Water Heater Size Calculator
The fact sheet’s filled household hour is 49.00 gallons. A first-hour rating within about 2 gallons of that peak is 47.00–51.00 gallons.
Last updated . First hour mode is selected. Chart gallons are filled in. Fixture flows are examples.
A water heater size calculator for a storage tank or a tankless heater. The tank line sums peak-hour uses from the 2001 DOE chart. The tankless line is GPM = sum of fixture flows, plus the temperature rise. BTU and kilowatts are information about that rise.
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Peak hour or tankless flow
Tank gallons per use open on the Gas Appliance Manufacturers Association chart printed in the August 2001 DOE fact sheet DOE/GO-102001-0785. The times match that sheet’s filled household example. Fixture flows of 2 and 1.5 are examples you replace. They are not ratings from that fact sheet. BTU and kilowatts are information about the rise, not a product rating.
Results
Peak hour from the DOE chart
The August 2001 fact sheet DOE/GO-102001-0785, “Energy-efficient Water Heating,” tells you to estimate the daily peak one-hour hot water demand, then shop a first-hour rating within about 2 gallons of that peak. It says to use the first-hour rating, not tank capacity. The first-hour rating is the hot water the heater can supply per hour starting with the tank full. The chart on that sheet is credited to the Gas Appliance Manufacturers Association and assumes no water conservation measures.
Gallons per use on the chart are 15 for showering, 20 for bathing, 2 for shaving, 2 for washing hands and face, 4 for shampooing hair, 2 for hand dishwashing, 14 for automatic dishwashing, 5 for preparing food, and 32 for clothes washing. The filled example is 2 × 15 + 2 × 4 + 1 × 2 + 2 × 2 + 1 × 5 = 49 gallons. You can change any chart figure. The PDF is www1.eere.energy.gov/buildings/publications/pdfs/building_america/26465.pdf.
Tankless flow and the rise
GPM = sum of fixture flows. Each row is a flow you type times how many of that fixture run together. The opening rows, 2 and 1.5 gallons per minute, are examples so the sum can be checked. They are not fixture ratings from the fact sheet. Rise is the target outlet minus the incoming water. The sheet illustrates a limited demand heater as a 70°F rise at five gallons per minute on gas and two gallons per minute on electric. That illustration is not the flow this form adds up.
The BTU and kilowatt lines are information only. BTU/h = GPM × 60 × 8.34 × the rise in °F, using 8.34 pounds per gallon as the planning weight of water. kW = BTU/h ÷ 3,412.141633. The fact sheet does not print that product, and the lines are not a burner rating, a room-heating load, or an electrical service size. At 3.50 gpm and a 70°F rise the information lines are 122,598 BTU/h and 35.93 kW.
Worked hour and worked flow
| Line | Result |
|---|---|
| Filled peak hour | 49.00 gal · 185.49 L |
| About 2 gallons either side | 47.00–51.00 gal |
| Example tankless flow | 3.50 gpm · 13.25 L/min |
| Rise, 120°F minus 50°F | 70.00°F · 38.89°C |
| BTU/h, information | 122,598 |
| kW, information | 35.93 |
Water heater questions
What is the filled peak-hour example?
Leave Heater on Storage tank, peak hour. The filled uses are two showers at 15 gallons, two shampoos at 4, one shave at 2, two hand-and-face washes at 2, and one food prep at 5. Peak hour reads 49.00 gal · 185.49 L. About 2 gallons either side reads 47.00–51.00 gal.
What if the clothes washer also runs in that hour?
Leave the other counts. Set Clothes washing times in the peak hour to 1. The chart figure stays 32 gallons. Peak hour reads 81.00 gal · 306.62 L. About 2 gallons either side reads 79.00–83.00 gal.
Why is the answer not a tank size in gallons?
DOE/GO-102001-0785 says the first-hour rating is the hot water the heater can supply in an hour starting with a full tank, and it says to choose the heater by that rating, not by tank capacity. It says to pick a model with a first-hour rating within about 2 gallons of the peak you estimated. The band line is that “about 2 gallons” turned into arithmetic on either side of the peak.
Where do the gallons per use come from?
The August 2001 fact sheet “Energy-efficient Water Heating,” DOE/GO-102001-0785, prints a Gas Appliance Manufacturers Association chart. The chart figures are showering 15, bathing 20, shaving 2, washing hands and face 2, shampooing hair 4, hand dishwashing 2, automatic dishwashing 14, preparing food 5, and clothes washing 32. The sheet’s filled household column uses two showers, two shampoos, one shave, two hand-and-face washes, and one food prep, which add to 49. The chart note says it assumes no water conservation measures. The PDF is www1.eere.energy.gov/buildings/publications/pdfs/building_america/26465.pdf.
What does the tankless example return?
Set Heater to Tankless, flow and rise. Leave fixture 1 at 2 gpm and count 1, fixture 2 at 1.5 gpm and count 1, fixture 3 blank with count 0, incoming 50°F, and target 120°F. Flow reads 3.50 gpm · 13.25 L/min. Temperature rise reads 70.00°F · 38.89°C. BTU/h, information reads 122,598. kW, information reads 35.93. GPM = sum of fixture flows.
What if fixture 3 is 2.5 gallons per minute?
On the tankless heater, type 2.5 for fixture 3 and set its count to 1. Leave the other two fixtures and the temperatures. Flow reads 6.00 gpm · 22.71 L/min. Temperature rise stays 70.00°F · 38.89°C. BTU/h, information reads 210,168. kW, information reads 61.59.
What are the BTU and kilowatt lines, and what if the target is not hotter?
They are information only. BTU/h = GPM × 60 × 8.34 × the rise in °F. kW = BTU/h ÷ 3,412.141633. The fact sheet does not print that product. It is not a burner rating, a furnace load, or an electrical service size. The sheet’s separate illustration, a 70°F rise at 5 gpm for gas and 2 gpm for electric, is an example of limited output, not these fixture boxes. Set incoming and target both to 120. Flow, rise, BTU/h, and kW read —. The note reads: The target outlet is not warmer than the incoming water.