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Design Load Hot water use is not spread out over the entire day. Instead, domestic use of hot water tends to peak in the morning hours and again in the early evening (supper time). As a result, a water heating system must be designed to have sufficient capacity to provide hot water during periods of peak use. An accepted method of determining the maximum demand on a water heater is to determine peak usage during a particular time of day. For single- and two-family dwelling units, this peak occurs during the morning or early evening. Refer to tbl.16 for typical household demands by use. TBL. 13.16 TYPICAL HOUSEHOLD HOT WATER DEMANDS BASED ON USE. UNITED STATES DEPARTMENT OF ENERGY. Hot Water (per Use) Household Hot Water Use, gal L Bath 20 76 Shower 20 76 Shaving 2 8 Face/hands washing 4 15 Teeth brushing 2 8 Hair shampoo in sink 4 15 Food preparation 5 19 Automatic dishwasher 14 53 Hand dish washing of family meal-major clean up 14 53 Hand dish washing of some dishes-minor clean up 4 15 Automatic clothes washer 32 122 == EX.13 Estimate the peak hot water demand of a household that has a morning routine of two showers, three hands/face washings, four teeth brushings, food preparation, and minor dishwashing. From tbl.16: 2 showers (20 gallons) 2 · 20 _ 40 3 hands/face washings (4 gallons) 3 · 4 _ 12 4 teeth brushings (2 gallons) 4 · 2 _ 8 1 food preparation (5 gallons) 1 · 5 _ 5 1 minor dish washing (4 gallons) 1 · 4 _ 4 total peak hot water demand: 69 gallons Typical hot water demands for various types of nonresidential buildings are provided in tbl.17. An accepted method of determining the maximum demand on a water heater is to determine peak usage. == EX.14 Estimate the hot water demand of a junior high school serving 850 students and 65 staff. From tbl.17: 1.5 gpm/student _ 850 students _ 1275 gpm == Sizing Storage Tank Water Heaters Residential water heaters are typically sized based on their first hour rating (FHR). This rating is contained on the U.S. Department of Energy Guide label found on all water heaters. The FHR relates to the gallons of hot water available for one hour of peak demand. It’s the maximum output of the water heater over an hour, in gallons of hot water in a 100°F temperature rise. The FHR is the sum of the "standby" hot water found in the tank plus the capacity of the heater to heat water during that first hour. Typically, standby water is taken as 70% of the tank capacity. Tank capacities are usually available on the water heater name plate. Recovery rate is indicated less frequently, but can be approximated from the heater input rating. Recovery rate is the quantity of water that the burner or element can heat to a 100°F (38°C) increase in one hour. This is referred to as a 100° temperature rise, which means that the water temperature is in creased 100°F. For example, if a burner can take 40 gallons of 40°F (4.4°C) water and raise its temperature to 140°F (60°C) in one hour, that burner/heater has a 40 gal recovery rate. _ recovery rate (gallons per hour) FHR (in gallons) _ 70% of tank capacity (gallons) -- Hot Water; Demand; Maximum per Hour; Maximum per Day; Average per Day; Type of Building; gal L gal L gal L Food service establishments, maximum 1.5 5.7 11.0 5.7 2.4 9.1 Type A: full meal restaurants and cafeterias 0.7 2.6 6.0 22.7 0.7 2.6 Type B: drive-ins, grilles, sandwich shops Apartment houses, per unit Number of apartments 20 or less 12.0 45.5 80.0 303 42.0 159 50 10.0 37.9 73.0 277 40.0 152 75 8.5 32.2 66.0 250 38.0 144 100 7.0 26.5 60.0 227 37.0 140 200 or more 5.0 19 50.0 195 35.0 133 Men's dormitories, per student 3.8 14.4 22.0 83.4 13.1 49.7 Women's dormitories, per