems require a dedicated outdoor air unit. The exception may be console type WSHP units where the wall box accessory can be used to introduce outdoor air. However, this requires a wall penetration at each unit and is not popular with many architects. Also, ASHRAE 62.1 requirements for ventilation air may be difficult to achieve without a separate ventilation system. Multiple Electrical Power and Control Wiring Connection Points Each heat pump unit requires a power supply and disconnect. Control wiring also is required between each space thermostat and the WSHP unit. 12

17 Multiple Access Points Many WSHP systems utilize horizontal above-the-ceiling type units. That means the impact on occupants must be considered if servicing is performed during the occupied time period. Limited Use Economizer Cycle If during colder weather conditions, the cooling load is being met with outdoor air instead of heat pump compressors, then there will be no heat rejected to the loop for those units requiring heating. Consequently, the use of an economizer can actually result in greater energy consumption for a WSHP system. Humidity at Part-Load Operation Most heat pump units are small single-compressor designs that cycle the compressor on and off to meet space dry bulb set point. During periods of light load, there may be a build-up of humidity in the space. Additional heat pump technology can be used to improve resulting relative humidity. Reheat, unloading compressors, and variable speed evaporator fans are described later in this TDP. Limited Filtration Capability Most WSHP units, other than large rooftop or vertical units, are limited to a 1 or 2-in. filter. The efficiency of this type filter may be at best MERV 7-8. Most WSHP units have low static fans incapable of accommodating the pressure drop of a higher efficiency filter. Poor Filter Maintenance Filter maintenance can be easy to neglect. However, an accessory dirty filter light can be integrated into the wall thermostat to tell the occupant when changing is required. 13

18 Product Overview This chapter describes the components that comprise a WSHP unit along with various equipment types and accessories that are available for use in WSHP systems. Its contents are intended to be an aid in the early stages of system design when it is necessary to decide which units are likely to be the best choice for a particular WSHP project. WSHP Components A WSHP unit is composed of the following internal components. The exact configuration may vary slightly from manufacturer to manufacturer. Compressor Most WSHP units contain a single hermetic compressor. Hermetic compressors are sealed in an enclosure and cooled by refrigerant gas. The three types of compressors used in the WSHP industry are scroll, reciprocating, and rotary. See Figure 16. The type of compressor used depends on the capacity of the WSHP and the brand. Premium efficiency WSHP models often offer multiple compressors for part-load staging. Larger sized units (above 10 tons) also feature multiple compressors. Figure 16 Compressor Types The following are characteristics of WSHP compressors: Fully hermetic design Non-field serviceable Highly efficient Extremely reliable Multiple Compressors Premium efficiency WSHP models often offer multiple compressors for part-load staging. Larger sized units (above 10 tons) also feature multiple compressors. Hermetic reciprocating compressors and rotary compressors can be used, but scroll compressors are most popular. They contain less moving parts and are very efficient. They are quieter and very reliable. Multiple compressor designs have better part-load efficiency and are better suited for larger units with more variable loads. Typically single and dual-compressor units are designed with separate electrical and mechanical refrigeration circuits. This avoids problems associated with multiple compressors on one circuit. 14

19 New Modulating Scroll Technology Some new WSHP units incorporate a scroll compressor with unloading capability. The basic scroll is modified by the addition of an internal unloading mechanism that opens a bypass port and allows an unloading step of 67 percent of full capacity. A single-speed, high-efficiency motor runs continuously while the scroll modulates between the two capacity steps. The result of this technology is better overall unit efficiency and control of humidity at part-load conditions. Reversing Valve The reversing valve reverses the flow of refrigerant through the cycle as shown in Figure 17. When the mode of operation changes, the reversing valve is repositioned. The reversing valve is located on the compressor discharge and directs the flow of refrigerant for the changeover from cooling to heating and heating to cooling. Figure 17 Reversing Valve Refrigerant-to-Water Heat Exchanger The refrigerant-to-water heat exchanger is usually a tube-in-tube design. See Figure 18. This design is also called a coaxial heat exchanger because it is wrapped in a circular fashion. The heat exchanger consists of a series of copper water tubes inside steel refrigerant tubes. The passages that the water flows through are small. The coaxial tube-in-tube design tends to be used on the smaller water source heat pumps, which are the majority of units produced. Figure 18 Refrigerant-to-Water Heat Exchanger 15

20 Brazed-plate heat exchangers are also used on WSHP units (depending on the manufacturer and unit size). They consist of a series of stainless steel plates brazed together with every second plate turned 180 degrees as shown in Figure 19. This design creates two highly turbulent fluid channels that flow in opposite directions over a surface area with a high heat transfer coefficient and good performance characteristics. Counterflow of the water and refrigerant maximizes heat transfer. Each layer or circuit is linked to an inlet and outlet via a manifold at either end. Closed Circuit On tube-in-tube and brazedplate heat exchangers, the better choice is to use a closed circuit for the loop water. On tube-in-tube and brazed-plate heat exchangers, the better choice is to use a closed circuit for the loop water. That is because the internal water passages on these types of condensers are not mechanically cleanable. A closed-circuit cooling tower (or open tower with an intermediate heat exchanger) is used with WSHP systems using these heat exchangers. Expansion Metering Device When liquid refrigerant flows from the higher pressure of the condenser to the lower pressure of the evaporator, a metering device must control its rate of flow. In the expansion device, the refrigerant expands as it enters the area of lower pressure in the evaporator. Figure 19 Brazed-Plate Heat Exchanger There are typically two kinds of expansion devices used in WSHP units. One device used is a TXV (thermostatic expansion valve). The TXV meters the correct amount of Figure 20 refrigerant based on actual load Expansion Device (Bi-Flow TXV) conditions. It tends to be used on the higher efficiency heat pump models. The thermostatic expansion valve on a heat pump is a unique bi-flow design that operates in both heating and cooling. See Figure 20. In the heating mode, refrigerant hot gas is condensed to a liquid in the refrigerant-to-air heat exchanger and then enters the TXV, which meters the refrigerant to the refrigerant-to-water heat exchanger. 16

21 In the cooling mode, the refrigerant hot gas is condensed to a liquid in the refrigerant-towater heat exchanger and then metered, by the TXV, to the refrigerant-to-air heat exchanger. The second type of expansion device, the capillary tube, is the simplest of all the expansion device designs. It is a small diameter tube selected to produce the desired pressure drop that allows for expansion of the refrigerant into a gas. This is accomplished by selecting the tube with the correct diameter and length to produce a certain flow at a given pressure. On a capillary tube there are no moving parts. Standard efficiency WSHP units often use capillary tubes as their expansion device. Premium efficiency models and larger units are equipped with TXVs. Refrigerant-to-Air Heat Exchanger The refrigerant-to-air heat exchanger is a copper tube, aluminum fin coil mounted in the draw-thru position. See Figure 21. The coil delivers air at approximately 55 to 60 F in the cooling mode, and 95 to 105 F in the heating mode. The coil is mounted in the draw-thru position, meaning the fan assembly draws the air through it and discharges air to the ductwork. Fan Assembly Most WSHP units use centrifugal fans with a forward-curved impeller as shown in Figure 22. These fans are capable of relatively low external static pressure applications. Typically, units 5 tons and under use a directdrive fan with a multi-speed tap. Larger models will use a belt-driven fan that allows for higher static pressure applications. Figure 21 Refrigerant-to-Air Heat Exchanger Figure 22 Fan Assembly 17

22 Condensate Drain Condensate drains are required on all units. Some heat pumps contain a built-in trap. Normally, horizontal models require an external trap. See Figure 23. For a discussion of the condensate system refer to the system application section. The height of the trap should be at least 1.5 times the expected negative static pressure (in. wg) in the drain pan of the WSHP. The outlet of the trap should be at least 1 in. below the trap inlet. Whether an internal or external trap is provided, a vent should be included in the condensate piping. Figure 23 Condensate Drain Filters Most WSHP units come with a 1-in. filter as standard. See Figure 24. An option to upgrade the filter from a 1-in. to a 2-in. type is common except on some console models. Rooftop WSHP units often have a 4-in. pleated optional filter offering. Unit Controls Figure 24 Typical Filters Each WSHP unit typically comes equipped with the following built in functions. See Figure 25 for an example of a unit control board. Figure 25 Unit Control Board 18

23 Anti-Short Cycle Timer This timer device prevents the compressor from turning on too quickly after turning off which can cause damage to the compressor. The timer delay is usually set at approximately 5 minutes. Random Start Feature The feature provides a random or staggered start up of individual heat pumps. It occurs at initial power up (or loss of power) but not at every compressor call. Otherwise, at the beginning of the occupied cycle, all units could start simultaneously, which would create a potentially large electrical demand. Over/Under Voltage Protection This device is contained in the unit control board. A voltage variation of plus or minus 10 percent of nameplate voltage is acceptable. If voltage is outside this range, the unit will be shut down by this protective safety device. High/Low Pressure Protection Pressure sensors are mounted in the suction line and the discharge line of the compressor. If line pressures exceed the high pressure set point, unit operation is terminated. A low-pressure switch provides loss of charge protection. Some units may use a temperature switch on the discharge line for loss of charge protection. High and low pressure protection is typically manual reset. Freeze Protection Most WSHP units have an air heat exchanger and water heat exchanger freeze protection device built in. For the water heat exchanger, a sensor is set at 30 F for standard closed loop applications, or approximately 10 F for colder ground loop applications. The sensor is used to shut off the compressor if water temperatures fall below this set point. For the refrigerant-to-air heat exchanger, a sensor set at 30 F is typically used. Condensate Overflow This sensor is located in the condensate pan and will shut down the compressor if condensate builds up in the drain pan indicating a potential overflow situation. This function is typically automatic reset. Unit-Mounted Controllers Two basic unit-mounted WSHP controllers are typically available: a non-communicating type (which means the controller is not capable of being connected to a building-wide network communications bus) or a communicating type for use on a building wide network. 19

24 Hydronic Accessories The following are several important accessories that are used with most WSHP installations. Supply and Return Water Hoses The most basic type of hose package is a supply and return hose with fittings and ball valves as shown in Figure 26. This package allows the simple and quick connection of a heat pump to a rigid pipe connection. The flexible hose provides vibration isolation. Balancing is accomplished with the return side ball valve. Valves are equipped with a memory stop. A manual kit includes accessories that allow the system or surrounding devices to be tested and manually adjusted for flow rate. The term kit refers to a supply or return hose with associated valves and accessories. Ball valves are generally included to enable the unit to be isolated and flow shut down. Pressure and temperature ports are included to determine the flow rates at predetermined levels. Temperature ports allow the system performance to be monitored and adjustments made for system balance. Automatic kits are similarly equipped, but the automatic-balance valves eliminate the need to manually adjust the flow rate. See Figure 27. A variety of options such as air vents, strainers, blow down valves, electric control valves, etc., are available to suit specific applications. Figure 26 Supply and Return Water Hose (Photo courtesy of Hays Fluid Controls) Figure 27 Hose Kit with Automatic Flow Control Valve (Photo courtesy of Hays Fluid Controls) 20

25 Automatic balancing valves in piping packages are growing in popularity as they are a major cost reduction method and offer constant flow control over a wide range of operating pressures and flow rates. See Figure 28. The major advantage is the constant adjustment to operating conditions. Automatic balancing valves maintain proper system flow control without periodic manipulation of the flow control devices to accomplish a balance in the system. That means there is a significant reduction in cost to setup and maintain the system. Condensate Hoses A condensate hose is often a clear, vinyl hose accessory. The hose comes with a pre-formed trap. They are usually 46 inches in length and can be field cut to a desired length. Hoses are UL 94 rated and treated with antifungicidal elements to allow for long life with undisturbed flow. Condensate hoses are available in kit form, with end fittings, blowdown fitting tee and clamps. See Figure 29. Ball Valves Ball valves are used for full open/closed service with limited capability for control. Their advantage is low cost, high capacity, low leakage, and tight sealing properties. Other valve types are best for balancing. Figure 28 Manual and Automatic Valves (Photo courtesy of Hays Fluid Controls) Figure 29 Condensate Hose with Pre-Formed Trap (Photo courtesy of Hays Fluid Controls) Gate Valves A gate valve can be used to isolate the WSHP system from the main supply and return. Gate valves are sometimes preferred on larger heat pump applications. Gate valves, also known as stop valves, are designed for shutoff duty. When the valve is in the wide-open position, the gate is completely out of the fluid stream, thus providing straight through flow and a very low pressure drop. 21

26 Strainers A strainer can be used on the supply line to the heat pump to keep the refrigerant-to-water heat exchanger free of debris. Use of strainers on individual WSHP units in closed loop applications is not always done, but a 16 to 20 rated mesh strainer is recommended for any open system like a well or pond application. On closed-loop systems, a central strainer is recommended at the circulating pump. Automatic Shutoff Valve Some WSHP systems utilize variable water flow in order to save pumping energy. When a unit cycles off on a constant flow system, water continues to circulate through the refrigerant-towater heat exchanger. An automatic shutoff valve will stop the flow of water to the exchanger when the compressor is deenergized, allowing the pump to save energy by only pumping water through active units. A bypass line is typically installed in the piping system to allow minimum pump flow. Automatic Balancing Valve This device contains a rubber diaphragm and an orifice plate. Unlike manual balance valves that have fixed-orifice openings, the automatic balance valve is a constant flow/variable orifice device. The hardness of the rubber and the area of the orifice openings determine the flow rate. The valve maintains constant flow by varying the orifice opening with changes in differential pressure. As the differential pressure increases, the rubber diaphragm distorts and presses further into the orifice opening creating a different opening size and maintaining constant flow. WSHP Unit Types Strainer A 16 to 20 mesh rating for the strainer is fine enough to strain out particles found in typical open-loop water systems. Water source heat pump equipment is available in the following unit types. Each type is intended for a specific application. A WSHP system may utilize multiple unit types depending on the building configuration. In this section we will examine these WSHP unit types: Horizontal units Vertical units Console units Stack units Rooftop units Water-to-water units 22

