Sizing water heaters to provide sufficient hot water efficiently LEARN THREE METHODS YOU SHOULD BE USING TO DESIGN EFFICIENT SYSTEMS

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1 LEARN THREE METHODS YOU SHOULD BE USING TO DESIGN EFFICIENT SYSTEMS BY DAVID E. DEBORD, CPD, LEED AP Sizing water heaters to provide sufficient hot water efficiently requires the designer to consider many aspects of the system, such as how much hot water is needed and at what temperature, the fuel source, equipment space requirements, storage-type heaters vs. separate heat exchangers, and many other decisions. Many ways to size water heaters are available, and the method that you select for a specific application may depend on the type of occupancy, the location, and other factors or even a combination of these factors. While this one article cannot tell you everything you need to know about water heaters, it presents three methods to derive the amount of hot water needed and explores some basic ways to meet those needs. It may be beneficial to use more than one of the three methods described in this article to determine a project s requirements and then decide which direction to take based on a comparison of the results. If you use the same design criteria for multiple sizing methods and get widely varying results, you may want to check the data and calculations for inconsistencies. Using a computer to perform the calculations makes it easier to double-check your results and to vary some of the parameters to compare different usage scenarios. You can compare different thermal efficiencies or different balances of storage and recovery rates to find the best solution for the application. This article focuses on three of the methods commonly used for sizing water heaters: 1. Fixture demand method 2. Population demand method 3. Hunter method The various types of water heaters, when to select one type over another, how to control temperatures, and other such details are not covered. As mentioned, many sizing methods are available, and some of your clients, such as large corporations, restaurants, or hotel chains, may have their own standards that you are required to follow. Figure 1 shows one example that a client may use as their standard basis of design. 14 Plumbing Systems & Design APRIL

2 FIGURE 1 EXAMPLE OF A TYPICAL CLIENT STANDARD Guest Rooms gph/room Total gph Redundancy Recovery ,400 4,800 Guest Rooms Gallons Total Gallons Storage Capacity ,000 Even if a client requires you to use their method, you also should perform calculations using one of the more familiar and standardized methods to see if you are comfortable with the results. You then may be motivated to consult with the client to recommend alternatives to their standard calculations. Of course, any time you deviate from an accepted standard or prescribed method of engineering, you must realize that you may be taking additional responsibility for the results. The consequences of such actions should always be considered before making the final decision, and the owner needs to be consulted and made part of this decision. WHAT IS THE GOAL? Regardless of the type of building or system, water heater sizing has some basic, common, and recurring goals: Determine the probable demand load Calculate the energy required to generate enough hot water to meet this demand Find a heater that is adequate to satisfy this demand in a safe manner Design an efficient system How do you do this? Where do you start? THE BASIC CONCEPT You can simplify the process by reducing the steps to the basic concept (see Figure 2) and more clearly defining your goals. As already stated, you first need to determine the demand. Then you balance the storage with the recovery (heating components) to determine the gallons per hour (gph) delivered. DEMAND Figure 2 The basic concept STORAGE + The next step in the simplification process naturally leads you to consider the four basic steps to sizing a water heater: 1. Determine the gph demand 2. Divide the demand between storage and recovery 3. Determine the storage capacity 4. Subtract the storage from the demand to get the recovery capacity required Once you know what you need, it will be easier to find the solution. Let s further refine each of these components of the design criteria. DEMAND What is demand? How do you measure and satisfy it? Domestic Water Heating Design Manual defines demand as a function of the anticipated hot water usage of the occupants of a particular building during the period being considered. It is affected by the population of a project as well as behavioral patterns of those occupants and the amenities offered them. Some engineers say that demand (or load) sizing is an art form because it requires a blend of science and art. Demand profiles vary for different types of occupancies, and they also can vary in any particular building. As always, when dealing with plumbing, the accuracy of the results