student 5.0 19 26.5 100 12.3 46.6 Motels, per unit number of motel units a 20 or less 6.0 22.7 35.0 133 20.0 75.8 60 5.0 19.7 25.0 95 14.0 53.1 100 or more 4.0 15.2 15.0 57 10.0 37.9 Nursing homes, per bed 4.5 17.1 30.0 114 18.4 69.7 Office buildings, per person 0.4 1.5 2.0 7.6 1.0 3.8 Elementary schools, per student 0.6 2.3 1.5 5.7 0.6 2.3 Junior and senior high schools, per student b 1.5 3.8 3.6 13.6 1.8 6.8 a Interpolate for intermediate values. b Per day of operation. --- Average gas-fired residential and light commercial heater sizes fall in the range of 30 000 to 75 000 Btu/hr, which equates to a recovery rate of about 27 to 68 gallons per hour. For a natural gas-fired water heaters, multiplying the burner input rating by 0.0009 can approximate recovery rate (e.g., A 35 000 Btu/hr burner has a recovery rate of 35 000 · 0.0009 _ 31.5 gallons). Water heaters powered by electricity will have recovery rates of about one gallon/hour per 250 watts. A common residential electric water heater is rated at 3000 to 4500 watts, which would be a recovery rate of about 12 to 18 gallon per hour. The recovery rate of a gas-fired water heater is typically about double the rate of electric-powered units having the same storage tank size. To compensate and provide a suitable FHR, electric water heater storage tanks are typically larger. In sizing a storage tank-type water heater, the FHR of a water heater must meet or exceed the peak hot water demand. tbl.18 is a guide to selecting the minimum recommended gas-fired storage tank water heater size, in gal. tbl.19 pro vides specifications for selected storage tank water heaters. TBL. 13.18 GUIDE TO SELECTING THE MINIMUM GAS-FIRED STORAGE TANK WATER HEATER SIZE, IN GAL. ADD 20 GAL FOR EACH ADDITIONAL BEDROOM. MOVE TO THE NEXT LARGEST VALUE FOR HIGH DEMAND. TBL. 13.19 SPECIFICATIONS FOR SELECTED TANK-TYPE WATER HEATERS. COMPILED FROM SEVERAL INDUSTRY SOURCES. Number of bathrooms 1 to 1.5 2 to 2.5 3 to 3.5 Number of bedrooms 1 2 323453456 Water heater tank size, gal 20 30 40 50 50 50 66 50 66 66 80 First hour capacity, gal/hr 35 45 55 70 70 70 86 70 86 86 100 W tank size, L 76 113 151 189 189 189 249 189 249 249 302 First hour capacity, L/hr 132 170 208 265 265 265 325 265 325 325 378 Sizing Instantaneous Water Heaters Instantaneous water heaters are selected based on the amount of hot water needed to meet the design load (peak instantaneous demand), in gpm or L/min, at a specific water temperature rise and other criteria. tbl.20 provides typical specifications for a selected residential gas-fired instantaneous (tankless) water heater. The design load (the flow rate of the instantaneous water heater) is determined by adding flow rates of fixtures used simultaneously. The following assumptions on water flow for various residential fixtures may be used to determine the size of unit: • Faucets: • Low-flow showerheads: • Older standard showerheads: • Clothes washers and 1 dishwashers: Most instantaneous water heaters are rated for flow rates at a variety of inlet water temperatures or temperature rises. A good assumption in design of instantaneous water heaters is that that the incoming potable water temperature is no warmer than 50°F (10°C). A temperature of 35° to 40°F (2° to 4°C) should be used in cold climates. Water must typically be heated to 120°F (49°C) for most residential uses. To determine the required temperature rise needed, subtract the incoming water temperature from the desired output temperature. The needed temperature rise is typically at least 70°F (39°C). tbl.21 provides test results of a single instantaneous water heater. Faster flow rates or cooler inlet (water supply) temperatures will reduce the water