A hydronic system controls comfort by delivering heated or cooled fluid to the conditioned space through pipes.

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Paula Wilkins
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1 Introduction to Hydronics A hydronic system controls comfort by delivering heated or cooled fluid to the conditioned space through pipes. Hydronic heating systems use hot water or steam to deliver the heat. A boiler is used to heat the water or to produce steam. Water has a greater specific heat than air, so it carries more heat per pound. Advantages of hydronic heating systems include enhanced comfort, and lower operating costs. Steam systems may be found in residential applications but are more common in commercial applications where steam is already available for other purposes. Steam systems do not require a circulating pump and temperature losses in the piping are low, making steam ideal for heating multiple buildings from a central steam plant. A cubic foot of water weighs pounds. It boils at 212 F and freezes at 32 F. Water entering a hydronic system can contain air that is made up of nitrogen and oxygen. The oxygen in the air can cause corrosion and rusting inside the system. When water is heated, the air is driven from the water and eliminated from the system by special devices. Chemicals can be added to water to prevent minerals and oxygen within the water from forming scale and rust. Pressure drop is the difference in pressure between two points. It is caused by the friction within the pipes and within other system devices. The change in pressure drop when there is an increase or decrease in the flow in gallons per minute (gpm) can be calculated using a formula. The formula states that final gpm divided by initial gpm squared, times initial pressure drop equals the final pressure drop. Power must be available in the system to overcome the effects of pressure drop. Pump manufacturers will provide pressure drop information that can be expressed in interchangeable units. Head pressure is another measure of pressure expressed in feet of water. A column of water 1 foot high exerts a pressure of 0.43 psi. A column of water 2.31' high exerts a pressure of 1 psi. Simple formulas are available for converting between head pressure in feet and pressure in psi.

2 Static pressure is created by the weight of the water in the system. Since a 1' column of water exerts a pressure of 0.43 psi, static pressure is equal to 0.43 psi for each foot of height above the system gauge. Static pressure is lowest at the highest point in the system, and gets increasingly higher at lower points in the system. Hot-water heating systems are used to provide comfort heating. The two main types of hot-water systems are gravity hot-water systems and forced hot-water systems. Gravity heating systems do not require a circulating pump. Thermal circulation or buoyancy causes movement as hot water rises and cool water falls. Water movement in gravity systems is slow, so the system responds slowly to changing load conditions. Gravity systems are very quiet. Forced hot-water systems use a pump to circulate the heated water. This feature allows different areas in a structure (called zones) to maintain different temperatures. When a zone thermostat calls for heat, the circulating pump turns on to supply water to the zone terminal device through a zone control valve. A check valve in the return line prevents gravity circulation of water when the pump is off. Hot-water heating systems contain a variety of special components. Hot-water boilers use the same fuels that forced-air furnaces use. They include natural gas, propane, and fuel oils. Some boilers use coal or wood. The burners and the way the fuel is ignited on hot-water boilers is generally the same as found on forced-air furnaces. Venting methods are also similar. Hot-water and steam boilers share some common characteristics but have differences including their operating controls. Hot-water boilers are entirely filled with water, while steam boilers are not. Low-temperature boilers, the most commonly used, are normally designed for a 30 psi maximum working pressure with 12 to 15 psig being normal. Operating temperature is limited to a maximum of 250 F. Medium- and high-pressure boilers are built to operate at pressures above 160 psi and temperatures above 250 F. Boilers are usually made of iron or steel. The use of copper and stainless steel in the heat exchangers is gaining in popularity. Units using copper tubes are often made with cast-iron headers and sidewalls. Modern materials enable boilers to be more compact and lightweight.