depends on the behavior of the end users. When calculating hot water demand, you should follow established demand allowances based on established procedures. STORAGE The storage capacities indicated in the various sizing guides and calculation methods are usually the net usable requirements. Assuming that 60 to 80 percent of the hot water in a storage tank is usable, the actual storage tank size should be increased by 25 to 66 percent to compensate for unusable hot water. This is due to stratification. As hot water is used, it is removed from the tank, and then cold water comes into the tank to replace the water that was removed. The cold water is denser and sinks to the bottom of the tank, pushing the hot water toward the top. Hopefully, you have piped the system to take advantage of this phenomenon, but that is outside the scope of this discussion. This is not to say that the tank will never be 100 percent filled with water that is at the design temperature only that you cannot depend on it being hot when it is needed. 75 percent generally is accepted as an average figure to use for this calculation. Any tank more than 119 gallons must be ASME certified, which requires an ASME-certified technician to sign off on the construction by stamping the vessel. Certified tanks tend to cost more than non-certified tanks, so you can pick a 120- gallon tank (or less), which is always actually 119 gallons per the manufacturers, to avoid this issue. However, some clients and applications require all water heaters and hot water storage tanks to be ASME rated, so this may be a moot point for specific projects. APRIL 2011 Plumbing Systems & Design 15

3 Recovery describes the rate at which the heated water that is removed from the storage tank is replenished. Once you know the demand and the storage capacity, you will know what amount is needed for recovery. The difference between the temperature of the incoming water and the temperature of the water leaving the heater is referred to as ΔT, or delta T. It also is called the rate of the temperature increase, which more commonly is called the rate of rise. You need to know the delta T to determine how quickly hot water can be recovered, which will help you determine the amount of energy needed. Thus, to determine the gph delivered, you add the storage capacity in gallons to the recovery rate in gallons per hour (see Figure 3). The goal is to deliver enough hot water to meet the demand. Let s look at some examples of this process. STORAGE + Figure 3 How to satisfy demand Example 1 If the demand for an application is 200 gph, is a 100-gallon tank big enough to meet the demand? 1. Divide the load between storage and recovery. Start with a 50/50 split for storage and recovery, or 100 gallons storage and 100 gph recovery. 2. Determine the actual hot water storage capacity. Remember that only 75 percent might be hot, so multiply 100 by 0.75 to get the actual gallons stored hot, which in this case is 75 gallons. Thus, the 100-gallon tank is not capable of meeting the 100-gallon storage capacity. 3. Subtract the storage from the demand to get the recovery capacity required, or 200 gph demand 75 gallons storage 125 gph recovery (see Figure 4). 75 STORAGE + Figure 4 Results of Example 1 DEMAND Keep in mind that this only works for applications that have a one-hour peak load. The storage capacity (as determined so far) most likely was depleted during the first hour, so if the maximum demand load is needed for more than one hour, additional capacity in storage or recovery may be required. Following is one way to account for these apparent storage anomalies. Example 1A If the load (demand) for an application is 200 gph, what size tank is needed to meet the required storage capacity? 1. Divide the load between storage and recovery. Start with a 50/50 split for storage and recovery, or 100 gallons storage and 100 gph recovery. 2. Determine the required storage tank size. Remember that only 75 percent might be hot, so divide 100 by 0.75, or 133 gallons. 3. Subtract the storage from the demand to get the recovery capacity required, or 200 gph demand 100 gallons storage 100 gph recovery (see Figure 5). Let s look at some examples using our three methods. 100 STORAGE + Figure 5 Results of Example 1A METHOD 1: FIXTURE DEMAND METHOD The steps in the fixture demand method are: 1. Determine the demand in gallons per hour for each fixture type 2. Determine the quantity of each type of fixture 3. Multiply the quantity of each fixture by the gph value of each This reveals the total gallon-per-hour demand. Demand is measured in different ways. When dealing with hot water, the demand rates are measured as: Gallons per minute (gpm) Gallons per hour (gph) Gallons per day (gpd) BUILDING TYPES VS. LOADS Different occupancies have different use patterns; thus, different criteria are used to evaluate the demand based on these occupancies. Basic building types include office buildings, hotels, schools, apartment buildings, and restaurants. The various ways of calculating the hot water demand rates of these different occupancy types include: Gallons per hour per fixture Gallons per hour per person Gallons per hour per bed Gallons per hour per unit, apartment, or hotel room Gallons per hour per meal Plumbing Systems & Design APRIL

