Water heating
Water heater sizing: first-hour rating, tankless GPM and temperature rise
Tanks are sized for the busiest hour, tankless units for the most taps open at once. Both start from how a household actually uses hot water.
Two kinds of water heater, two sizing methods
A tank water heater stores hot water and reheats it as it is used. It can deliver a lot of hot water quickly, until the tank runs down, then slowly while it recovers. A tankless water heater stores nothing and heats water as it flows, so it never runs out, but it can only heat so many gallons per minute. A tank is sized for the busiest hour; a tankless unit for the busiest moment.
Tanks: the first-hour rating
The Department of Energy’s first-hour rating (FHR), printed on the yellow EnergyGuide label, is how many gallons of hot water the heater can deliver in an hour starting with a full tank. It combines the stored water with what the burner or element can heat during the hour. The right tank has an FHR at or above the household’s peak-hour demand.
The DOE’s sizing worksheet estimates that demand from the uses in the busiest hour:
| Use | Hot water per use |
|---|---|
| Shower | 10 gallons |
| Bath | 20 gallons |
| Shaving | 2 gallons |
| Hand and face washing | 4 gallons |
| Dishwasher | 6 gallons |
| Hand dishwashing | 4 gallons |
| Food preparation | 5 gallons |
| Clothes washer | 7 gallons |
The bath figure is this site’s addition; the others are from the DOE worksheet. The water heater tank size calculator adds up the peak hour and compares it with an estimated FHR for each common tank.
The DOE example household
Three showers, one shave, four hand and face washes, one dishwasher load, one food preparation and one clothes washer load in the busiest hour, with a gas tank.
- Peak hour: 30 + 2 + 16 + 6 + 5 + 7 = 66 gallons
- 30-gallon gas tank: first-hour rating about 63, too small
- 40-gallon gas tank: about 74, covers it
- Tank: 40 gallons gas
Gas tanks recover faster than electric ones, because a 36,000 to 40,000 BTU burner puts in more heat than a 4,500 W element, which is about 15,000 BTU per hour. An electric tank therefore needs to be larger for the same household. The water heater recovery time calculator shows the difference: a 40-gallon gas tank with a 40,000 BTU burner recovers in about 44 minutes, a 50-gallon electric tank in nearly two hours.
Daily use
Peak-hour demand sizes the tank; daily use sets the running cost. The daily hot water use calculator counts showers by minutes and flow, mixing hot with cold to the shower temperature, and adds appliances and sinks:
A family of four with four 8-minute showers at 2.0 GPM, one dishwasher load and one warm laundry load uses about 70 gallons a day. A couple with two 10-minute showers at 2.5 GPM uses about 49. Shower flow and length are the largest factors by far.
Tankless: flow and temperature rise
A tankless unit is sized by the flow of hot water it must supply at once and the temperature rise, the difference between the incoming cold water and the setting:
The 500 is 8.34 lb per gallon × 60 minutes. The rise depends on the groundwater temperature, which ranges from about 40 °F in the northern states in winter to about 75 °F in the far south.
Two showers in a mid-US winter
Two 2.0 GPM showers at once, 50 °F groundwater, 120 °F setting, a condensing gas unit at 0.95.
- Flow: 4 GPM; rise: 70 °F
- Heat output: 4 × 500 × 70 = 140,000 BTU/hr
- Gas input: 140,000 ÷ 0.95 = 147,368 BTU/hr
The tankless water heater size calculator also converts to the flow at the 77 °F rise most manufacturers quote. Whole-house electric tankless units need very large currents: one shower and a kitchen faucet at a 50 °F rise need 26.2 kW, about 109 A at 240 V, which many houses’ electrical service cannot supply. In cold climates a gas or heat pump water heater is usually the practical choice.
Heat pump water heaters
A heat pump water heater is a tank with a small heat pump on top. It heats much more efficiently than an electric element but more slowly, so its FHR depends on whether its backup element runs. Check the FHR on the label for the mode you will use, and choose a size up if the household’s peak hour is close to it. Running costs are covered in the guide to water heater running costs, and the guide to comparing fuels explains why the heat pump wins on energy.
Reading the EnergyGuide label
The yellow label on a tank shows the first-hour rating and the estimated yearly energy cost at national average prices; on a tankless unit it shows the maximum GPM at a stated rise instead of an FHR. Compare FHRs between models, not the nominal gallons, since two 50-gallon tanks with different burners or elements can have quite different first-hour ratings.
Getting the size right
Too small runs out of hot water in the busiest hour. Too large stores and keeps hot more water than the house uses, wasting standby heat, and costs more to buy. Size to the peak hour with a small margin. Replacement prices are covered in the guide to what installations cost and in the water heater replacement cost calculator.