3 Boilers are often installed in parallel for increased capacity and to provide redundancy. Small, gas-fired boilers can be wall-mounted with direct through-the-wall venting. Condensing boilers with efficiencies above 95% are now available. They can be vented through PVC pipe like forced-air furnaces. Modern gas-fired boilers often use the same gas valves, burners, and ignition systems found on forcedair furnaces. Sophisticated electronic controls and sensors ensure efficient operation. Cast-iron is still widely used to make boilers. They are assembled by bolting together individual cast-iron sections. The more sections used, the greater the heating capacity of the boiler. The hot-water heating system and the boiler are equipped with several controls and devices to ensure safe and correct operation of the boiler and system. A gauge that combines boiler water temperature and boiler operating pressure monitors those two important conditions. A pressure relief valve protects the boiler from excessive water temperature or steam pressure. It is designed to fully open at 30 psi. Relief valves must be installed according to the manufacturer s instructions. They should be checked periodically for proper operation. Aquastats are thermally operated switches used to control boiler water temperature or to operate other devices based on water temperature. Aquastat sensing elements can be inserted in the water directly or they can sense water temperature by being surface-mounted on the boiler or pipe. Electronic probe-type controls use an electrode placed in the water to sense water level. It uses the conductivity of the water to complete a circuit to ground. Water expands when heated. An expansion tank allows for the safe expansion of water as it is heated, preventing a dangerous condition and helping to maintain system pressure. In addition to standard expansion tanks, pressurized diaphragm expansion tanks are available. The water and the air in the tank are separated by a flexible diaphragm. Pressurized expansion tanks often come pre-charged with air, but the pressure can be field-adjusted through the charging valve to meet system design requirements. Air can be trapped in a system as it is being filled with water or released as heat drives air out of water. Air-control devices can remove the trapped air manually or automatically.

4 The circulating pump forces the hot water from the boiler, through the system, and then back to the boiler. The pump can be installed either in the supply side or return side of the system. On larger systems with high pump head, it is recommended to install the pump in the supply side. The term head pressure is used to give the capacity of a circulating pump expressed in feet of water. Pump performance curves are useful in determining pump performance under different conditions. The horizontal scale indicates pump delivery in gpm while the vertical scale shows head pressure in feet. At zero gpm, there is no delivery on the curve. At this point, the power of the pump is equal to the pressure drop opposed to it. With no pressure difference between two points, the pump will not deliver water at this point. Examination of the chart capacity curves for various impeller sizes shows that the lower the volume of water delivered by the pump in gpm, the higher the head pressure against which the pump can deliver water, and vice versa. Review the examples in paragraph and study Figure 23 for a better understanding of pump performance curves. Hot-water heating terminals transfer the heat carried by the hot water to the conditioned space. They come in several styles and shapes. A convector is a heating device that depends mainly on gravity conductive heat transfer. They are typically mounted near the floor on an outside wall. Baseboard units are often a finned-tube design and are usually installed in a continuous run along the perimeter of an outside wall. Finned-tube units are similar to baseboard units and are used for perimeter heating, often in large glassed-in areas. Radiators that are often made of cast-iron, transfer heat by radiation. They are typically found in older systems. Unit heaters consist of a finned-tube heating element and a fan contained in an enclosure. The fan moves air over the heating element, allowing heat to be transferred to the conditioned space. Unit ventilators are similar to unit heaters except they are used for both heating and cooling. Cast-iron boilers are often equipped with an internal heating coil called a tankless heater. It is used to provide domestic hot water. The tankless coil heat exchanger is immersed in the hot boiler water. Cold water passing through the tankless coil is heated by the boiler water.