4 TABLE 1 HOT WATER DEMAND PER FIXTURE FOR VARIOUS TYPES OF BUILDINGS ( [LITERS] OF WATER PER HOUR PER FIXTURE, CALCULATED AT A FINAL TEMPERATURE OF 140 F [60 C]) Fixture Apartment Club Gymnasium Hospital Hotel Industrial Plant Office Building Private Residence School YMCA Basins, private lavatory 2 (7.6) 2 (7.6) 2 (7.6) 2 (7.6) 2 (7.6) 2 (7.6) 2 (7.6) 2 (7.6) 2 (7.6) 2 (7.6) Basins, public lavatory 4 (15) 6 (23) 8 (30) 6 (23) 8 (30) 12 (45.5) 6 (23) 15 (57) 8 (30) Bathtubs 20 (76) 20 (76) 30 (114) 20 (76) 20 (76) 20 (76) 30 (114) Dishwashers a 15 (57) ( ) ( ) ( ) (76-380) 15 (57) (76-380) (76-380) Foot basins 3 (11) 3 (11) 12 (46) 3 (11) 3 (11) 12 (46) 3 (11) 3 (11) 12 (46) Kitchen sink 10 (38) 20 (76) 20 (76) 30 (114) 20 (76) 20 (76) 10 (38) 20 (76) 20 (76) Laundry, stationary tubs 20 (76) 28 (106) 28 (106) 28 (106) 20 (76) 28 (106) Pantry sink 5 (19) 10 (38) 10 (38) 10 (38) 10 (38) 5 (19) 10 (38) 10 (38) Showers 30 (114) 150 (568) 225 (850) 75 (284) 75 (284) 225 (850) 30 (114) 30 (114) 225 (850) 225 (850) Service sink 20 (76) 20 (76) 20 (76) 30 (114) 20 (76) 20 (76) 15 (57) 20 (76) 20 (76) Hydrotherapeutic showers 400 (1,520) Hubbard baths 600 (2,270) Leg baths 100 (380) Arm baths 35 (130) Sitz baths 30 (114) Continuous-flow baths 165 (625) Circular wash sinks 20 (76) 20 (76) 30 (114) 20 (76) 30 (114) Semicircular wash sinks 10 (38) 10 (38) 15 (57) 10 (38) 15 (57) Demand factor Storage capacity factor b a Dishwasher requirements should be taken from this table or from manufacturers data for the model to be used, if this is known. b Ratio of storage tank capacity to probable maximum demand per hour. Storage capacity may be reduced where an unlimited supply of steam is available from a central street steam system or large boiler plant. Reprinted from Plumbing Engineering Design Handbook, Volume 2, Table 6-1 How do you determine the demand in gph per fixture? This information can be found in charts and tables published by industry associations and manufacturers (see Table 1). Example 2 Let s look at an example that would be considered a social club. Assume the fixtures as indicated in Table 2 to calculate the loads. According to the table, the total demand for this project is 240 gph. The next step is to divide the demand between storage and recovery. Using the example procedure from above, start by dividing the demand (load) between storage and recovery. Start with a 50/50 split for storage and recovery, or 120 gallons storage and 120 gph recovery. If the storage capacity requirement is 120 gallons, does that mean you need a 120-gallon tank? Not exactly. Only 75 percent of the water in the tank may be hot when water is needed, so if you multiply 120 by 0.75, the actual gallons stored in a 120-gallon tank is only 90 gallons. To size a tank large enough to store 120 gallons of hot water, divide 120 by 0.75, which results in a 160-gallon tank. To determine the recovery capacity required, subtract the storage from the demand (see Figure 6). SIZING THE WATER HEATER Assuming that you chose the 160-gallon tank, now you need to determine how to recover 120 gph. First, you need to determine how much energy is needed to deliver the required recovery rate. TABLE 2 FIXTURE DEMAND FOR SOCIAL CLUB EXAMPLE Fixture Demand Quantity Total Demand Basin, private lavatory Basin, public lavatory Dishwasher Kitchen sink Laundry, stationary tub Pantry sink Service sink Total STORAGE STORAGE + Figure 6 Results of Example APRIL 2011 Plumbing Systems & Design 17

5 1 kilowatt (kw) will provide 4.1 gallons of hot water per hour, at a 100 F delta T. 1,000 British thermal units per hour (Btuh) will provide about 1 gph at a 100 F delta T. 1 kilowatt equals 3,412 Btuh (approximately). Next, determine what type of energy source will be utilized. Many choices are available, but the most common are natural gas and electricity. Other options are steam and high-temperature hot water from a boiler. The water heating formula is: { q q gph [( 1 Btu ) ( 8.33 lb ) ] lb/ F gal ( T) m3 [( kj ) ( kg ) ( T) ]} h kg/k m 3 where: q Time rate of heat transfer (Btuh) For natural gas, the equations are: Btu/Input gph For electricity, the equations are: kw gph gph x 8.33 x T x 1 % Efficiency of heater Btuh Input x % Efficiency T x 8.33 gph x 8.33 x T x 1 gph x T OR 3, kw x 3,412 T x 8.33 OR kw x 410 T Example 3 You need 120 gph. Assume that you are using electricity to heat the water and that the incoming water temperature is 40 F. Remember that 1 kw will provide 4.1 gallons of hot water per hour at a 100 F delta T. You need 140 F water out of the heater, which is a 100 F delta T. Since 