27 Horizontal Units Horizontal units are available in capacities from 1/2 to 10 tons, and are used for ceiling mounted applications. Typically, applications include office buildings, apartments, schools, hotels, and medical clinics. See Figure 30. Figure 30 Horizontal Unit Units are hung from the ceiling with isolation hangers to minimize vibration and the transmission of noise to the occupied spaces as shown in Figure 31. Typically, horizontal WSHP units are suspended by their lifting bracket using threaded connector rods and a rubber grommet isolator. On installations where mostly larger units are used (over 3 tons) and vibration is a concern, the units may be installed with springs. Large capacity units (3 tons or more) are typically located over corridors, over public spaces, or in utility closets. These locations isolate the units acoustically from the conditioned space and make them accessible for service. Office Buildings Horizontal units are the most commonly used WSHPs for large office buildings. They occupy no floor space and are positioned above standard T-bar ceilings, which allow access for maintenance. Figure 31 Typical Horizontal Unit Installation Horizontal units are the most commonly used WSHPs for large office buildings. They occupy no floor space and are positioned above standard T-bar ceilings, which allow access for maintenance. Horizontal units are utilized for both interior and perimeter zones, and are easily integrated with a separate ventilation system to provide fresh air to the zone. Air from the conditioned zone is returned to the horizontal unit through either a ceiling plenum return or direct-ducted return system. 23

28 Vertical Units Vertical units are typically available in capacities from 3/4 to 25 tons, and are commonly located in utility closets or small equipment rooms. Some very large tonnage units are also manufactured as variable air volume units. Units are installed in interior zones of office buildings, or in schools, apartments, and condominiums. See Figure 32. Vertical units require little floor space. Mounting the units on vibration-absorbing pads minimizes sound transmission to the conditioned space. Units typically come with a removable service panel, providing service access to all major components. See Figure 33. Vertical units are easily integrated with a separate ventilation system to provide fresh air to the zone. Air from the conditioned zone is returned to the vertical unit through return air ductwork, or the utility closet itself can be utilized as a return plenum. Console Units Console units are typically available in capacities from 1/2 to 2 tons. See Figure 34. Console units require minimal use of the conditioned floor space. Units typically come with a removable front service panel, providing service access to all major components. Console units are suited for installations beneath windows in perimeter zones or in entryways. Units are commonly used in perimeter offices, hotel rooms, hospital rooms, apartments, dormitories, and condominiums. Figure 32 Vertical Unit Figure 33 Typical Vertical Unit Installation in Utility Closet Figure 34 Console Unit 24

29 Separate filter access is usually provided for easy filter changes. See Figure 35. Ventilation air can be provided integral to the unit through a wall box accessory that introduces air into the back of the unit. However, a separate ventilation system is a more common solution. Stack Units Stack units are typically available in capacities from 1/2 to 3 tons, and are usually built into the wall with only the face panel exposed to the conditioned space. See Figure 36. Exposed cabinet models are also available. Units may also be located in the conditioned space with the pipe risers located in the wall. Stack units are commonly used in high-rise hotels, condominiums, and apartments where floor space is limited and the vertical-stacking feature of the product can simplify piping installation. Supply, return, and condensate pipe risers are built into the rear of the unit at the factory, thereby minimizing field installation labor. Figure 35 Typical Console Unit Installation Figure 36 Stack Unit 25

30 Supply air can be ducted from the top of the unit or supplied from a grille built into the cabinet. Air is typically returned from the zone to the unit through a grille in the front of the cabinet. Units typically come with a removable service panel to provide service access to all major components. Ventilation with stack units is typically provided by a separate ventilation system. Shown in Figure 37 is an example of stack units installed in a multi-floor building. Direct return piping is shown, but reverse return piping is also utilized. Rooftop Units Figure 37 Typical Stack Unit Installation Rooftop water source heat pump units are available in capacities from approximately 3 to 20 tons, and are ideal for applications where it is not desirable to install equipment in a ceiling, utility closet, or small equipment room. See Figure 38. Units are commonly used in low-rise office buildings, schools, hotels, motels, nursing homes, and hospitals. Since the units are located on the roof, no floor space is required, and outside walls need not be penetrated. Units are typically located on the roof over less acoustically critical areas like storage rooms or rest rooms. Units typically come with removable service panels, providing service access to all major components. Figure 38 Rooftop WSHP Unit 26

31 A rooftop WSHP is used for applications where a larger tonnage unit is necessary and no ceiling or floor space is available for other WSHP unit configurations. Unlike a conventional aircooled rooftop, a water source rooftop can be connected to the building water loop. See Figure 39. No Available Space A rooftop WSHP is used for applications where a larger tonnage unit is necessary and no ceiling or floor space is available. Outdoor air is easily provided through the outdoor air intake of the unit, and is controlled by either a twoposition damper or an airside Typical Rooftop WSHP Unit Installation Figure 39 economizer option. Supply air is provided through ductwork to the conditioned zone. Air is returned to the unit through a ceilingplenum return or a direct-ducted return. A standard rooftop WSHP unit is normally applied with approximately 20 percent outdoor air as a maximum, unless preconditioning the outdoor air is accomplished by another means, such as energy recovery. See the Ventilation System Energy Recovery section on page 45. For a detailed discussion, see TDP-910, Energy Recovery. Water-to-Water Heat Pump Units The water-to-water heat pump is typically available in 3 to 30 ton capacities and utilizes two refrigerantto-water heat exchangers. A refrigeration cycle transfers heat from water in the source to water in the load heat exchanger. The water for the load heat exchanger can be used to provide chilled or hot water for airhandling units, fan coils, hydronic baseboard, radiant in-slab piping, or swimming pools. See Figure 40. Figure 40 Water-to-Water Unit 27

32 These units can be piped in multiples in a parallel or series configuration. These arrangements are meant to provide additional capacity beyond what a single unit can offer. If large capacity is required, units may be piped in parallel as shown in Figure 41. Total pressure drop is the same as a single unit. In a series arrangement, lower leaving water temperatures are available than could be accomplished by a parallel or single unit. The leaving load water temperature of the first unit becomes the load entering water temperature of the second unit. This arrangement provides an additional decrease in water temperature beyond the capability of a single unit. Capacity control is accomplished by cycling the units on or off. Preconditioning Ventilation Air Water-to-water WSHP units are used in several applications. One is to precondition the ventilation air in the dedicated outdoor air unit by extracting heat or cool energy from the common loop. Figure 41 Series and Parallel Piping For piping diagrams showing integration of waterto-water heat pumps in a WSHP system, see the System Variations section of this TDP. System Overview Components In this section, we will discuss the components of a closed-loop, commercial water source heat pump system. Cooling Tower The purpose of the cooling tower is to reject excess heat from the common WSHP water loop. The cooling tower is staged to be on full capacity when the loop starts to approach its upper value of 90 F. There are two types of cooling tower designs that are used for a closed-loop WSHP system: Closed-circuit cooling tower Open cooling tower with a heat exchanger Both options ensure that the loop water never comes in contact with the atmosphere. This is accomplished with the open tower by using a plate and frame heat exchanger. The WSHP loop passes through one circuit and the tower loop through the other. Because loop water does not 28

33 come in contact with the atmosphere, both corrosion and scaling in the WSHP refrigerant-towater heat exchangers are eliminated. Scaling degrades WSHP unit performance and reduces equipment life. Both options make use of the evaporative cooling effect, which takes advantage of both sensible and latent heat exchange. The cooling capability of a tower is dependent on entering wet bulb temperature of the outdoor air, not the entering dry bulb temperature as with a dry cooler. Since wet bulb temperatures are always less than the associated dry bulb temperature, the potential for a greater cooling effect is realized. Closed-Circuit Cooling Tower The more popular choice for WSHP systems is the closed-circuit cooling tower. A closed-circuit cooling tower has the WSHP loop water circulated through its coil. See Figure 42. In this design, the water coil is an integral part of the cooling tower. The water coil isolates the loop water from the atmosphere. The potential for contamination is reduced. The tower may be located outdoors or indoors. When it is located indoors, centrifugal Figure 42 type fans are often provided to Closed-Circuit Cooling Tower overcome the additional pressure drop associated with discharge ductwork. The closed-circuit tower is usually equipped with inlet and discharge dampers that can be closed when the tower is off thus minimizing heat loss. Closed-circuit cooling towers for WSHP systems can be justified based on the benefits they supply over open towers: less maintenance, ability to run dry in winter, less down time, and limited fouling on the outside of the tubes. The closed-circuit design results in a higher first cost than open cooling towers for the same tonnage. Much of this cost is from the large water coil and the centrifugal type fan that is required. It is important to realize that the use of closed-circuit cooling towers results in less condenser and piping fouling than with an open cooling tower because the water in the loop is not being aerated and is not exposed to the environment. Also, a relatively small spray pump is used to wet the coil for an evaporative cooling effect versus a larger system loop water pump that an open tower requires. Loop temperature is maintained by staging a spray over the coil and regulating airflow. There is still a requirement for water treatment with a closed-circuit tower. 29

34 Open Cooling Tower with Heat Exchanger For a closed-loop WSHP application on an open tower arrangement, as shown in Figure 43, the following are required: a separate tower water pump sized at approximately 3 gpm/ton a plate and frame heat exchanger, which can be located indoors for freeze protection a field-installed piping run between the heat exchanger and the open tower The pumping costs are greater on Open Cooling Tower with Heat Exchanger an open tower than a closed design because the full loop flow is being pumped through the tower. The open loop part of the system is limited to the circuit on the tower side of the heat exchanger. This open cooling tower arrangement is sometimes used instead of a closed circuit cooling tower on larger projects. With an open cooling tower, the tower sump can be located indoors for freeze protection. In this arrangement, the tower water drains by gravity into the sump when the tower is off. See the Freeze Protection section of this TDP for more information. One negative of an open cooling tower is that it introduces an additional inefficiency into the heat transfer process with the heat exchanger. Also, additional controls are required. However, between the sizes of approximately 200 to 300 tons, the open tower and plate heat exchanger is a cost effective alternative to a closed-circuit tower. Preferences of one type over the other may be regional. An open cooling tower is subject to scaling and corrosion, therefore, professional water treatment is required. Also the evaporation of the circulating water will tend to build up concentrations of the minerals in the water, so bleeding some water off and replacing it with fresh water will help limit concentration. See the Water Conditioning section on page 44 for more information. Boiler (Heat Adder) Figure 43 Open Cooling Tower Between the sizes of approximately 200 to 300 tons, the open tower and plate heat exchanger is a cost effective alternative to a closed circuit tower. If the rejected heat available from the WSHP units operating in the cooling mode is not sufficient to meet the needs of zones requiring heating, a boiler (also known as a heat adder) is required to maintain the loop temperature. The boiler may run on fossil fuel (oil or gas), or electricity depending on the availability, convenience, and relative cost of the various forms of energy. 30

35 If the boiler runs on fossil fuel, it will operate at much higher temperatures (140 F and higher) than the WSHP loop temperature of 60 to 90 F. The introduction of full flow of colder loop water could cause internal heat exchanger condensation and damage (also known as boiler shock). The piping arrangement shown in Figure 44 illustrates a bypass arrangement that allows the system to operate at any temperature above 60 F without condensation forming in the boiler. As a general rule, copper-tube type fossil fuel boilers do not require the piping arrangement shown in Figure 44 to prevent boiler shock. Cast-iron type boilers, however, do require this piping arrangement. If the heat adder is an electric type, thermal shock and condensation within the heat exchanger is not a concern. The heater may be piped as shown in Figure 45. This type of boiler allows are large portion of the WSHP loop to pass through it. Multiple staged heaters are usually supplied for capacity control. Figure 44 Fossil Fuel Boiler Piping Figure 45 Electric Boiler Piping 31

36 Air Distribution System Water source heat pump units (other than console and water-to-water type) deliver a constant volume of air through a low-pressure duct system to the conditioned space. This duct system typically terminates at one or more diffusers located in the ceiling of the conditioned space. In order to provide proper room air distribution during both cooling and heating modes of operation, care must be given to the sizing of the ductwork and the selection and location of the diffusers. Water source heat pump systems benefit from using simple symmetrical duct layouts as shown in Figure 46. One of the benefits of WSHP systems is that the ductwork is a simple low velocity, low pressure design. The design is simplified further if a return duct system is not used. Figure 46 Symmetrical Duct Layouts The basic WSHP air distribution design should accomplish the following objectives: Produce a low pressure drop within the confines of the space available for the ductwork Minimize sound levels by reducing airflow velocities Maintain low cost by using simple layouts and direct duct runs Ventilation Air Distribution Units The dedicated ventilation air distribution unit can be a central station air-handling unit, packaged DX unit, gas-fired unit, or an energy recovery unit. See Figure 47. The use of energy recovery for the ventilation system is highly recommended. ASHRAE Standard 90.1 requires that individual fan systems that have both a design supply air volume of 5,000 cfm or greater and bring in at least 70 percent of the supply air from the outside be equipped with energy recovery. There are some exceptions to this. Refer to TDP-910, Energy Recovery for details. Figure 47 Ventilation Air Distribution Units Packaged rooftop WSHP units connected to the water loop have been used for dedicated ventilation air, but some designers do not like to position the heat exchanger on the roof because of the potential to freeze. Also, additional heat is required since the WSHP refrigerant-to-air coil 32

37 can only accomplish approximately 20 F rise in the air. An energy wheel may be used with the rooftop unit to precondition the outdoor air. Water Pumps Centrifugal water pumps are the most common type of pump used in comfort air conditioning applications. There are several types of centrifugal pumps used in the HVAC industry, such as inline, close-coupled, end suction, vertical split case, and horizontal split case. The type of pump to use for a WSHP system is typically determined by the flow rate and head pressure requirements, available space, serviceability, and first cost. For a detailed discussion on pump types refer to TDP-502, Water Piping and Pumps. Usually two pumps piped in parallel are provided for a WSHP system. See Figure 48. One pump is a 100 percent standby. The pump location within the WSHP loop pushes water into the WSHP units. Heat generated by the pump enters the loop water prior to the flow entering the WSHP refrigerant-to-water heat exchangers. Air Separator and Expansion Tank Air separators are used in addition to an expansion tank in a closed system. Air separators eliminate entrained air from the system. Circulation of the water through an air separator can remove a large percentage of this air. This will improve the overall heat transfer efficiency of the system (air is an insulator) and also reduce corrosion caused by dissolved oxygen. An expansion tank should be sized to handle the excess volume of water that is a result of temperature change. A water expansion tank should be part of every closed-loop system. See Figure 49. The change in volume for a WSHP system is approximately 2 percent for the range of temperatures of a typical closedloop system. The change in water system volume is usually about 1 percent for chilled-water systems and about 3 to 4 percent for normal hot water systems in the 180 to 200 F range. Figure 48 WSHP Loop Pump and Standby Figure 49 Air Separator and Expansion Tank 33