5 An indirect heater is a heat exchanger installed in a hot water storage tank. Hot boiler water passes through the heat exchanger and heats the water in the tank. Both heaters require year-round boiler operation. Radiant floor heating systems consist of a continuous loop of PEX tubing installed under a floor. Warm water circulating through the tubing warms the floor. The floor then radiates heat into the room. The four general classifications of water piping systems are one-pipe, two-pipe, three-pipe, and fourpipe. One-pipe systems are used primarily for hot-water heating systems and can be either a series-loop or single-loop configuration. A series-loop one-pipe system has a continuous run of pipe from the supply to the return connection, allowing all hot water to flow through all the terminals. It is used in residential systems and may have more than one piping circuit. A variation of the series-loop one-pipe system installs special tees at each terminal that diverts some of the water from the main into each terminal. Two-pipe systems have terminals connected across separate supply and return main piping. Two-pipe, direct return systems have the supply water and return water flowing in opposite directions. Circuit-balancing valves are used with this type of system. Two-pipe, reverse-return systems have the supply water and return water flowing in the same direction through parallel pipe runs. This type of system is more easily balanced than direct-return, two-pipe systems. Zoned hydronic heating systems allow the loads in different parts of the structure to be controlled separately. Simple zoned systems use multiple zone valves and a single circulator pump. The pump is sized to accommodate total system flow at the required head of the longest or most resistant zone. More complex zoned systems use multiple circulating pumps. Each zone pump is sized to accommodate the flow rate and head pressure in its own zone piping. A dual-temperature system is a water system that circulates hot and chilled water to heat or cool with common piping and heat transfer terminals. Almost all the components used in a dual-temperature system are the same as those used in the individual heating or chilled water systems. The main differences are in the system piping and the valves or other system controls used to select heating, cooling, or both.

6 Water balancing ensures that the proper amount of hot water, chilled water, or both is delivered to the conditioned space to maintain comfort. To balance a system, water flow and pressure drop measurements are made and then valves and flowcontrol devices are adjusted. Pressures are often measured with a differential pressure gauge or manometer. The system pump must be able to provide the rate of flow (in gpm) required by the system. The rate of flow can be calculated using the formula gpm = hourly heat loss (HL) in btuh TD TD represents temperature differential, 8.33 is the weigh in pounds of a gallon of water, and 60 is the number of minutes in an hour. Once the flow rate is determined, consult a pump performance curve chart to select the circulator pump. Water flowing through pipes and fittings produces friction loss. Friction loss in any piping system should be kept to a minimum. Fittings and other components are assigned a friction loss in equivalent length of pipe. With this information, total equivalent length of pipe and fittings can be determined. Using the pump head pressure at a known flow rate, and the total equivalent length of pipe for the longest run in the system, friction loss for the system can be calculated. Friction loss per 100' of pipe can be calculated by dividing permissible head by equivalent length of pipe times 100. Review the Study Example in for a better understanding of how to calculate friction loss. The amount of head pressure added by a circulating pump at a given flow rate can be determined by measuring the pressure differential between the pumps suction and discharge ports. A formula is available that converts differential pressure measured across a pump to head. Head = (ΔP) 144 D. ΔP represents pressure differential and D represents the density of the fluid being pumped in pounds per cubic foot. The calculated total pump head pressure in feet and the pump impeller size on a pump performance curve chart can be used to determine the amount of water flow through the pump in gpm. Introduction to Hydronic Systems (Final gpm initial gpm)2 initial pressure drop = final pressure drop (10 gpm 5 gpm)2 5 psi = final pressure drop (2 gpm)2 5 psi = final pressure drop