120 gph 4.1 gph per kw kw, you will specify 30 kw. Thus, the water heater will be 160 gallons at 30 kw/480 volt/3 phase. Keep in mind that all electricity is not equal. Regarding amperage draw: 1 kw 2.5A, 230V, 3 Phase 1 kw 4.4A, 230V, 1 Phase Amp(3 Phase) kw x 1,000 Volts x Amp(1 Phase) kw x 1,000 Volts Always coordinate this specification and design with the electrical engineer. Example 3A Now let s see what is needed if you are using natural gas to heat the water. You need to recover 120 gph, and the incoming water temperature is 40 F. You need 140 F water out of the heater, which is a 100 F delta T. Table 3 shows an example of a simple chart indicating the difference in Btuh requirements based on different efficiencies. It is based on the basic formula: Btuh/Output gph x 8.33 pounds per gallon x T Efficiency TABLE 3 EFFECT OF THERMAL EFFICIENCIES Btuh (gph) ( T) (8.33) Efficiency 119, , , Remember that 1 Btu will raise 1 pound of water 1 F in one hour. Thus, 1 Btuh will heat approximately 0.01 pound per hour at a 100 F ΔT, and 1,000 Btuh will heat 10 pounds of water at a 100 F ΔT. Since 1 gallon of hot water weighs 8.33 pounds, 1,000 Btuh 1 gph (at 84 percent efficiency). Note that the 84 percent basically cancels out the weight of the water. Then, 120 x 1, ,000 Btuh, so the water heater will be 160 gallons at 120,000 Btuh. However, nothing is ever that simple. The exercises in this article have been using a 50/50 split, but note the last two rows in Table 1, the demand factor and the storage factor. Notice that they seldom come close to 50/50. Also notice that they usually add up to more than 100 percent. This is partially to compensate for peak demand periods longer than one hour. Read those rows carefully! Table 1 may be used to determine the size of the water heating equipment based on the number of fixtures. To obtain the probable maximum demand, multiply the total value for all of the fixtures by the demand factor in line 19. The heating capacity of the water heater should equal this probable maximum demand. The storage capacity should equal this demand multiplied by the storage capacity factor on line 20. Let s demonstrate this procedure for the example of a social club in Example 2: Probable maximum demand 240 gph x gph Heater or coil capacity 72 gph Storage tank capacity 72 x gallons Using this method, the water heater would be: gallons If electric: kw/208 V/3 Phase If gas: 72,000 Btuh This result is quite a bit different than the 50/50 split result and presumably more accurate. However, the engineer makes the final decision. This example was a rather small load. With larger loads, you have more comfort with the diversity factors, which in reality are a function of curves rather than linear relationships, just as you see when sizing water supply piping. This is another consideration to include in the design process: the relative size of the system. 18 Plumbing Systems & Design APRIL

6 METHOD 2: POPULATION DEMAND METHOD The steps in the population demand method are: 1. Determine the demand in gallons per day for each occupant 2. Determine the number of occupants 3. Multiply the number of occupants by the gallons per day required for each This reveals the total gpd demand. Using Table 4, you can determine: Maximum hourly demand Duration of peak demand Storage capacity required Heating capacity (recovery) required METHOD 3: HUNTER METHOD This method uses curves that were developed with empirical data from actual studies to determine the demand in gph for each occupant or unit. The basic steps to sizing a water heater per the Hunter method are: 1. Find the chart and curves that fit the occupancy type 2. Find the storage capacities on the bottom of the chart or the recovery capacities on the left 3. Follow this perpendicularly to find the other factor. Now you have the storage and the recovery requirements for each unit. 4. Multiply these values by the number of units to get the storage and recovery capacities required for the system Building Type gpd per Person TABLE 4 FACTORS FOR POPULATION DEMAND METHOD Maximum Total gpd Peak Duration Hourly Demand Total Occupants Storage Capacity in Relation to Day s Use Heating Capacity in Relation to Day s Use Factor 1/7 4 1/5 1/7 Residence, hotel, or apartment 20 min ,000 1,716 6,864 2,400 1, max ,000 3,432 13,728 4,800 3,432 Factor 1/5 2 1/5 1/6 Office building 2 min 600 1, max 600 1, Factor 1/3 1 2/5 1/8 Factory ,000 1,000 1,000 1, Example 4 For this example, the project is a 600-room hotel with an expectation of using the higher end of the range of hot water indicated in Table 4. Maximum daily requirement 600 x 40 24,000 gallons Maximum hourly demand factor 24,000 gallons x 1/7 3,452 gph Duration of peak load Four hours Water required for four-hour peak 4 x 3,452 13,728 gallons If four 1,000-gallon tanks are used and 75 percent of the water in the tanks is hot, the available hot water is 4,000 x 75 percent 3,000 gallons. Water to be