38 Controls The system control requirements for WSHP are in addition to the unit-mounted controls discussed earlier. The system level controls should have the ability to control the following: Loop water temperature by sequencing the heat adder and heat rejecter Occupied and unoccupied time scheduling Pumps and their sequencing Ventilation air supply Alarm, emergency shutdown, maintenance Warm up or cool down cycle Communications to other building systems like lights and fire safety (optional) Integration to outside building automation controls (optional) For a discussion of the WSHP controls, refer to the Controls section of this TDP. Example System Configurations This section illustrates several types of buildings and their configuration for WSHP systems. If the building is divided into a number of separate temperature control zones, some of which call for heating at the same time that others need cooling, that building is an ideal candidate for a WSHP system. There is a good possibility for heat recovery from core cooling units to perimeter heating units. Whenever there are concentrated heat loads (solar, lights, or equipment) in zones that can be used for heating of other zones, a WSHP system should be considered. Reclaim Possibilities Many WSHP systems are used in buildings with limited or no actual heat reclaim possibilities. It should be noted that many WSHP systems are used in buildings with limited or no actual heat reclaim possibilities. If the building configuration does not generate a simultaneous heating and cooling call, then heat recovery cannot take place. However, there are other reasons why a WSHP system may still be desirable. When the occupancy is variable, a WSHP system can shut off units for zones not in use. Also, off-hour occupancy (evenings or weekends) can be accommodated by operating only those units required. Water source heat pump systems are desirable if the building owner wishes to defer a substantial portion of his HVAC investment until individual tenants sign leases for their respective areas. In that case, individual heat pump installation can occur at a later date. If the cost of energy is to be billed to tenants, the WSHP system is able to accommodate that need. There will still be some common component energy to proportion like towers, pumps, and boilers. However, the decentralized approach of the WSHP system lends itself to separate billing by usage. Buildings where floor space is at a premium with limited room for mechanical equipment are good candidates for WSHP systems. If horizontal WSHP units in the ceiling areas are used, only the pumps, a tower and a boiler must be located in or on the building. In buildings where ceiling space is limited, large duct runs may be impossible. Vertical units in closets, or console units under windows are a good solution. 34

39 Single-Story Buildings Single-story buildings are usually good candidates for WSHP systems. They may contain areas of different sizes and usage patterns requiring different air conditioning needs. Larger areas may require their own independently metered HVAC systems. A single-zone cooling/heating unit installed on the roof supplies ventilation air. The arrangement in Figure 50 shows how the major components of a WSHP system can be configured on ground level to serve a one story building. Air is distributed by ductwork in the ceiling plenum space. Horizontal heat pump units can be suspended above the ceiling to leave more available floor space for rental. Two to Four-Story Buildings Figure 50 Single-Story WSHP Layout Here in Figure 51 is a modern looking high-rent, medium size building that can be handled economically by a WSHP System. On this building the WSHP system incorporates a central station air-handling unit with energy recovery located in a rooftop penthouse. The boiler, tower, and pumps have also been located in the rooftop penthouse. Horizontal WSHP units provide heating and cooling throughout the rest of the building. Figure 51 Two to Four-Story WSHP Layout 35

40 High-Rise Buildings In the building shown in Figure 52, horizontal WSHP units provide heating and cooling throughout the building. Outdoor air is supplied by a roof-mounted central station unit with a modulating gasfired heat exchanger with an energy recovery wheel incorporated into the configuration. The tower has been located on the roof to minimize stack and tower discharge ductwork. The boiler and pumps have been located in the basement. System Variations Figure 52 High-Rise WSHP Layout This section discusses variations to the basic closed loop system, which incorporates a boiler, cooling tower, constant speed pumps, WSHP units, control system, and a dedicated ventilation air unit. Variable Volume and Temperature (VVT ) System A VVT system can provide a small, separate zoning system for individual WSHP units. The VVT concept would utilize a standard WSHP unit coupled to a variable volume and temperature computerized control system. The system includes zone dampers controlled by special zone controllers and zone sensors. All controllers communicate to their respective zone damper and then to the WSHP unit through an air source unit controller. A complete VVT system will have numerous zone controllers arranged in a linkage coordinator/zone controller relationship. A VVT system uses a system pilot user interface to provide access and to program the VVT system as well as any other device Figure 53 residing on the communication bus. VVT System with WSHP See Figure 53. VVT Systems For a complete discussion on VVT systems, refer to TDP-704, Variable Volume and Temperature Systems. The WSHP unit with VVT delivers a variable volume of either cold or hot supply air to each zone as load dictates. Constant airflow, however, is maintained through the WSHP unit by use of a bypass damper and controller. For a complete discussion on VVT systems, refer to TDP-704, Variable Volume and Temperature Systems. 36

41 VAV in the Interior Zone Indoor water-cooled packaged VAV units can be incorporated into the WSHP loop. When a load analysis confirms that a large interior area will require varying amounts of cooling all year long, the individual space loads within it benefit from zoning. See Figure 54. This scheme offers the advantage of individual space temperature control and fan energy savings at part load operation. Since it is unlikely that all the spaces served by the by the VAV unit Figure 54 will reach their peak cooling Self-Contained VAV Units in the Interior Zone requirements at the same time, the water-cooled packaged unit is selected for less capacity and less airflow than the sum of the peaks in the individual spaces. This, coupled with the fact that a single water-cooled packaged unit can replace several smaller WSHP units, may reduce cost. This scheme tends to be used in larger WSHP applications where the core has a year round cooling load. Variable Flow Systems The use of variable flow in the WSHP loop is widespread and can provide pump energy savings for many buildings that have temporary occupancy areas like hotels, motels, or multipletenant offices. Because the occupancy varies significantly during the occupied period, many of the WSHP units may be in the off mode for significant periods of time. When individual WSHP compressors are off, water flow to the unit s refrigerant-to-water heat exchanger is controlled by an automatic isolation valve. The result is a reduction in overall system flow and a savings in pump energy. The main loop pump must be provided with a VFD (variable frequency drive), controlled by a differential pressure sensor located in the piping. See Figure 55. The two-way automatic isolation valves must be provided on the return lines for each WSHP unit. The valves should be slow response type to prevent water pressure damage. The WSHP manufacturer can normally supply these valves as an accessory. At the end of the water loop, some automatic three-way valves (or a single-bypass water line) should be used to provide a minimum flow as required by the boiler. Figure 55 Variable Flow WSHP System 37

42 If PVC pipe is used for the loop piping in conjunction with a variable flow system, it is important to avoid high loop water temperature. Piping could sag at high temperatures and low flow. Again, a bypass line for minimum constant loop flow should be considered. The purpose of using a few automatic three-way isolation valves is to bypass the WSHP unit when the compressor is off and still maintain some water flow in the system. In this way, a minimum circuit flow is ensured to protect the pump and/or boiler. Hot Water Pre-Heating Buildings in warm climates typically have year-round cooling loads and are a good supply for low-grade heat. Rather than reject this heat through the cooling tower, the rejected heat may be used as pre-heat for the building s domestic hot water needs in applications like hotels, hospitals, and schools. A separate water-to-water heat exchanger is required to pre-heat the domestic water with loop water. Sprinkler Piping Integrated with the WSHP Loop When allowed by local building and fire codes, combining the building fire-sprinkler with the WSHP water loop can lower installed cost. The design can be used with either single or double riser systems. The piping to the WSHP is tapped into the sprinkler grid. The supply line to each unit must include a shut-off valve and an automatic balancing valve. See Figure 56. The return line requires a check valve and a shut-off valve. In addition, the piping layout for the two-riser system includes a bypass with check valve (around the cooling tower, boiler, and pump) to provide a direct path for the sprinkler water coming in from the city in case of fire emergency. During fire emergency, when sprinklers are activated, the pump of the hydronic circuit is shut off. Each sprinkler head is then supplied directly with the necessary amount of water under pressure from the city main or from a fire pump. During normal operation, the loop water is circulated. Figure 56 Sprinkler System Integrated with the WSHP Loop The system has various other requirements related to codes governing the fire sprinklers and may need a considerable amount of engineering work, as well as close coordination between the HVAC, control, and sprinkler specialists during design, installation, and start-up. Applications are subject to proper qualification by a consulting engineer and the applicable codes. 38

43 System Without a Boiler In some circumstances, it may be possible to omit the use of a central heat adder altogether in a closed-loop system as shown in Figure 57. For instance, in mild climates, where the total heating loads are relatively small, the boiler is sometimes eliminated. However, supplementary heat is still required and can always be provided by an electric heating coil in the WSHP discharge for those units on the perimeter zone. In a system without a boiler, if the Figure 57 water loop temperature stays above System without a Boiler 60 F, the system works in cooling and heating mode in the usual manner. But if the loop water temperature approaches the lower limit of 60 F, more heat is being absorbed from the loop by units in the heating mode than is being added to it by units in the cooling mode. At that point WSHP units must be prevented from absorbing more heat from the loop. A water temperature sensor deenergizes the compressors in the heat pumps requiring heating and the electric heat is cycled on in those zones. If, at the same time, other WSHP units in the system are on the cooling cycle rejecting heat into the loop, the loop water temperature will stabilize and rise. After the temperature rises to an acceptable level, the corresponding perimeter WSHP units are allowed to return to the heat pump heating mode. System With a Storage Tank During the winter or intermediate seasons, some WSHP systems with substantial interior cooling loads may generate more rejected heat to the loop than can be absorbed by perimeter heating units. If that is the case, excess heat can be stored in a tank instead of rejected to the atmosphere through the tower. In the heating season, this excess heat can be used to minimize the use of the boiler. See Figure 58. A storage tank increases the fluid volume in the WSHP system. The storage tank acts as a heat sink whose heated or cooled water can be released whenever it is needed. Depending on the load profile of the building and the local utility s demand structure, a storage tank can help minimize the electrical demand charges caused by an electric boiler. Figure 58 System with Storage Tank 39

44 Storage tanks are sometimes used to maintain loop temperature during the morning warm-up or cool-down period. During the morning warm-up period, all the WSHP units simultaneously require their THA (total heat of absorption) from the water loop. The unit THA is shown in WSHP printed literature or can be generated by their selection software. During this period, the storage tank can help maintain the loop temperature from falling too low by adding heat to the loop. If the beginning of the occupied cycle requires a cool-down mode instead a warm-up mode, all the heat pumps will start in the cooling mode. The rejected heat for each unit will enter the water loop simultaneously. This value (in Btuh) is available from the manufacturer for each heat pump. During this period, the storage tank can help maintain the loop temperature from rising too high by adding cooled water to the loop. Water-to-Water Heat Pump Water-to-water heat pump units can use heating or cooling from the common water loop for various applications like pre-conditioning the ventilation air or for reheating supply air. A typical application during the cooling season is for the heat pump to pre-cool and dehumidify building s central ventilation air. On the load side of the water-to-water heat pump, chilled water is circulated to the air-handling unit where it cools and dehumidifies ventilation air. The water absorbs heat from the ventilation air and returns back to the water-to-water heat pump where the heat is removed via the refrigeration circuit and then transferred to the source heat exchanger. Heated water from the source heat exchanger is used to reheat air in the ventilation unit. The heat is then absorbed from the water by the air in the reheat coil and, if necessary, returned to the cooling tower, boiler, Figure 59 or ground loop for further heat Water-to-Water Heat Pump in Cooling Season Applications rejection. See Figure 59. During the heating season, the water-to-water unit can extract heat from the building s exhaust system by use of a coil in the exhaust air. The unit uses the heat to provide hot water to the ventilation unit s heating coil to temper the air entering the building. On the load side of the heat exchanger, hot water is circulated to the air-handling unit for heating ventilation air. As the heat from the water is released to the air, the water is circulated back to the unit. On the source side of the unit, water is circulated through the heat recovery Figure 60 Water-to-Water Heat Pump in Heating Season Applications 40

45 coil, is heated by the exhaust airstream, and then is circulated to the water-to-water unit. Control valves, as shown in Figure 60, maximize the process of extracting heat from the exhaust. Control valves on the source heat exchanger supply and return water work to minimize or eliminate the need for additional heat from boilers. System Application Topics WSHP Units With Reheat Dehumidification of the supply air can be handled in a number of ways. One method is with a modulating reheat coil. This technology takes advantage of the transfer of heat through the water piping loop. Loop water is diverted during the cooling mode to a hydronic reheat coil positioned downstream of the refrigerant-to-air coil. If the water from the coaxial heat exchanger is not warm enough, the water is sent back for another pass through the heat exchanger to pick up more heat via an internal loop. Proportional reheat is controlled to the desired leaving air temperature regardless of the loop water temperature. See Figure 61. Figure 61 Reheat WSHP Reheat with Loop Water Another reheat method uses a conventional refrigeration cycle based hot gas reheat coil. In many instances, dehumidification can be required under less than full load operating conditions a large part of the time. Therefore it is important that the reheat function is capable of 100 percent capacity in the intermediate seasons of Spring and Fall. During these times the loop temperature can be relatively cool. The desired reheat is achievable by controlling the number of passes through the heat exchanger. Typical applications are classrooms, theatres, auditoriums, or applications where humidity can be a problem. Another reheat method uses a conventional refrigeration cycle based hot gas reheat coil. This method depends on an elevated refrigerant temperature in order to work to full capacity. However, with hot gas reheat, an elevated refrigerant temperature may not be available during intermediate seasons when reheat is required. That is because the unit may be partially loaded at these times. Freeze Protection Water source heat pump systems that have piping exposed to loop temperatures below 32 F should be protected from potential freeze-up. The major component of a conventional WSHP system that may be susceptible to freezing is the water coil inside the closed circuit cooling tower. If an open tower with heat exchanger is 41