temperature available. Using low flow showerheads and water-conserving faucets are a good idea with instantaneous water heaters. TBL. 13.20 SPECIFICATIONS FOR SELECTED GAS-FIRED INSTANTANEOUS (TANKLESS) WATER HEATERS IN CUSTOMARY ( U.S.) UNITS. COMPILED FROM INDUSTRY SOURCES. TBL. 21 TEST RESULTS OF FLOW RATES AT VARIOUS RISES IN TEMPERATURE FOR A SELECTED RESIDENTIAL GAS-FIRED INSTANTANEOUS (TANKLESS) WATER HEATER BASED ON AN INLET TEMPERATURE OF 66°F (19°C).WITH LOWER SUPPLY TEMPERATURE, THE FLOW RATE WOULD BE LOWER AT THE SAME DELIVERY TEMPERATURES. THE HEATER MODULATES DELIVERY OF HEAT FROM 19 000 TO 180 000 BTU/HR (5.6 TO 53 kJ/s) IN 13 INCREMENTS ACCORDING TO INLET WATER TEMPERATURE AND HOT WATER DEMAND. Delivery Temperature; Temperature Rise Flow Rate gpm (L/s) Sizing a Large Multifamily Water Heating System As with one- and two-family water heaters, a hot water system must be designed to provide a sufficient supply of hot water for use by building occupants during peak periods of use. In larger buildings, it’s less likely that a large share of plumbing fixtures will be in use at a given time. In large multifamily dwellings, such as apartments or condominiums with 10 or more units, hot water demand tends to peak between 6 and 9 AM and again between 5 and 8 PM. An approximation method used to determine hot water demand in large multifamily buildings (10 or more dwelling units) is based on the demand unit (DU). See tbl.22. For apartments or condominiums, a DU is counted for each bathroom and clothes washing machine served by the water heating system. For apartments or condominiums with 10 or more units: MPD _ 350 _ 11 (DU) Maximum probable demand (MPD) of hot water, in gal/hr, s This expression assumes 11 gal/hr for each demand unit plus a 350 gal/hr reserve capacity. TBL. 23A CAPACITIES OF CYLINDRICAL TANKS, IN GALLONS. CAPACITIES PROVIDED ARE TANK SIZE EXCLUDING INSULATION THICKNESS. TBL. 23B CAPACITIES OF CYLINDRICAL TANKS, IN LITERS. CAPACITIES PROVIDED ARE INTERIOR TANK SIZE EXCLUDING INSULATION THICKNESS. TBL. 24 CAPACITIES OF SELECTED GAS-FIRED WATER HEATING BOILERS. Water Heating Rate; gal/hr Output for Temperature Rise L/hr Output for Temperature Rise EX.16 Estimate the peak hot water demand of a 24-unit condominium having a single water heating system. Each unit has two bath rooms, and there are four clothes washing machines in the common area. A hot water boiler is typically sized to heat water for buildings containing 10 or more dwelling units. It’s customary to include a storage task with the boiler to act as a reserve for times when instantaneous demand exceeds boiler capacity. Size of the storage tank is again based on the number of DUs served by the water heating system. See tbl.23. TBL. 22 WATER HEATING DEMAND UNITS (DU) VERSUS STORAGE TANK CAPACITY PER DEMAND UNIT. tbl.24 provides capacities of selected gas-fired water heating boilers. To meet the 922 gal/hr hour demand (from EX.17), a gas-fired boiler with a capacity of 900 000 Btu/hr input (960 gal/hr) at a 90°F temperature rise could be selected. An alternate solution would be to presume that some of the 840 gal storage tank capacity (from EX.17) could be shared over the 3-hr peak demand period: 840 gal/3 hr _ 280 gal/hr. Therefore, the boiler capacity can be decreased, based on 922 gal/hr hour demand minus 280 gal/hr or 642 gal/hr. At a 90°F temperature rise, a gas-fired boiler with a capacity of 650 000 Btu/hr input (693 gal/hr) would be selected. The alternate solution is a more cost-effective solution because of lower initial cost and lower operating costs. It is, however, a less conservative approach to design. Prev: DOMESTIC WATER
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