What size water heater do you need?
Count your busiest hour or your showers at once, get the size instantly.
How many times each happens in your busiest hour of the day:
What size water heater do I need?
For a tank heater, add up the hot water your household uses in its busiest hour and pick a heater whose first-hour rating (FHR) is at least that. With AHRI's worksheet figures, three showers at 20 gallons, a shave, a shampoo and hand dishwashing come to 70 gallons — so look for an FHR of 70 gallons or more.
What is the first-hour rating?
The number of gallons of hot water a tank heater can supply in an hour, starting full. It is printed on the EnergyGuide label and depends on the tank size and how fast the burner or elements reheat — a 50 gallon gas tank often has a higher FHR than a 50 gallon electric one.
How do I size a tankless water heater?
By flow and temperature rise. Add the flow of everything that may run at once — a shower is at most 2.5 gallons per minute under federal rules, a faucet 2.2 — and subtract the incoming water temperature from the temperature you want. Two showers from 50 °F water to 120 °F need 5 gpm with a 70 °F rise: 5 × 500 × 70 = 175,000 BTU per hour of output.
Why is an electric tankless heater so large?
Heating water on demand takes a lot of power: 175,000 BTU per hour is about 51 kW, more than many homes' electric service can spare. That is why whole-house tankless heaters are usually gas, and electric ones serve one or two fixtures.
What about water-saving fixtures?
AHRI's worksheet assumes no water-saving measures, so it errs on the large side. Low-flow showerheads and newer dishwashers and washers use less; if you have them, you can size a little smaller than the result.
How do I find the temperature rise?
The rise is the temperature you want at the tap minus the temperature of the incoming cold water. The incoming temperature varies by region and season, so use the coldest water of the year for a safe size. A heater set for 120 °F with 50 °F incoming water has a rise of 70 °F; with 70 °F water in summer, only 50 °F.
What do BTU per hour mean in kilowatts?
Divide by 3,412, since one kilowatt-hour is 3,412 BTU. One 2.5 gpm shower with a 70 °F rise needs 87,500 BTU per hour of output, which is 25.6 kW. A gas heater must have an input rating higher than the output by its efficiency losses.
How the two methods work
For a tank heater the calculator adds up the hot water in gallons that your household uses in its busiest hour, using the figures of the AHRI peak-hour worksheet (20 gallons for a shower or a bath, 14 for a dishwasher load, 32 for a clothes washer load, 4 for hand dishwashing, 4 for a shampoo, 2 for a shave, 4 for hand and face washing and 5 for food preparation). A heater should have a first-hour rating at least equal to that total. For a tankless heater it is flow and rise: gallons per minute times 500 times the temperature rise gives BTU per hour, where 500 is 8.33 pounds of water per gallon times 60 minutes times 1 BTU per pound per degree.
Worked example: a busy morning
Two showers (40 gallons), three hand washings (12), two shampoos (8), one dishwasher load (14) and one food preparation (5) in the same hour add up to 79 gallons, so a tank heater with a first-hour rating of at least 79 gallons is the target. For a tankless unit serving two showers at 2.5 gpm and one 2.2 gpm faucet at the same time, the flow is 7.2 gpm. Heating from 50 °F to 120 °F, a 70 °F rise, takes 7.2 × 500 × 70 = 252,000 BTU per hour, or 73.9 kW. With 70 °F summer water the same flow needs 180,000 BTU per hour, 52.8 kW. Tankless sizing must satisfy the coldest water of the year.
Tank or tankless
A tank heater holds a store of hot water and recovers slowly, so it is sized for the peak hour. A tankless heater has no store and is sized for the highest flow you will run at once. The same household can therefore need a 60-gallon tank or a unit of a quarter of a million BTU per hour; neither is better in general. Fuel, venting, gas line size, electrical service and the cost of installation matter as much as the capacity, and the calculator does not weigh them. If the heater is far from the fixtures, the wait for hot water at the tap is a separate issue from capacity.
Common mistakes
Sizing to the average day rather than the busiest hour leaves the family with cold showers on mornings when everybody is up. Using the warm incoming water temperature of summer undersizes a tankless unit for winter. Counting the full flow of every fixture in the house, including ones that are never on together, oversizes it. Reading the capacity of a tank (gallons stored) as the first-hour rating is another: they differ with the reheat rate. And the AHRI figures assume no water-saving fixtures, so a house with low-flow showerheads can size somewhat smaller.
Sources: Air-Conditioning, Heating, and Refrigeration Institute (AHRI), worksheet for estimating peak hour demand (first hour rating); federal maximum flow rates of 2.5 gpm for showerheads and 2.2 gpm for faucets (10 CFR 430.32); BTU conversion factors from NIST SP 811. Related: BTU calculator, heating cost calculator and water softener calculator.