46 used, the critical part of the system for freeze protection is the heat exchanger. To ensure proper freeze protection, locate the heat exchanger indoors. Closed circuit towers are normally winterized (protected from freezing) by one or more of the following methods: Full Tower Coil Flow Maintaining full tower coil flow year round is the easiest and most typical way of preventing freeze-ups. Moving water should not freeze. Indoor Located Tower Locating the closed-circuit tower indoors reduces heat loss of the system loop and minimizes the potential for freeze-up. Sump Heater Shown in Figure 62 is a tower sump heater. Steam, hot water, and electric heaters are available. If a heater is not available, the sump can be located indoors to avoid potential freeze-up. Intake and Discharge Dampers When the tower fan is not operating, positive-closure dampers are used to prevent cold air circulation and to seal off the opening to the outside air. See Figure 63. Figure 62 Tower Heating Element Heat Tape on Exposed Piping It is good practice to provide heat tape on water loop lines that run outside or in freezing areas. Antifreeze in the Closed Loop An antifreeze solution can also be added to the closed loop to protect it from freezing. The concentration requirement rises as the required freeze protection temperature is reduced. This translates into an increase in pumping horsepower over the identical fresh water flow rate, Figure 63 Intake and Discharge Dampers 42

47 especially when the concentration of the anti-freeze exceeds 10 to 15 percent. Use of anti-freeze solution also results in a reduction of in WSHP capacity. As the degree of concentration rises, the protection in the fluid increases from burst level protection (where slush is going to form), to freeze protection (where no crystals at all are going to form). A decision must be reached as to the level of protection desired. This can vary with the type of system under consideration. Refer to TDP-622, Air-Cooled Chillers for a detailed discussion of freeze versus burst protection. Antifreeze Solutions Antifreeze solutions are used more frequently in closedloop, geothermal systems where the piping is buried underground than in conventional loop piping systems inside the building. Their use can result in an increased pressure drop through the refrigerant-to-water heat exchanger and a decrease in capacity over fresh water. The term fresh water applies to water that contains no antifreeze. Fresh Water Water may contain chemical additives for corrosion, scale, and bacteria and still be considered fresh water. Glycols The ability of glycols to lower the freezing point of water is the main reason that glycol based heat transfer fluids are popular. Ethylene glycol based fluids are widely used and consist of ethylene glycol (EG), water, and corrosion inhibitors. The term inhibited means that additives have added to prevent (inhibit) the effects of corrosion. Ethylene glycol s disadvantage is that it is toxic if ingested and cannot be used where contact with food or potable water may occur. Ethylene Glycol The disadvantage of ethylene glycol is that it is toxic if ingested and cannot be used where contact with food or potable water may occur. Brines Propylene glycol consists of a mixture of propylene glycol (PG), water, and corrosion inhibitors. Propylene glycol fluids are recommended for use where incidental contact with potable water is possible, or where use of a propylene glycol-based fluid is required by state or local regulations. Brine solutions are corrosive and leakage can cause damage to surrounding materials. It is not recommended for use with WSHP systems. While salt solutions provide satisfactory performance especially in low temperature applications like ice rinks, they should be avoided in WSHP applications because of their potential for corrosion. The use of salt solutions will void some manufacturer s warranties. Brine Salt solutions should be avoided in WSHP applications because of their potential for corrosion. 43

48 Methanol This freeze protection fluid consists of a mixture of methanol and corrosion inhibitors and it is sometimes used in closed-loop ground source heat pump applications. Methanol is also used with industrial refrigeration. It has a low viscosity at the lower temperatures, which makes it attractive from a pumping and heat transfer standpoint. If antifreeze is used, methanol or propylene glycol solutions are the chosen fluids. Freeze protection should be maintained to 15 F below the lowest expected entering loop temperature. For example, if on a geothermal application the lowest expected entering loop temperature is 30 F, the freeze protection solution should offer protection down to 30 F - 15 F = 15 F. So either 21 percent methanol or 30 percent propylene glycol is required. See Table 1. Water Conditioning Table 1 Antifreeze Percentage by Volume Minimum Temperature for Freeze Protection ( F) Antifreeze Methanol (%) % USP Food Grade Propylene Glycol (%) As we have seen, WSHP systems typically utilize small coaxial refrigerant-to-water heat exchangers that must be kept clean to maintain proper heat transfer and system efficiency. The entire loop should be cleaned and flushed before initial start-up. The refrigerant-to-water heat exchangers are bypassed with hoses during this process. Water quality varies from location to location and is unique for each project. Water characteristics such as ph value, alkalinity, hardness, and specific conductance are important in WSHP systems. See Appendix A. A low ph and a high alkalinity can cause system problems. The term ph refers to the acidity, basicity, or neutrality of the water supply. A ph below 7.0 means the water is acidic. A ph above 7.0 means the water is basic. Neutral water has a ph of 7.0. Water typically includes impurities and hardness that must be removed. The required treatment will depend on the water quality and the system type. The three main problems that can result from poor quality water are: Scale formation caused by hard water reduces the heat transfer rate and increases the water pressure drop through the heat exchanger. As water is heated, minerals and salts are precipitated from a solution and are deposited on the inside surface of the pipe or tube. See Figure 64. Corrosion is caused by absorption Figure 64 of gases from the air coupled with Scale water on exposed metal. Organic growths such as algae can reduce the heat transfer rate by forming an insulating coating on the inside tube surface. Algae can also promote corrosion by pitting. 44

49 A water treatment specialist can help with these problems. See Figure 65. One of the advantages of the closed-loop WSHP system is that only normal water corrosion inhibitors are necessary to maintain proper water quality inside the loop system. Some designs also incorporate a solids separator to remove particles. The formation of scale and organic growth is not normally an issue since the loop water does not come in contact with the air. Figure 65 Water Treatment However, the spray water on a closed-circuit cooling tower is open to the environment and requires water treatment and bleed off to maintain chemical balance. For a complete discussion on water treatment, see TDP-641, Condensers and Cooling Towers. If the system is a once-thru design as described in the Geothermal Systems section on page 55, scaling and the growth of algae and slime must be addressed depending on the source of the water. Frequent cleaning must be done. Use of a cupro-nickel heat exchanger is an option depending on water quality. Water testing should be performed prior to using refrigerant-to-water heat exchangers on any well, lake, or river once-thru system. Ventilation System Energy Recovery For energy recovery to take place, the exhaust airstream must exchange heat with the incoming ventilation airstream. Exhaust from the building is accomplished by positive means from toilets, conference rooms, and other spaces within the building. If the building design allows, exhaust air can be ducted to the same location as incoming ventilation air, making it convenient to use most types of recovery products. Figure 66 shows a representation of side-by-side recovery. When the two airstreams cannot be positioned close together, recovery is still possible by using a runaround loop. Figure 66 Side-by-Side Recovery Testing Water testing should be performed prior to using refrigerant-to-water heat exchangers on any well, lake, or river once-thru system. 45

50 ASHRAE Standard 90.1 requires that individual fan systems that have both a design supply air volume of 5,000 cfm or greater, and bring in at least 70 percent of the supply air from the outside be equipped with energy recovery. Dedicated outdoor air units on WSHP systems are a prime candidate for energy recovery. In this section we will describe several energy recovery options that may be incorporated into the ventilation air unit for a WSHP system. For more detailed information, see TDP-910, Energy Recovery. Energy Wheel An energy wheel is a revolving exchanger filled with an air-permeable medium. This medium has a large internal surface area. The exchanger is designed to be installed between two adjacent ducts with opposing flow directions. This establishes a counterflow heat exchange pattern similar to that illustrated in Figure 67. The wheel rotates between 10 and 60 revolutions per minute depending on the application. When the wheel passes through the higher-temperature airstream, the media temperature increases as heat is transferred and stored in the media material. When the media wheel rotates into lowtemperature airstream, the media are cooled and release heat. The wheel can be coated with a desiccant, which allows for total recovery. Total recovery means the transference of both latent and Figure 67 sensible heat between the ventilation Energy Wheel with Rooftop WSHP and exhaust airstreams. In this type application, the building ventilation and exhaust systems are routed into single ducts and brought in close proximity to one another. The wheel then rotates between the two airstreams. Effectiveness is a term used to describe the ability of the energy recovery device to change the condition of supply air from outdoor air to indoor air. The higher the effectiveness, the greater heat or energy transfer occurs. Effectiveness for a rotary wheel is typically in the 60 to 80 percent range. This is high relative to other forms of air-to-air recovery. Wheels are often mounted inside the cabinet of the ventilation system air handler or rooftop packaged unit. They are also available mounted in a separate cabinet with a supply and an exhaust fan for use next to or remote from the ventilation unit. When packaged this way the unit is called an ERV, or energy recovery ventilator as shown in Figure

51 Heat Pipes Heat pipe exchangers have the appearance of ordinary finned coils, but each successive tube is independent and not connected to other tubes. Each tube is built with an internal capillary wick material. The tube is evacuated, filled with a fluid (like refrigerant or water) and individually sealed. See Figure 68. With the tubes installed horizontally, one-half of the heat exchanger will act like an evaporator and the other half acts as a condenser. There is a partition between the halves of the heat exchanger. In summer, the high-temperature outside airstream passes over half of the tubes. As the internal refrigerant is warmed and vaporized in the evaporator half, the incoming outdoor air is cooled. The internal vapor pressure drives the gas to the condenser end of the tube. In the condenser end, the fluid releases the latent energy of vaporization as it condenses, thereby transferring the heat from the incoming outdoor air to the cooler building exhaust air. Heat pipes are useful for sensible heat transfer only. However, some small amount of latent effect is achieved if the hot, humid outdoor airstream is cooled sufficiently to condense moisture on the evaporator end of the unit. A heat pipe s typical effectiveness is approximately 40 to 50 percent. Heat pipes, like all recovery devices, are most effective when the airflows are balanced. Heat pipes can also be mounted in rooftop units and air handlers to precondition the ventilation air. Fixed-Plate Heat Exchangers Figure 68 Heat Pipes (Photo courtesy of Heat Pipe Technologies) There are many different configurations and sizes of fixed-plate heat exchangers. The heat transfer core of a fixed-plate heat exchanger is made from alternate layers of plates, formed and sealed at the edges to create two adjacent, but separate, airflow paths as shown in Figure 69. The most distinct advantage is that the fixed-plate heat exchangers have no moving parts. Sensible heat transfer across the plates from one airstream to the next is driven by the thermal gradient only. Therefore, fixed plate heat exchangers transfer sensible heat only. Figure 69 Fixed-Plate Heat Exchanger (Photo courtesy of AEX USA) 47

52 Fixed-plate heat exchangers are manufactured so there is little or no leakage between the airstreams. Air leakage, however, may occur outside of the fixed-plate exchanger, such as within the air-handling unit casing. Effectiveness of heat transfer is about 60 to 75 percent. Airside pressure drops range from approximately 0.35 in. wg to over 1.5 in. wg of pressure drop in each airstream. Fixed-plate exchangers can be factory-mounted in air handlers or used in ventilation air distribution units. However, wheels are used more often than fixed-plate heat exchangers because they transfer sensible and latent heat. Runaround Loops A coil energy recovery runaround loop is a heat recovery system and distinct from the individual pieces of equipment like energy wheels or heat pipes. Runaround systems circulate a fluid (usually a glycol mixture) between two airstreams to transfer heat using standard finned tube coils. See Figure 70. In the winter, the warm exhaust air from the building is transferred to the circulating fluid, which warms the supply airstream containing incoming cold ventilation air. In summer, the cool exhaust is transferred to the circulating fluid, which then cools the warmer supply air. The supply air coil may be installed in an air handler in a precool and pre-heat position. There may also be multiple coil locations in multiple exhaust and supply ducts. The runaround loop piping connects all the coils together. This is the major advantage of this approach to heat recovery. The major components of the system are: a pump, expansion tank, interconnecting piping, exhaust and supply coils, and controls, which include a 3-way valve. Runaround loops are very flexible and well suited to industrial applications or comfort-tocomfort applications with remote supply and exhaust ductwork. The airstreams do not need to be next to one another. There are no cross contamination issues. Runaround loops have an effectiveness of approximately 50 to 60 percent in transferring sensible heat. System IAQ Topics Figure 70 Runaround Loop Indoor air quality (IAQ) involves the maintenance of the indoor air and is considered acceptable if there are no harmful concentrations of known contaminants and if less than 20 percent of the occupants express dissatisfaction. All participants in the design, installation, and operation of a building share the responsibility of good IAQ. The design engineer, however, has the responsibility to design the HVAC system in 48

53 accordance with construction codes and the generally accepted IAQ standards (ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality). In this section, we will discuss various topics that affect indoor air quality for a WSHP system. See TDP-902, Indoor Air Quality for detailed discussion of this topic. Ventilation and Ductwork The ventilation air system should deliver the recommended amounts from ASHRAE Standard If a ceiling plenum is used to distribute ventilation air, design the ductwork so that the ventilation air is as evenly distributed as possible. Ducting the ventilation air directly to the return duct of each WSHP or directly to the conditioned space ensures the proper distribution of fresh air throughout the building. Unit Filters Use the best filter the unit can safely accommodate. On many WSHP units, this means upgrading to a 2-in. filter. Confirm that unit fans can accomplish the required static pressure of the system with thicker filters. On the ventilation system unit, extended surface filters like bag filters or cartridge filters can be used to accomplish greater efficiencies. Many WSHP thermostats include a dirty filter light option. This is a good feature since it is often difficult to track filter loading on systems with hundreds of units. The dirty filter light on a standard water source heat pump wall thermostat is usually lit after a cumulative number of fan run hours has been surpassed. The number of run hours before the light is activated is usually adjustable on programmable thermostats. This does not take into account the actual filter loading condition. Filter loading can vary depending on airflow and environmental conditions. On some control systems, the filter may be equipped with a pressure switch or differential pressure sensor that measures actual filter pressure drop. Filter pressure drop is an actual indication of filter loading. The building control system can post an alarm when filters need to be changed out. If a ducted return is used, provide a suitable slot in the duct for filter access. Condensate Removal The entire condensate line should be properly vented to prevent fan pressurizetion from causing a blockage in the line. It is also a recommended practice to fieldinstall a secondary condensate drain pan under units above finished ceilings to prevent damage to the ceiling in the event of a plugged condensate drain. See Figure 71. Provide chemical treatment for algae in the condensate pans and drains in geographical areas that are conducive to algae growth. Document the recommendation for periodic flush-out of the condensate system to rid the system of sludge and dirt. Figure 71 Secondary Drain Pan 49