7 4 gpm 5 psi = 20 psi The construction and operation of hot-water and steam boilers are similar, with two exceptions. The operating and safety controls used with hot-water boilers are different than those used with steam boilers. Also, hot-water boilers are entirely filled with water, while steam boilers are not. Lowtemperature boilers are the most widely used type of boiler. They are used for residential, apartment, and commercial buildings. Low-pressure hot-water boilers can be constructed to have working pressures of up to 160 psi. Normally, they are designed for a 30 psi maximum working pressure, but are frequently operated below that pressure level, with 12 to 15 psig being common. Low-pressure hot-water boilers are limited to a maximum operating temperature of 250 F. Above this temperature, even water under low pressure will begin to boil and begin the change of state to steam, creating a dangerous condition. A pressure (safety) relief valve is used to protect the boiler and the system from high pressures caused by either water thermal conditions or steam pressure conditions in the boiler. It does not operate unless an overpressure condition exists. The typical hot-water boiler is constructed for a maximum working pressure of less than 30 psi, at which point the valve is designed to be fully open. An expansion tank, also called a compression tank, is used to maintain system pressures. It allows for the safe expansion of water as it is heated. The volume of water in a piping system varies with temperature changes. When water is heated or cooled, it expands or contracts. When the system water is at maximum operating temperature, the resulting excess water volume caused by expansion is stored in the expansion tank. When the system water temperature drops and the water volume in the tank contracts, water is returned to the system. An expansion tank must be able to store the maximum volume of excess water that exists when the system is operating at maximum temperature. It must do this without exceeding the maximum system operating pressure. It also must maintain a required minimum level of system pressure when the system is cold. The circulating pump is sized to overcome the system pressure drop and to supply the necessary amount of water in gpm at the proper temperature to each terminal device. Pressure drop results from the friction caused by the system piping, fittings, boiler, terminals, and other heating components. The term head pressure is used to give the capacity of a circulating pump. It is just another way of expressing pressure drop.the maximum head of a pump is actually the maximum pressure drop against which the pump can cause water to flow. It is usually expressed in feet of water. High/low pump head are terms often used to indicate the relative magnitude of the height of a column of water that a circulating pump is moving, or must move, in a water system. Globe valves are used to adjust water or steam flow within limits that depend on system pressure variations. They are also called compression valves. Globe valves come in many configurations, but all

8 have a plug that allows gradual throttling of the water or steam. Flow through a globe valve moves in a Z-pattern that allows throttling without excessive wear on the valve seat. An angle valve is a form of globe valve, and is made with its outlet port rotated at an angle, typically 90 degrees relative to its input port. Angle valves are commonly used in various applications in place of a standard globe valve and elbow combination. Zone control valves manage the flow of water in each zone of a zoned system. They are two-position, thermostatically controlled valves. Zone control valves can be operated by heat or with an electric motor. In a heat-operated valve, a resistance wire around the valve heats the valve s bimetal element when the zone thermostat calls for heat. This causes the bimetal element to expand and slowly open the valve. When the zone thermostat is satisfied, the bimetal element cools, allowing the valve to slowly close. Electric-motor operated valves also operate to open and close the valve slowly, under control of the zone thermostat. They may contain a switch to operate the circulating pump. Slow opening and closing of zone control valves is necessary to reduce expansion noise and prevent water hammer. The time required for a typical zone control valve to open and/or close ranges from about 15 to 30 seconds. Some zone control valves have a feature that enables the valve to be opened manually in the event of a power failure. This feature is also useful when troubleshooting heating problems in a zoned system. Some zone control valves have a flow indicator dial that aids in system balancing. Many cast-iron boilers used for residential and commercial hot-water systems are equipped with an internal heater coil called a tankless heater. It is a water-to-water heat exchanger used to supply domestic hot water. The domestic hot water is contained in the coil and is quickly heated by the boiler water. In most cases, this eliminates the need for a hot-water storage tank. This type of system requires a special aquastat that maintains boiler water temperature at a certain level. The boiler runs all year long because it heats domestic water. In the direct-return system, the supply water and return water flow in opposite directions. Also, the return water from each terminal takes the shortest path back to the boiler. This means the water flowing through the terminal nearest to the boiler is the first back to the boiler, because it has the shortest run. The water flowing to the terminal farthest away from the boiler has the longest run and is the last to return. This arrangement results in an unequal length of travel for water flow through the terminals. This causes an imbalance in the system, with the result being poor heat distribution. Circuit balancing valves are normally used with direct-return systems to aid in balancing the system. Systems using zone valves generally have a single pump. In the figure, the installation of an indirect domestic water heater that often uses its own dedicated circulating pump is also shown. As individual zone thermostats call for heat, the zone valves open and allow water to flow through to the zone. To prevent the pump from dead-heading or attempting to pump with the flow fully restricted, a differential bypass valve or three-way zone valves can be used. This provides a path for some water flow when all zone valves are closed. Occasionally, this is necessary when only a very small zone is in operation as well. Otherwise, the circulating pump should operate only when one or more zone valves are in their heating position. The pump is sized to accommodate total system flow at the required head of the longest, or most resistant, zone.

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