heated in four hours 13,728 3,000 10,728 gallons Heating capacity per hour 10, ,862 gph (Note that depending on how some numbers are rounded when you are performing these calculations, even with a calculator or spreadsheet, some minor discrepancies may be reflected in your calculations. These minor discrepancies will have no detectable effect on your system performance.) Using this method, the water heater for this hotel would include: Four 1,000-gallon storage tanks 4,000 gallons Heating capacity 2,862,000 Btuh Note that for steps 2 and 3 above, it does not matter if you start with the storage factor or the recovery factor. The intersection of the graph will be at the same point. Figure 7 shows the relationships between recovery and storage capacity for various building categories. Any combination of storage and recovery rates that falls on the proper curve will satisfy the building requirements. TIP Using the minimum recovery rate and the maximum storage capacity on the curves yields the smallest hot water capacity capable of satisfying the building requirement. With a higher recovery rate, you get greater 24-hour heating capacity and smaller required storage capacity. STORAGE CAPACITIES The storage capacities shown are net usable requirements, as previously mentioned. Assuming that 75 percent of the hot water in a storage tank is usable, the actual storage tank size should be increased by 33 percent to compensate for unusable hot water. Example 5 A high school has 1,000 students, and the demand is 3 gallons of storage per student. Locate 3 gph on the bottom of APRIL 2011 Plumbing Systems & Design 19

7 FIGURE 7 AND STORAGE CAPACITIES FOR VARIOUS BUILDINGS A. DORMITORIES B. MOTELS E. FOOD SERVICE F. APARTMENT BUILDINGS the high school chart in Figure 7h, follow up to the curve, and then follow left to find the required recovery at 0.15 gph per student. 1,000 x 3 3,000 gallons storage 1,000 x gph recovery If electric, kw Example 5A For the same high school, the demand is 1 gallon of storage per student. Find 1 gph on the bottom of the high school chart in Figure 7h, follow up to the curve, and then follow left to find the required recovery at 0.5 gph per student. 1,000 x 1 1,000 gallons storage 1,000 x gph recovery If electric, kw Example 5 provides 3,150 gph, while Example 5A provides 1,500 gph. Which is correct? They both are. The apparent discrepancy is in the fact that this method relies on curves, so the relationships between storage and recovery are not linear. The hot water storage capacity provides a buffer that reduces the recovery capac- ity required. When more is stored, you can draw that down when the demand exceeds the recovery. When less is stored, you don t have that extra cushion, and you need to recover at a rate closer to the demand rate. The total maximum gph delivered may not be the same because Example 5 provides a greater reserve storage buffer, whereas Example 5A does not. However, Example 5A has a greater recovery capacity and may be more efficient, depending on the particular usage parameters of the individual project. Then again, maybe you do not have a large enough electrical service available for the second scenario. SUMMARY The mission of this article was to provide three distinct methods of evaluating domestic hot water demand loads and to demonstrate some methods of satisfying that demand. While enough information was provided to do that, you should know that some thought is required beyond pure calculations. This is where the engineer takes precedence. The computer can only take it so far the engineer needs to make the final selection. Be the engineer! 20 Plumbing Systems & Design APRIL

8 FIGURE 7 AND STORAGE CAPACITIES FOR VARIOUS BUILDINGS C. NURSING HOMES D. OFFICE BUILDINGS G. ELEMENTARY SCHOOLS H. HIGH SCHOOLS David E. DeBord, CPD, LEED AP, ARCSA AP, is a plumbing engineer and senior associate at Environmental Systems Design in Chicago and has more than 30 years in the consulting business. He currently serves as the Legislative Vice President of ASPE. He is also an Adjunct Assistant Professor at Illinois Institute of Technology, is a member of the American Solar Energy Society, American Rainwater Catchment Systems Association, Geothermal Heat Pump Consortium, and USGBC, and serves on ICC and IAPMO code committees. For more information or to comment on this article, articles@psdmagazine.org. ASPE Webinars Did you miss a webinar? Want to brush up on your knowledge? Now is the time to visit ASPE s archived webinars, which allow you to view and listen to previously recorded events at a time that is most convenient for you. Pay a small fee to earn quick CEUs. Residential fire sprinkler systems Clean agents for fire suppression Natural gas and propane sizing Siphonic roof drainage Inside the Green Codes Grease interceptors Graywater Systems Water heater sizing Medical gases LEED BIM Visit for more information. APRIL 2011 Plumbing Systems & Design 21