54 Overflow Protection Units can be equipped with condensate overflow protection that shuts the unit down if the condensate level in the drain pan rises too high. Water source heat pump units can be equipped with a solid-state electronic condensate overflow protection that shuts the unit down if the condensate level in the drain pan rises above a safe level. Access For Coil and Condensate Pan Maintenance Regardless of the type of WSHP used, the installation location must provide easy access to inspect and replace filters, inspect and clean the condensate pan, and make unit repairs. When installing horizontal units, do not run rigid piping under any part of the unit. Install a hinged access door in the ceiling beneath the unit. In tee-bar or lay-in ceilings, removal of ceiling panels can be substituted for an access door. See Figure 72 Figure 72. Part-Load Humidity Control Humidity control is not typically a problem at full load. The WSHP unit refrigerant-to-air exchanger will be at its coldest temperature and moisture is being removed from the space. Most humidity problems occur at part-load. If the WSHP has one stage of cooling and is cycled on/off to control room temperature, the room relative humidity may rise during the off cycle. This phenomenon is common to all standard WSHP units up to approximately 8 tons in capacity since they all typically have one compressor and use on/off control to maintain room temperature. Units above 10 tons usually have multiple compressors to provide better part-load humidity control. Some new WSHP units incorporate a single scroll compressor with unloading capability. An internal unloading mechanism provides an unloading step of 67 percent of full capacity. The compressor modulates between the two capacity steps. The result of this technology is better overall unit and system efficiency and control of humidity at part-load conditions. See Figure 73. Some manufacturers provide an optional electronic fan speed control matched to thermostat or to a Access to Unit Figure 73 Compressor Unloading Technology 50

55 humidistat demand. Varying the fan speed in this manner keeps the cooling coil active for much longer periods of time and significantly improves part-load humidity control. Reheat capability with each WSHP unit is an excellent way to control space relative humidity. The air is initially cooled by the refrigerant-to-air heat exchanger, then the air heated as required to the desired space dry bulb and relative humidity. ASHRAE Standard 90.1 allows for reheat as long as the source is site-recovered energy. This is the case with hot gas or loop water reheat. Lastly, the ventilation air handler can pre-cool the outdoor air to a neutral condition using recovery coupled with an on board cooling coil as required. If outdoor air is ducted, it can be conditioned to the unit supply air temperature and sent directly to the unit for mixing with return air. This relieves the WSHP unit of any latent outdoor air loads and helps with part load humidity and control. Demand Controlled Ventilation The design ventilation airflow for each space in the building is established by ASHRAE Standard The design rates ensure adequate dilution of room contaminants and a healthy occupant environment. The ventilation equipment is then set to provide the design airflow rate as long as the building is occupied. However, the occupancy rate for individual spaces, as well as the building as a whole, varies throughout the day. For example, there can be a large drop in building occupancy around noon as people leave for lunch. If the current occupancy is lower than the design occupancy, then a lower ventilation airflow can be used. Operating at a lower ventilation airflow during these times can save significant operating cost dollars, while maintaining adequate building IAQ. Direct digital control systems are capable of utilizing CO 2 sensors to track the occupant density in the building and match the ventilation rate to occupancy needs. See Figure 74. This can be done on a space-by-space level, and the ventilation source is continually adjusted to match space needs. With a WSHP system, the ventilation air duct from the dedicated ventilation air unit can be routed into the unit inlet duct. The inlet duct is a small length of ductwork provided by the contractor. An internally lined elbow is often used. This scheme directs the correct amount of ventilation air to each WSHP zone instead of just spilling it in the proximity of the unit. Certain areas of a multi-unit WSHP system can be designed for DCV usage, such as conference rooms, cafeterias, or other large zones where heavy ventilation amounts may exist. Figure 74 CO 2 Sensor 51

56 Acoustics Considerations Following are recommendations for three major types of WSHP units with respect to sound. Manufacturers offer software programs to assist in detailed analysis. Measured sound power ratings on the equipment and a software program can be used to determine the sound attenuation effect of the ceiling and room. In addition to the software analysis, there are many suggestions that, if followed, can minimize unwanted noise reaching the conditioned space. See TDP-901, Acoustics for detailed information on this subject. Horizontal Units To minimize sound from horizontal WSHP installations (see Figure 75): Use common sense in locating units. Do not locate larger WSHP units over a sound sensitive space. Try and position units above hallways, utility closets, restrooms, or storage rooms. Provide at least 10 ft between WSHP units to avoid the additive effect of two noise sources. Hang the unit as far above the ceiling tile as practical. Size the sheet metal supply duct with velocities no greater than 1000 fpm. Figure 75 Locate the supply duct balancing damper as far away from the Sound Control for Horizontal Units outlet diffuser as possible. Locate the balancing damper at the trunk duct exit. If return air is drawn through a ceiling plenum, provide an acoustically lined return duct elbow or L shaped boot at the WSHP to eliminate line-of-sight noise into the ceiling cavity and return air grilles. Face the elbow or boot away from the nearest adjacent WSHP unit to prevent additive noise. Use the factory-available unit sound treatment. Vertical Units Minimize Sound Use spring isolators on WSHP units located above and below or where lightweight floor construction is used Vertical units tend to be installed in small equipment rooms or closets. To minimize sound from vertical WSHP installations: Mount the unit on a pad made of high-density sound absorbing material such as rubber or cork. Prevent line-of-sight noise into the space if there is a grille mounted in the closet door for nonducted return applications. Use an elbow with turning vanes in the direction of the fan rotation to minimize discharge turbulence. 52

57 Console Units With console units, the fan and compressor are located within the space, and only the unit casing design attenuates the transmission of these sound sources. The designer should review the decision to utilize console units in general, and if concerned, use lowest fan speed to make unit selections. Refrigerants As of this writing, most manufacturers of WSHP units utilize R-22 refrigerant. R-22 has been the refrigerant of choice for many years; however, this situation is in transition. R-22 contains chlorine, which, if released to the atmosphere, has detrimental environmental effects. Therefore R-22 is being phased out under current legislation. By the year 2010, R-22 cannot be used in new equipment. Consequently, newer WSHP designs will utilize new blends like R-410A (Puron ) and R-407c. With the year 2010 in mind, manufacturers are modifying their products in phases to use new environmentally sound refrigerants. It should be understood that it is not possible to replace R-22 with R-410A in an existing WSHP unit. R-410A has far different pressure, temperature, and heat of vaporization characteristics than R-22. It is acceptable to have WSHP units of different refrigerants on the same water loop. For instance, new R-410A units can share the same water loop as R-22 units. Please consult TDP-402, Refrigerants for a full discussion concerning refrigerants. See Figure 76. Refrigerants Figure 76 Refrigerants Not Scheduled for Phase-Out The phase out date for R-22 is 2010 when supplied in new equipment and 2020 for service. Airside Economizer An airside economizer is a standard energy saving feature on most commercial air-handling units, rooftop units, and indoor packaged units. ASHRAE Standard 90.1 mandates the application of economizers in most regions of the country (with exceptions). An airside economizer provides free cooling with outdoor air whenever the temperature and relative humidity conditions outside are acceptable. One type of economizer control is a simple dry bulb changeover economizer that uses outdoor air for cooling whenever it is approximately 55 F or colder outside. At this changeover temperature, the compressors are turned off to save energy. Shown in Figure 77 is an arrangement for an airside economizer with water Figure 77 source heat pumps. Economizer for Horizontal WSHP Units 53

58 On a WSHP system, however, the use of an airside economizer is not required and might actually result in greater overall energy consumption. Water source heat pump units providing cooling with an economizer are not operating their compressors and, therefore, not rejecting heat into the loop. The potential exists for the loop temperature to drop below the lower limit, since some perimeter units may be absorbing heat from the loop. This may require the boiler to be activated to maintain loop temperature. Running the boiler and consuming new energy may cost more than is saved from shutting off the compressors. Economizers If the loop water temperature during the economizer mode of operation stays above the boiler activation temperature (approximately 60 to 65 F), the air side economizer operation with WSHP units is acceptable. Water-Regulating Valves If the loop water temperature during the economizer mode of operation stays above the boiler loop water activation temperature (approximately 60 to 65 F), then airside economizer operation with WSHP units is acceptable. In a WSHP system, the economizer would have to be locked out to allow for the normal operation of the heat recovery from the interior zone cooling units to the perimeter zone heating units. Coupled with the additional cost of duct outlets to each WSHP unit sized for full unit airflow (and the cost of controls), an economizer is not often utilized. A water-regulating valve can be used to maintain the proper head pressure for WSHP units installed on widely fluctuating entering water temperature applications such as open loops. For closed-loop applications, water-regulating valves are not typically necessary since the loop temperature is maintained by the boiler and tower through the system controls. The waterregulating valve responds to rising and falling refrigerant pressure and controls the flow through the refrigerant-to-water heat exchanger for proper unit operation. Maintenance Some designers feel the maintenance aspects of a WSHP system are a disadvantage because a typical commercial installation has many units positioned throughout the building. Some may be under windows, while some may be above ceilings, while others may be in utility closets. However, WSHP units are designed to operate while requiring no greater amount of maintenance than any other watercooled air-conditioning unit. See Figure 78. The components of a WSHP that require maintenance are the refrigerant-to-air coil, the blower assembly, the filters, the refrigerantto-water heat exchanger, and the drain pan. Let s discuss maintenance requirements of each of the components. Figure 78 Maintenance of Ceiling-Mounted Unit 54

59 Refrigerant-to-Air Coil The coil should be inspected yearly and any lint or dirt should be removed from the fins. The drain pan should be kept clean and free of any bacteria producing growth. The condensate drain line must be open for the complete removal of any water that condenses on the coil in the cooling mode. Blower Assembly Most units have direct drive blower assemblies so maintenance of belts is not an issue. Larger units with belt drive blowers should have an annual inspection. Filters Filters must be changed at regular intervals in order to promote good IAQ. Dirty filters restrict the airflow and degrade fan performance. Overall unit efficiency is affected. Using a dirty filter warning light on the unit thermostat can help the operating staff maintain clean filters. Refrigerant-to-Water Heat Exchanger Since most closed-loop systems use a closed-circuit cooling tower, sediment inside the loop after initial commissioning is generally not an issue. However, the individual refrigerant-to-water heat exchangers should be inspected and chemically cleaned if waterside fouling has occurred. Since all units are on the same loop, a representative unit can be inspected for film formation or deposits inside the tubing. The strainers in a system loop require cleaning if the water source is a well, river, or pond. This is discussed in the geothermal section. A water quality test must be conducted prior to using well, river, or pond water with a WSHP refrigerant-to-water exchanger. The services of a water treatment specialist are recommended on systems where an open tower has been used. Deposits of minerals and contaminants must be kept at an acceptable level. Since some WSHP systems incorporate an antifreeze mixture, the mixture should be checked periodically to maintain the desired solution percentage. Major pieces of equipment like boilers, cooling towers, and pumps have manufacturer s procedures that should be followed. Geothermal Systems Up to now we have been discussing closed-loop WSHP systems where the loop piping is run inside a commercial building. With closed-loop systems, a heat adder and heat rejecter are used to keep the loop within a normal operating range of 60 to 90 F. This is the majority of applications for WSHP systems. Limited Applications Geothermal systems are often used for residential and medium to small commercial applications. However, some WSHP systems can be designed to take advantage of the fact that the Earth s resources (ground or water) remain at a relatively constant temperature at a certain depth all year long. Instead of using a conventional heat adder like a boiler and a conventional heat rejecter like a closed circuit tower, the ground or water can be used as the heat sink for absorption or rejection of heat. 55

60 The term geothermal is used to describe using the Earth s ground or water as a heat sink for WSHP systems. Geothermal systems tend to be used for residential and medium to small commercial applications. This is because land areas, boring costs, and once-thru water quantities are limited on most applications. Geothermal heat pumps can work with closed loops or open loops. Open loops are used with well, river, or lake water applications. Open-loop designs are also known as ground water systems. Closed-loop designs are often called ground-coupled or ground-loop systems. Most manufacturers offer extended range WSHP units for use in geothermal applications. These extended range units can handle entering fluid temperatures from 20 to 120 F (instead of the standard 60 to 90 F range) and incorporate factory-installed insulation on the coaxial coil and refrigerant and water piping to prevent condensate from dripping. The units also have fieldselectable freeze protection for well or loop application. Units used with ground water are usually available with an optional cupro-nickel coaxial heat exchanger when there is concern about the water quality. Thermostatic expansion valves are the required metering device. Extended range water source heat pumps typically are used for all geothermal applications. There are several factors that affect the design of a geothermal WSHP system. For instance, assuming the same soil type, a small capacity system will require a relatively small ground-source loop, while a larger capacity system will require a larger ground-source loop length. Some of the application factors that are evaluated prior to designing a geothermal system are: Ground water availability and quality Loop installation costs Land area availability Subsoil conditions Local codes Owner preferences Many regions have contractors specializing in the installation of the ground loop portion of the earth-coupled system. Most heating and air conditioning contractors are not experts at soil identification. There are two choices available: become proficient at soil identification through study and field experience or have an independent soil testing laboratory or geologist perform the identification. For the contractor not familiar with earth-coupled comfort systems, this second choice will remove any doubt about this aspect of system design. Closed-Loop Systems Closed-loop systems consist of an underground (or underwater) heat exchange network of sealed, high-strength, polyethylene plastic pipes and a pumping module. When in the cooling mode, the loop fluid temperature will rise, and rejected heat is dissipated into the ground or water. Conversely, while heating, the loop fluid temperatures fall, and heat is absorbed from the ground or water. The pump module circulates the water/anti-freeze fluid within the piping system. Closed loops do not require a ground water supply or drain, and they are not subject to mineral build-up. Closed loops can be installed in vertical or horizontal ground configurations, or submerged in a pond or lake. When designed properly, all three alternatives operate with similar efficiency. Typically high-density polyethylene (HDPE) pipe is used for all closed loop installations. Pipe 56

61 connections are heat fused to form strong joints. The heat-fusion process requires special tools and training and should not be attempted without these items. The pipe manufacturer can provide additional literature and training on this subject. Loop piping has a life expectancy in excess of 50 years. High-density polyethylene is the recommended pipe material that should be used in the ground-source loop. Materials such as PVC should not be used for a ground-source loop. Polyethylene pipe is available in a variety of diameters in straight lengths and coils. Fittings to perform various functions are also available. For closed-loop systems, if the fluid may reach freezing temperatures, an antifreeze solution is used instead of fresh water in the pipes. Refer to the Freeze Protection section of this TDP. Horizontal Loops Life Expectancy Loop piping has a life expectancy in excess of 50 years. High-density polyethylene is the recommended pipe material that should be used in the ground-source loop Horizontal loops are often considered when adequate land area is available. The pipes are placed in trenches, excavated by a backhoe or chain trencher to a depth of 4 to 6 ft. Depending on design, one to six pipes are installed in each trench. Multiple pipe and coiled spool configurations are often used to conserve land requirements and reduce overall installed loop costs. See Figure 79. Trench lengths range from 100 to 400 ft per system ton. Pipes are spaced from 6 to 10 ft apart. See Figure 80. The overall land area required ranges from about 750 to 1500 ft 2 per system ton. Horizontal designs can use a series or parallel flow path. Series paths offer higher performance per pipe length, but a large pipe size must be used and the pressure drop can become too high. Figure 79 Horizontal Closed-Loop System Parallel Piped Figure 80 Horizontal System Trench 57

62 Vertical Loops Vertical loops are the ideal choice when available land area is limited. See Figure 81. Drilling equipment is used to bore small diameter vertical holes. Two pipes joined together with a U-Bend fitting are inserted into the vertical bore. Vertical systems use piping installed in bore holes. The space around the pipe is filled with a grout material. This provides support and also promotes heat exchange between the pipe and the ground. Bore hole depth ranges from 100 to 300 ft per system ton. Bores should be spaced about 20 ft apart and properly grouted. The land space that is required ranges from 100 to 200 ft 2 per system ton. The number of loops requires depends on ground conditions, depth of each hole, and load requirements. See Figure 82. Figure 81 Vertical Loop Figure 82 Vertical Loop Installation Pond and Lake Loops Pond or lake loops are very economical to install when a body of water is available. See Figure 83. The water serves as the source for absorption and rejection of heat. Local codes may not permit the use of a lake or pond for heat transfer. This must be checked before using a lakeloop design. Figure 83 Pond and Lake Loops 58

63 One popular design uses a spiral loop or spool. These designs require less area than straight pipe systems. A pond or lake is attractive as a heat sink because excavation costs are virtually eliminated. Coils or spooled mats of pipe can be placed in the pond or lake. A typical residence would require ¼ to ½ acre of water surface at a depth of 8 to 10 ft. See Figure 84. The coils should not rest on the bottom of the lake so heat transfer can occur on all sides of the coil. Figure 84 Coil Loop Installation in Pond Open-Loop Ground Water Systems Open-loop systems utilize ground water as a direct energy source when good quality water is available at a reasonable pumping depth. It is very important to examine the water quality and quantity first. A well must have enough capacity to deliver a minimum of 1.5 gpm per ton during peak operation. On every open-loop system, after the water absorbs or rejects the building heat, the water must be returned. When two wells are used, the source is called the production well and the discharge is called the injection well. Ditches, ponds, or streams are the most common discharge systems. See Figure 85. Recirculation wells can also be utilized in some regions. Local codes will often govern how and where water may be returned to the earth after use in a WSHP system. In ideal conditions, an open loop application can be the most economical type of system to install. See Figure 86. Water quality is an issue on these systems. Mineral build-up inside the refrigerant-to-water heat exchanger is a concern. Usually an intermediate plate and frame heat exchanger is employed. An intermediate heat exchanger causes a slight decrease in overall efficiency and must be kept clean and excessive fouling avoided. Figure 85 Open Loop Figure 86 Recirculation Well (Photo courtesy of Oklahoma State University Boring and Environmental Thermal Systems Group) 59

64 Hybrid Ground Water System This system utilizes two independent refrigerant circuits on the heat pump unit. A closed loop of approximately 60 percent of the typical length is installed as the source for the first stage refrigerant circuit with ground or city water as the source for the second stage refrigerant circuit. See Figure 86. The unit will operate the majority of the time on the earth loop and only use the well or city water a small percentage of the time. Benefits include a reduction in required land area, less expensive earth loop, increased overall cooling and heating capacity and efficiency, and much lower water usage than a standard once-thru ground water system. The annual well/city water consumption is approximately 20 percent of a typical ground water application. Geothermal System Advantages Now that we have seen many of the configurations possible with geothermal systems, let s discuss why its popularity is growing. High Efficiency The extremely high levels of efficiency are possible because a geothermal heat pump only uses power to move heat, not produce it. A geothermal WSHP unit typically supplies about 4 kilowatts of heat for every kilowatt of electricity used. Three of these kilowatts of heat come Hot Water Generator Some geothermal units also include a hot water generator, which diverts a portion of the supplied heat to the domestic water heater. High Comfort Levels directly from the earth itself, and are clean, free and renewable. Some geothermal units can also include a hot water generator, which diverts a portion of the supplied heat to the domestic water heater. This option is used for residential systems and can provide a substantial portion of a family s hot water needs at a very low cost. Geothermal heat pumps can provide high comfort levels for the conditioned space. By using a relatively warm source of heat such as the Earth, supply air temperatures are maintained. Geothermal heat pumps may also cycle less often than other forms of heat like fossil fuel boilers, creating a more consistent indoor temperature. Environmentally Sound The environmental advantages of geothermal systems appeal to governmental agencies such as the Environmental Protection Agency (EPA) and the Department of Energy (DOE). Because it is lowest in CO 2 emissions, geothermal technology provides a solution to global warming by primarily using the natural energy of the earth. In contrast, traditional space conditioning systems depend upon burning of fossil energy sources with the resultant greenhouse gas emissions. 60

65 Low Operating Costs Attractive life-cycle costs are provided by the low operating and maintenance costs of geothermal systems, even when the higher initial installation costs are considered. Life expectancy of the WSHP unit exceeds 20 years. Electric utilities, recognizing the dual benefits of high efficiency and low electric peak demand, often provide incentives to purchase these systems. Codes and Standards There are a number of codes and standards applicable to water source heat pumps that are important to understand. They have been divided into performance and safety related categories in this section. Performance Related Codes and Standards Air Conditioning and Refrigeration Institute (ARI) The ARI is a trade association for the industry that has established a water source heat pump certification program with defined testing procedures and tolerances. Manufacturers whose equipment bears the ARI label participate in a program of random audit certification testing. See Figure 87. The test checks that the water source heat pump performs per the manufacturer s ratings as represented in product rating literature such as selection software and catalogs. The ARI standard for water source heat pumps is ARI/ISO Standard This was the first ARI Subsection to incorporate an international standard into its certification activities Figure 87 when it was adopted on January 1, This standard ARI/ISO Seal covers all water-to-air and brine-to-air heat pumps and replaced the former ARI Standards 320 (boiler/tower), 325 (ground water) and 330 (ground loop). ARI/ISO Standard covers those heating and cooling systems usually referred to as water source heat pumps. A system may provide cooling, heating, or both functions. The system is typically designed for use, within one or more of the following liquid heat source/sink applications: Water-loop heat pump using temperature-controlled water circulating in a common piping loop Ground-water heat pump using water pumped from a well, lake or stream Ground-loop heat pump using brine circulating through a subsurface piping loop Standard uses different operating rating conditions than the previous standards. The standard also uses a consistent methodology for including fan and pump energy to calculate cooling capacity, heating capacity and energy efficiency ratio (EER). Tables 2 and 3 compare ARI and ISO rating and performance test conditions. 61

66 Table 2 Comparison of ARI ISO Rating Test Conditions Rating Tests Water-Loop Heat Pumps Ground-Water Heat Pumps Ground-Loop Pumps ARI/ISO ARI 320 ARI/ISO ARI 325 Hi ARI 325 Lo ARI/ISO ARI 330 Standard Cooling Air dry bulb, F Air wet bulb, F Airflow rate, cfm per mfr per mfr per mfr per mfr per mfr per mfr per mfr Liquid full load, F Liquid part load, F Liquid flow rate, gpm per mfr per mfr per mfr per mfr per mfr per mfr per mfr Standard Heating Air dry bulb, F Air wet bulb, F Airflow rate, cfm per mfr std clg std clg std clg std clg per mfr std clg Liquid full load, F Liquid part load, F Liquid flow rate, gpm per mfr std clg per mfr per mfr per mfr per mfr std clg External Static Air H2O Liquid, ft H2O 0 na Table 3 Comparison of ARI ISO Performance Test Conditions Performance Tests Water-Loop Heat Pumps Ground-Water Heat Pumps Ground-Loop Pumps ARI/ISO ARI 320 ARI/ISO ARI 325 ARI/ISO ARI 330 Maximum Cooling Air dry bulb, F Air wet bulb, F Liquid, F Maximum Heating Air dry bulb, F Liquid, F Minimum Cooling Air dry bulb, na Air wet bulb, F na Liquid, F na Minimum Heating Air dry bulb, F 59.0 NA Liquid, F 59.0 NA Enclosure Sweat Air dry bulb, F Air wet bulb, F Liquid, F Notes: All ratings based upon 208v operation Ground loop heat pump ratings based upon 15% antifreeze solution For ease of comparison, most of the U.S. manufacturers list ARI/ISO performance for their WSHP units. Please note that a rating based on ARI conditions will not compare equally with a rating based on ARI/ISO conditions. 62

67 ASHRAE Standard 90.1 The ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) organization establishes and maintains standards for the industry. ASHRAE Standard 90.1 defines minimum energy efficiency standards for a variety of building components, including air conditioning equipment. This standard also applies to water source heat pumps. The standard defines the minimum EER, IPLV (integrated part load value), and COP of the units. Below in Table 4 are the current water source heat pump efficiency requirements. Table 4 WSHP Efficiency Requirements Subcategory or Rating ASHRAE 90.1 Minimum Efficiency (1989) ASHRAE 90.1 Minimum Efficiency (2004 Code) Size Category Standard Condition OBSOLETE APPROVED <17,000 Btuh ARI F EWT 9.3 EER ISO F EWT 11.2 EER 17,000 Btuh and 210/ F EWT 9.3 EER < 135,000 Btuh ISO F EWT 12.0 EER <135,000 Btuh 210/ F EWT 11.0 EER 59 F EWT 16.2 EER <135,000 Btuh 210/ F EWT 10.0 EER 77 F EWT 13.4 EER EER (Energy Efficiency Ratio) Standard efficiency WSHP units have an EER range (which varies by size) of about 12.0 to 13.0 for closed water loop (boiler/tower) applications. Premium efficiency R-22 models have an EER range of about 12.5 to Premium efficiency Puron refrigerant two-stage models have full-load EERs from 15.0 to 16.0 and part-load EERs from 17.5 to 18.5 when used in closedwater loop applications. All of the EERs are higher for geothermal applications. That is because the entering water temperature for EER calculations in geothermal systems (set by the ISO Standard) are cooler than for conventional boiler/tower systems. Energy Star Energy Star is a voluntary labeling program of the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Energy that identifies energy efficient products. Qualified water source heat pump products exceed minimum federal standards for energy consumption by a certain amount, or where no federal standards exist, have certain energy saving features. These units may display the Energy Star label. Energy Star models exceed all levels of legislated efficiency and are designed for utility and/or local and federal government rebate and energy-driven incentive programs. 63

68 As an example, a state energy agency may offer $140/ton for 14.0 SEER units up to 5 tons and $500/ton for using Energy Star rated units on a geothermal application. These figures are for illustration only. Actual rebate programs will vary based on the local situation. ASHRAE Standard 90.1 was developed to establish building efficiency requirements in the U.S.A. Energy Star efficiency standards exceed ASHRAE Safety Related Codes and Standards UL/CSA and ETL Several safety standards apply to water source heat pumps. Agencies such as Underwriters Laboratories, Inc., Canadian Standards Association, and ETL Testing Laboratories, test for compliance to those standards. When a water source heat pump is approved by these agencies from the required testing, it will bear a mark or label from the certifying agency. UL (Underwriters Laboratories, Inc.) is an independent nonprofit organization that tests products for safety and certifies them. The Canadian Standards Association (CSA) is a non-profit association serving business, industry, government, and consumers in Canada. Among many other activities, CSA develops standards that enhance public safety. For heating, ventilating, and air conditioning, UL/CSA Standard UL 1995 / CSA C22, Heating and Cooling Equipment applies. The ETL Testing Laboratories, like UL, conducts electrical performance and reliability testing. OSHA (Occupation Safety and Health Administration) recognizes ETL and UL as nationally recognized testing laboratories. The ETL Listed Mark and Canadian-ETL Listed Mark are accepted throughout the United States and Canada as compliance with nationally recognized standards such as ANSI (American National Standards Institute), UL, and CSA. This certification mark indicates that: The product has been tested to and has met the minimum requirements of a widely recognized U.S. product safety standard. The manufacturing site has been audited. The applicant has agreed to a program of periodic factory follow-up inspections to verify continued conformance. If the mark includes a small US and/or C, it follows product safety standards of United States and/or Canada respectively. System Sizing and Layout Tips In this section we will review the sizing and layout of the major components that comprise a typical closed-loop WSHP system. The reader is encouraged to consult with the Carrier System Design Guide Water Source Heat Pumps for detailed sizing and a sample layout of an example building. 64

69 WSHP Units Most manufacturers offer computerized selection programs to aid in the selection of individual WSHP types. See Figure 88. Manual selection can also be made from product catalogs. Regardless of the method involved, the following information is required to make a selection for a waterto-air WSHP unit: Total capacity required Voltage Airflow or static pressure Entering dry bulb and wet bulb temperature Loop entering water temperature during cooling and heating Flow rate Unit configuration The final selection of the unit should ensure that it meets the zone sensible heat and latent heat loads. See Figure 89. Office applications typically have an 85 percent sensible heat ratio (15 percent latent, 85 percent sensible), and WSHP units are well suited for this need. Applications with a lower sensible heat factor (higher latent load) require selection at a lower cfm per ton in order to meet the sensible and latent needs of the space. Oversizing of units by more than 20 percent is discouraged since the units will cycle more often and lose control of zone relative humidity at part load. When the unit s sensible capacity is close to the required zone sensible load, minimal cycling is assured. Figure 88 Selection Program Base Unit Screen Figure 89 Unit Performances Screen 65

70 Many selections for WSHP units are based on a relatively low external static pressure of about 0.25 in. wg for each unit. This is because the WSHP units are located close to the area they serve and have minimal duct lengths. The final duct layout is usually not known at time of selection. When the final duct design is finished, the unit fan can be checked. Since most units are direct drive, different fan speeds are available. If the unit selections are based on low fan speed, and slightly more capacity is required, the unit can always be switched to higher fan speed. See Figure 90. The following water temperature assumptions are often used to select the units. During cooling, the loop can rise to as high as 90 F. Thus, for cooling duty, the WSHP units can be selected to make capacity at 90 F entering water temperature. During the design heating load, the building will be unoccupied, and the loop temperature could drop to 60 F as an operational low temperature. Thus, the units heating performance can be based on 60 F entering water temperature. Cooling Tower Selection Figure 90 Final Selection Screen The cooling tower selection is usually based on the heat of rejection of the block cooling load. It may need to be sized based on the total heat rejection of all the installed units in the building. The engineer must examine the building loads and determine whether or not all units can operate at full cooling together. This may be the case on smaller WSHP projects. When the tower size is based on the heat of rejection of the block cooling load, as opposed to the heat of rejection of the total installed units, a stagger start up of heat pumps to reduce morning pull-down load is recommended. That way, all WSHP units are not simultaneously rejecting heat to the loop. Figure 91 Closed Circuit Tower Selection Screen (Photo courtesy of Baltimore Air Coil) 66

71 On larger jobs, the cooling tower size is typically selected based on the following inputs (as shown in Figure 91): Fluid type (fresh water, propylene glycol, etc.) Water flow (gpm) this is the total loop water flow for all the units in the system. Entering fluid temperature calculated as follows: EWT = 90 F + [(THR) (gpm 500)] THR = block total heat rejection load, in Btuh. gpm = total loop water flow 500 = conversion factor Leaving fluid temperature this is the temperature of water exiting the tower. We will use 90 F since that is the upper loop temperature Entering wet bulb temperature this is the outdoor wet bulb temperature that exists when the block load is at peak, and it will be found on the printout of the block cooling load. It can also be found from the ASHRAE outside design conditions for the location being considered. As shown in Figure 92, typical computer selection results are tower model, dimensions, airflow, fan motor hp, coil volume, connection sizes. Sound data is available as an option also. Boiler (Heat Adder) Figure 92 The auxiliary boiler adds heat to the hydronic circuit to maintain the water temperature above the lower limit (usually 60 F). See Figure 93. In most commercial buildings, the maximum demand on the boiler takes place at morning warm-up on a winter day. Tower Computer Selection Screen (Photo courtesy of Baltimore Air Coil) Figure 93 Boiler 67

72 To simplify the boiler selection and avoid under-sizing, three assumptions are often made: 1. There is no ventilation air load at start-up, and no heat gain credit from lights, people, or any other zones. 2. The cooling tower is a closed-circuit type. It is installed indoors and does not contribute heat loss to the loop. 3. There is no storage tank. For systems without night set-back, the installed boiler capacity should sized between 70 and 90 percent of the block heating load. This varies based on the building configuration and amount of perimeter space versus core space. Buildings with greater areas of perimeter spaces use the 90 percent rule. Boiler Sizing A second method for boiler sizing with night setback is to multiply the block heating load of the building by Ventilation System The ventilation air load contains both a sensible and a latent component. This load is best handled by a dedicated ventilation air unit. See Figure 94. If the load is handled by the ventilation air system, then the individual WSHP units only need to be sized for their space loads. Since the outdoor air load is variable, a dedicated system can be selected to maintain reasonably close control of the ventilation air exit conditions. The heat pumps will handle their individual space loads leading to better humidity control. The units will also be smaller to save valuable space. For systems with night set-back, the boiler capacity should be sized for the heat of absorption of all connected WSHP units. This assumes that all units will be in heating mode simultaneously. Please note that a staggered start would avoid all the units starting simultaneously. A second method for boiler sizing with night setback is to multiply the block heating load of the building by The 25 percent extra is to account for morning heating allowance. Figure 94 Dedicated Ventilation System The ventilation system fan should be sized to supply the required outdoor air necessary based on building occupancy and use per ASHRAE Standards. The cooling capacity for the ventilation air unit typically is sized to deliver neutral air. The cooling capacity of the ventilation air unit can be downsized by the use of energy recovery. For an example of this procedure, see TDP-910, Energy Recovery. A typical ventilation air unit design incorporates an energy recovery device such as an energy wheel to precondition the outdoor air. Another popular design uses an indirect gas-fired heat exchanger with modulating blowers and discharge air control to maintain a neutral discharge air temperature of about 65 to 70 F in the cold weather months. 68

73 During the cooling season, a separate DX cooling system in the ventilation air unit takes the preconditioned air from the energy recovery device and cools it to about 50 to 60 F for delivery to the individual WSHP units. Other methods of delivery are used and they are discussed next. Method of Delivery of Ventilation Air One of the important considerations involved in the design of a WSHP system is the way ventilation air is to be provided to the conditioned space. Regardless of the chosen method of delivering the ventilation air, the ASHRAE Standard 62.1 recommended ventilation rates should be maintained. There are several methods that can be used. An outside wall grille with console WSHP units is one method. However, this method typically may not introduce the required ASHRAE 62.1 ventilation amounts. The other methods we will discuss all use a dedicated ventilation air system and they are: direct ventilation to the conditioned space ventilation using the interior units only ventilation using a ceiling plenum ventilation with ductwork to each WSHP Outside Wall Grille with Console Unit Console type WSHP units must be located along the outside wall. An optional air intake accessory is available. The accessory requires a wall penetration to the outside behind each console unit. Wall penetrations are subject to unpredictable airflows. High winds and stack effects may cause the outside air to bypass the WSHP coil altogether and enter the space untreated. To help minimize the wind effects, wind deflectors should be installed over the outside grilles. Units are typically provided with motorized dampers to close off the wall penetration when the unit is not operating. Also, as mentioned above, the design engineer must evaluate the ability of the console unit selected to introduce proper ventilation air amounts. Wall penetrations with console type WSHP units should only be used in low-rise buildings (less than three stories) that are protected from the wind. Hotels, motels, and schools may lend themselves to the use of console type units. From an IAQ standpoint, the quality of the filter used for the outdoor air on a console unit is usually less efficient than that in a central unit. For these reasons, on WSHP systems used in most commercial jobs, the ventilation air is handled by a dedicated system. Delivery Direct to the Conditioned Space In applications that utilize console or stack type WSHP units, outside wall penetrations may not be feasible. A constant volume of outside air, tempered to room conditions by a ventilation air handler or rooftop unit, is delivered to the conditioned space ceiling diffuser. See Figure 95. Tempering the air to neutral or room conditions prevents overcooling of the space when no cooling load is present, and prevents reheat when the space requires heating. A ceiling plenum or ducted return with exhaust fans is used to remove air from the space. Figure 95 Direct Delivery to Conditioned Space 69

74 The WSHP unit in the space handles the room sensible and latent loads, while the separate ventilation air unit handles the ventilation air sensible and latent loads. Delivering to Interior Units Only In applications involving open floor plans, it may be acceptable to supply ventilation air for the entire floor through interior WSHP units. In this arrangement, large vertical WSHP units are located in central mechanical rooms on each floor and utilized to condition the interior spaces. Smaller WSHP units located in ceiling cavities are utilized to condition the perimeter spaces. With open floor plans, supply air has a chance to mix and spread the ventilation air throughout the entire floor. With a large vertical WSHP located in a mechanical room on each floor, a centrally located vertical building shaft is used to provide ventilation air to the mechanical room. In this design, the mechanical room itself is used as a mixture plenum. Outdoor air is brought into the room and mixed with return air entering the room through grilles from adjacent ceiling plenum returns. The vertical WSHP units condition the mixture of untreated fresh air and return air. Should the interior spaces have floor-to-ceiling partitions, this method will not prove feasible, and ventilation air must be ducted to the perimeter horizontal WSHP units. Ceiling Plenum Near Each WSHP This method requires less installation cost. It is used with horizontal WSHP units that are located in a ceiling return air plenum. The outdoor air is ducted in close proximity of each horizontal WSHP unit s return air inlet. The WSHP unit fan draws the fresh air in along with return air from the plenum and delivers it to the conditioned space. See Figure 96. During summer operation the outside air will be cooled and dehumidified by the central unit. Care should be taken to ensure that this cold air does not enter in an uncontrolled manner into the conditioned space through ceiling tiles or return air grilles. During winter operation, the air should be heated to neutral temperatures (about 65 F). Figure 96 Ceiling Plenum Return Some designs deliver untreated outdoor air directly to the ceiling plenum. This approach is not recommended as it can introduce warm moist air or very cold air into the building and the WSHP units must then be sized to handle the outdoor air load along with the local space loads. A better solution would be to utilize energy recovery in the dedicated ventilation air unit to provide preconditioning of the outdoor air. 70

75 Direct-Connected to Each WSHP An effective way to deliver outside air in a controlled manner throughout the building is to direct connect the outside air duct to each WSHP unit. This method can be used with both large vertical WSHP units that serve the interior of a building and smaller horizontal units above the ceiling. The outdoor air is delivered through a vertical building shaft to ductwork on each floor. The ductwork provides fresh air to each vertical WSHP. The ductwork also passes through the ceiling cavity to the return air inlet of each horizontal WSHP. See Figure 97. Final duct connections to each WSHP (vertical or horizontal) are made with runout ducts with balancing dampers. If the runout ducts penetrate a firewall, a fire damper will also be required. The balancing damper is typically motorized and tied to the WSHP unit fan so it opens only when the WSHP runs. Ventilation Air Duct Design Ventilation air ductwork should be designed and sized based on standard low velocity duct design. Use an equal friction rate of 0.10 in. wg per 100 ft. Maintain duct velocities of 1000 to 1400 fpm to keep the system quiet and the static drop low. If the fans of vertical WSHP units located at each floor have to pull the fresh air down the ventilation shaft, the shaft should be designed with a very low pressure drop. This can be accomplished by sizing the shaft between 1000 and 1200 fpm for sheet metal ductwork. Piping Systems Loop Piping Figure 97 Direct Connection Ventilation Air As discussed earlier in this TDP, loop risers should be arranged for reverse return. In some installations, this will be the natural layout. In other systems, reverse return will require an additional length of vertical pipe, but will eliminate the need for balancing valves and balancing work. Typically the piping used in a WSHP system is schedule 40 black steel. Type L copper or threaded schedule 40 black steel pipe is normally used for 2-in. diameter and smaller. In some closed-loop water source heat pump applications, schedule 40 PVC piping has been used where local codes and inspectors permit. 71

76 The procedure for sizing the pipe and determining pump horsepower is as outlined below. For a complete example problem, refer to the TDP-502, Water Piping and Pumps. A pipe sizing chart is shown in Table 5. Pipe Size 2 and smaller Over 2 Table 5 Water Pipe Sizing Design Parameters Basic Design: Pressure Loss of 1 to 4 ft wg/100 ft Velocity Limit: 2 to 4 fps Can exceed 4 fps only is system has good air eliminator and low turbulence Basic Design: Pressure Loss of 1 to 4 ft wg/100 ft Upper Limit: 4 ft/100 ft Pipe Sizing and Pump Horsepower 1. Layout the piping in a reverse return arrangement. Use one of the suggested configurations from this TDP that best fits the building height and size. 2. Determine pipe water velocity to be used for sizing. For WSHP systems consider limiting velocities to 2 to 4 fps. 3. Note the gpm for each section of loop piping. These values are based on the selected WSHP units, tower, and other major system components. 4. Size all pipe on a pipe friction chart. A separate pipe friction chart exists for steel piping, and copper piping. The charts and procedures are reproduced in Carrier System Design Guide Water Source Heat Pumps and in TDP-502, Water Piping and Pumps. For closed-loop WSHP systems, a pressure loss of 1 to 4 ft wg per 100 ft is recommended. 5. For pump sizing, find the length of the highest pressure drop piping circuit in the loop. Add valves and elbows to convert it to total equivalent pressure drop. Some designers simply multiply actual length by 1.4 to find equivalent length. 6. Multiply the total equivalent length by the pressure drop per 100 ft for the highest pressure drop circuit. This is the friction loss in the piping. Sum all other components like the tower, boiler, etc. 7. Use the formula in the pump sizing section to find required pump horsepower. Condensate Piping In the cooling mode, the evaporator coil will condense moisture from the airstream into the drain pan. This condensate must be collected from each unit and removed. Since the condensate piping often runs above finished ceilings, it is important to maintain a clean, debris-free path for the water to flow to a central disposal point. Disposal of condensate is regulated by local codes. In some locations, the condensate lines from the individual WSHP units can be run into the storm drain system. In other areas it is permissible to connect into the building sanitary system. In either case, a design practice that is recommended is the use of an air gap between the unit condensate discharge and the building drain system. This prevents the possible entrance of vapors from the building system into the WSHP unit drain pan. If condensate is not free to flow, the backup of water could lead to damaged ceilings and walls. Therefore, all piping runs must be sloped to drain properly. Any areas where gravity drainage is not possible should be equipped with a condensate pump. 72

77 Often, PVC piping is used for the condensate system. Insulation is required if the pipe surface temperature may drop below the dewpoint of the air in the ceiling plenum. Insulation extending 6 to 8 ft from the trap is common practice. Copper piping may be required depending on local codes. Secondary Drain Pans Many building codes call for a secondary drain pan under the horizontal WSHP unit regardless of ceiling access. The condensate piping should be sloped 1 in. per 10 ft of pipe. If the slope is less, then the pipe size should be doubled. The pipe sizes are based on the normal collected tonnage of the WSHP units. Normally a one ton WSHP unit will produce about 3.0 pounds of condensate per hour during cooling. When used in high latent areas, up to double the amount of condensate will be produced by the WSHP coil. Double the unit capacity in tons when sizing the condensate pipe for a high latent unit. A condensate pipe sizing chart is shown in Table 6. All condensate-piping connections to WSHP units should be properly vented to prevent the unit fan suction from pushing the flow of condensate away from the drain pan. When hard ceilings without service panels are installed below WSHP units, a secondary drain pan should be installed below the WSHP unit to prevent damage in the event of a plugged unit drain. Many building codes call for a secondary drain pan under the horizontal WSHP unit regardless of ceiling access. Condensate pipe sizing is a simple matter of adding the cooling capacities of the WSHP units as the condensate piping is connected. Double the unit capacity for high latent load applications. All units typically come with at least a ¾-in. condensate connection. No condensate line should be less than ¾ inches in diameter. Pumps Table 6 Condensate Pipe Sizing Maximum Connected Cooling Load (Tons) Minimum Pipe Size (in.) 2 ¾ ¼ 50 1 ½ After the loop piping system has been laid out, and the total pressure loss for the pumps is calculated, the selection of pumps can be made. In terms of performance, a pump should be selected to provide the required flow rate at the design head pressure while trying to achieve the lowest possible horsepower. Refer to TDP-502, Water Piping and Pumps for a pump type comparison chart to help in the selection process of one centrifugal pump type versus another. Figure 98 Typical Computerized Pump Selection (Photo courtesy of Bell and Gossett) 73

78 Pump catalogs, with pump performance curves, allow the proper pump to be selected. Most pump manufacturers also have software programs that can select the optimal pump for your application. Figure 98 shows the selection of a pump producing 150 gpm at 66 feet of head pressure. The total head pressure of the pump will consist of the following: pipe friction loss, valves including control valves, any accessories, equipment such as closed circuit coolers and boiler, the WSHP unit that establishes the highest pressure drop circuit, air separators, etc. Liquid Horsepower Liquid horsepower is obtained by the formula, gpm head specific gravity 3960 (for standard water sp gr = 1.0), where 3960 converts the equation units into horsepower (33,000 ft lb per minute, divided by 8.33 lb per gallon). Brake Horsepower Brake horsepower (bhp) is the power required to drive the pump and equals the liquid horsepower divided by the overall efficiency of the pump. As a rule of thumb, the efficiency is usually about 65 percent. Air Separator and Expansion Tank The air separator is selected from the manufacturer s catalogue, on the basis of the required flow. For an expansion tank, the variation of water volume caused by temperature changes can be calculated. Determine the total water volume in the system and multiply it by the change in specific volume of water for the highest and lowest temperatures expected. As a rule of thumb, the change in water system volume is usually about 2 to 3 percent for a WSHP system. This volume, however, is not the volume of the expansion tank. It represents the expansion volume of the system. Bladder type tanks have an acceptance volume of approximately 95 percent, which means if the system expansion were calculated to be 200 gallons, the actual tank size required would be approximately 210 gallons. Controls This section covers basic principles and general guidelines that apply to the design and selection of controls for WSHP systems. See the Carrier System Design Guide Water Source Heat Pumps for additional information. The following control system components and functions will be discussed in this section. WSHP thermostats and unit controllers Loop control panels Water sensors and switches Pump controls Cooling tower and boiler controls Ventilation systems System safety and alarms Methods of reducing operating costs Overall system controls 74

79 WSHP Thermostats and Controllers Several types of thermostats are available for WSHP units. A programmable thermostat is used to establish the occupied and unoccupied periods in a zone with its own internal timeclock. A programmable thermostat can have seven day occupancy and set point programming, holiday programming, security levels, and the ability to be wired to a remote room sensor. A programmable thermostat can be used in a stand alone control strategy. This means the thermostats for the individual WSHP units are non-communicating and each one controls the operation of its respective unit in its zone. See Figure 99. The thermostat can also be a Programmable Thermostat communicating type and can be wired to a building wide communications bus. The thermostat provides all the capabilities of the programmable thermostat for zone occupant flexibility, but through communications, allows an interface to a building automation system. This provides external linkage to the WSHP unit and room temperature control information. Controllers Two basic unit-mounted WSHP controllers are typically available. The first type of controller is non-communicating and can be wired to a standard thermostat, a programmable thermostat, or a communicating thermostat. A non-communicating thermostat is not capable of being connected to a building-wide network communications bus. A non-communicating thermostat is the normal choice for many WSHP applications where internal operational and safety information about the WSHP unit is not needed on a building-wide basis. The other type of WSHP controller is capable of communicating on a building-wide network bus and may be connected to a space sensor, a thermostat, a communicating sensor, a linkage thermostat, or a CO 2 sensor for indoor air quality monitoring and control. See Figure 100. The WSHP controller s ability to communicate allows all internal operational and safety information of the WSHP to be available to an overall building management system. This is useful for report gathering, tracking, service and maintenance activities, and energy management options. Figure 99 Figure 100 PremierLink Communicating Controller 75

80 Linkage Thermostat New technology has led to a linkage thermostat. A linkage thermostat is a programmable communicating thermostat designed to control multiple WSHP units equipped with communicating controllers. With functionality similar to a programmable thermostat, the linkage thermostat can control up to eight WSHP units. Linkage is a term used to describe a sophisticated handshake between the major components that comprise a total system. This connection actually passes packets of information back and forth between the players, such as the WSHP units and the loop controller. The WSHP system utilizes linkage software to link the other system components with their source of air (WSHP units) to form a coordinated system. Loop Control Panel A loop control panel is usually offered by the heat pump manufacturer for use with WSHP units in either a stand-alone or a DDC WSHP system. See Figure 101. The panel monitors and controls the operation of the closed water loop consisting of the WSHP units, the cooling tower, the heat adder, and the system pumps. Functionality includes the ability to control up to two water circulating pumps and monitor water temperature to initiate up to eight stages of cooling tower and boiler operation. Figure 101 Loop Control Panel The panel can work with variable speed pumping arrangements. It also has control of loop alarms, safeties, and can signal an energy management system for central shutdown on system fault. See Figure 102 for a system schematic. Figure 102 Loop Control Panel Schematic 76

81 Summer Operation During the summer, the loop water temperature is maintained between 60 and 90 F for efficient system operation. The loop controller sequences the heat rejector when loop water temperature rises to about 83 F and provides full tower operation with high fan speed if loop water temperature continues to rise to 90 F. Winter Operation In winter, the loop controller sequences the heat adder when loop water temperature falls to about 65 F and provides full boiler capacity if loop water temperature continues to fall to 60 F. Alarms The loop controller can also provide an alarm and emergency shutdown signal as follows: Alarm if loop temperature rises to 100 F, shutdown at 105 F (high limit) Alarm if loop temperature falls to 55 F, shutdown at 50 F (low limit) Alarm and shutdown for lack of loop water flow. Water Sensors and Switches Loop sensors and switches for system control functions are contractor installed. The loop water temperature sensor is installed in an immersion well in front of the WSHP units. A water pressure sensor is used to provide a signal for use with a VFD-equipped pump for variable flow systems. The water pressure sensor is installed on the discharge side of the pumps in a common location. A loop water flow switch is also installed on the pump discharge in a common location. Pump Control It is normal practice is to have the circulating pump in operation 24 hours per day. The pumps should be automatically sequenced by the loop control panel. If the main pump does not start or produce adequate flow, the standby would be energized. In the event of no flow, a shutdown signal is sent to all WSHP units. This prevents the WSHP units from tripping on a safety device. Standby Pump The use of a standby pump is highly recommended. Water source heat pump systems cannot cool or heat without flow unless each unit has an auxiliary electric heater. Cooling Tower and Boiler The cooling tower is staged by a water temperature controller. On a rise in loop temperature the tower is staged in the following order: 1. The dampers are opened. 2. The spray pump is started. 3. Low speed fan is energized. 4. High speed fan is energized. When the loop temperature falls, the reverse staging is initiated. 77

82 The boiler control depends on the type of boiler used. For instance, many fossil fuel boilers require isolation from the WSHP loop because they operate at higher temperatures. Condensation inside the heat exchanger is prevented by regulating a valve to divert sufficient water through the boiler to maintain inlet water temperature at approximately 140 F while allowing the WSHP loop to operate at normal temperatures. A water temperature controller will energize the boiler and a separate boiler-circulating pump, if provided. The on-board boiler control regulates the burner. Hot water will be blended into the loop as required to maintain loop temperature. If the heat adder is electric, the heating element is staged by the water temperature controller to maintain loop temperature. Most loop control systems have a temperature reset option that automatically raises the minimum loop water temperature during periods of extremely cold weather. This allows the WSHP units in heating mode to deliver more capacity during extreme conditions. Ventilation System There are several ventilation air unit types that can be used, such as: DX packaged units, gasfired make-up units with cooling, or central station air-handling units with heating and cooling. Regardless of the unit type, the objective of the ventilation unit in summer is to deliver outdoor air that has the sensible and latent load (moisture) removed. That way, the outdoor air does not impose a load on the heat pumps units and they can be sized for the loads in the spaces they serve. In winter the objective is to deliver neutral air. To help accomplish this, the ventilation air unit may incorporate sensible or latent recovery, which preconditions the outdoor air in both summer and winter. A separate ventilation air unit controller that maintains discharge air temperature is required. The controller modulates or stages the cooling and heating to maintain a ventilation air temperature of approximately 60 F. At the onset of the occupied mode, the ventilation system remains inoperative for a pre-determined period of time (during building warm-up). Then the outdoor damper opens and the fan starts. Conditioned air is sent to the heat pump inlets, or conditioned space. Details on these three most common methods of delivery are discussed earlier in this TDP. Reheat, if available from an onsite energy source may be used to temper the air. When the WSHP system switches to the Unoccupied Mode, the cooling or heating mode is deenergized, the fan stops, and the air damper closes. Two-speed fan motors or VFDs can be used to adjust outdoor air amounts based on schedule or occupancy rates. System Safety and Alarms Operational safety is ensured by proper interlocks and safety controls for each piece of equipment as well as for the system as a whole. Figure 103 illustrates an overall schematic for controlling loop elements. The boiler is equipped with all internal operational controls and safeties by its manufacturer. Once energized, the boiler s controls vary the heat input and protect the boiler Figure 103 Typical Stand Alone System 78

83 against possible malfunctions. In case of malfunctions of either the boiler or the tower, the temperature limit controls (high and low) will initiate an emergency shutdown of the system. The cooling tower dampers are of the positive shutoff type, and will drive to the closed position in case of power failure to protect against coil freezeup at low ambient temperature. For WSHP units, in the event of a system fault, the general shutdown should be made from a central control point. If this is not done, the WSHP units will continue to operate until each one goes off on its own safeties. This would require manual resetting of each unit when the system fault has been corrected. Reducing Operating Cost Night Setback Night setback lowers the heating set point of the local thermostats during the unoccupied mode. As a result, the WSHP units will be required to do less heating, which means less power consumption and lower operating cost. Night setback should permit temperatures at least 5 F below room design conditions. Reduced Demand The starting of individual WSHP units should be staggered upon start-up or after an emergency shutdown, to reduce peak simultaneous demand. A random start timer in the on-board WSHP controls typically accomplishes this. Also, a storage tank can assist in the heating the loop at morning start-up, reducing demand if an electric heat adder is used. Optimal Start/Stop Optimal Start/Stop calculates the WSHP start and stop times to match anticipated occupied/unoccupied times based upon load conditions, previous start/stop history, outdoor air temperature and how long the building has been unoccupied. This reduces the amount of energy used. Demand Controlled Ventilation (DCV) Demand controlled ventilation varies the amount of ventilation air to match the actual building occupancy at any point in time. With the recent development of low-cost, maintenancefree, carbon dioxide sensing technology and direct digital controls, implementation of demand controlled ventilation has become practical. Water source heat pump units equipped with carbon dioxide sensors can track people density and communicate this information to the ventilation air handler. The air handler modulates the outside air dampers to match the amount of outdoor air needed to keep the building supplied with the proper ventilation rate. Overall System Control Loop Flow Alarms Loss-of-system loop flow is critical and interlocks should result in stoppage of boiler and tower operation. An alarm should notify building personnel so corrective action can be initiated as the system may now be using the standby pump. System level control typically utilizes a network communication bus that connects microprocessor controllers on all equipment located throughout the building. The network bus is used to implement system control strategies between the controllers. Control functions can generally be arranged into three categories for a WSHP system: master control panel functions, computer access functions, and energy management and building automation control functions. 79

84 Master Control Panel Functions The Master Control Panel provides centralized control for the entire WSHP system in the building. See Figure 104. The panel coordinates and monitors all these functions: Occupied or unoccupied mode Pumps Ventilation system. Individual unit thermostats Night setback Manual override, when in the unoccupied mode Alarms and emergency shutdowns Staggered start of the WSHP units (this function may be handled by random timers in the WSHP controllers and not needed in the master control panel) Outdoor air and loop water temperature Status of all equipment System emergency shut down Optimized start and stop of the building Computer Access In order to provide for human interface to the WSHP system, computer access may be added to the building network. In addition, remote computer access may also be provided from outside the building through the use of the telephone system or the internet. A central computer on site, directly connected to the communications bus, can provide central access, monitoring, calibration, remote troubleshooting, and manual override of any controller in the building. Other user functions include the ability to execute central holiday scheduling, central occupancy programming, and central set point modifications. See Figure 105. Figure 104 DDC Network Figure 105 User Interface 80