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Tankless water heater size calculator

Updated

A tankless heater has to heat everything you run at the same time. Enter the fixtures you might use at once and your incoming water temperature, and the calculator gives the flow rate and heating power to look for.

Fixtures running at the same time

How the calculator works

The DOE guide Sizing a New Water Heater sizes tankless (demand) heaters with two numbers:

  • Flow rate = the gallons per minute (gpm) of every hot water fixture you expect to run at once, added together.
  • Temperature rise = the hot water temperature you want minus the incoming water temperature. DOE's example assumes 50°F incoming water and 120°F hot water, a 70°F rise.

To turn that into heating power, the calculator uses the definition of a BTU, the heat that raises one pound of water by 1°F. A gallon of water weighs about 8.33 pounds and there are 60 minutes in an hour, so:

Heat output (BTU per hour) = gpm × 500 × temperature rise (°F)

A gas heater's input rating is higher than its output, so the calculator divides by the efficiency you enter. For electric models, it divides by 3,412 BTU per kilowatt-hour to get kW, and shows the total current at 240 volts. Electric tankless heaters often need more than one breaker and heavy wiring, so an electrician should check your panel first.

Worked example

Two showers at 2.5 gpm and one bathroom faucet at 1.5 gpm make 6.5 gpm. With 50°F water heated to 120°F, the rise is 70°F. Heat output is 6.5 × 500 × 70 = 227,500 BTU per hour. A gas heater at 90% efficiency would need about 252,800 BTU per hour of input. An electric heater would need about 66.7 kW, which is beyond what most homes can supply.

For scale, DOE says gas-fired demand heaters typically manage a 70°F rise at 5 gpm and electric ones at 2 gpm.

Flow rates for common fixtures

The EPA WaterSense showerhead page puts standard showerheads at 2.5 gpm and WaterSense labeled ones at 2.0 gpm or less. The EPA WaterSense bathroom faucet page gives 2.2 gpm as standard and 1.5 gpm or less for labeled faucets. The flow is usually stamped on the showerhead or the faucet aerator.

Undersizing shows up as temperature swings when a second fixture opens. Our guide on a shower that turns cold after 5 minutes covers the tankless version of that problem, including flow dropping below the burner's minimum. For a tank heater, use the water heater size calculator, which works from first hour rating.

Frequently asked questions

What size tankless water heater do I need for two showers?
Two 2.5 gpm showers need 5 gpm. At a 70°F rise, that is 175,000 BTU per hour of heat output, or about 194,000 BTU per hour of gas input at 90% efficiency. With 2.0 gpm WaterSense showerheads, it drops to 4 gpm.
Why does incoming water temperature matter so much?
The heater has to raise the water from its incoming temperature to your set point. Colder water in winter means a bigger rise, and every heater delivers fewer gallons per minute at a bigger rise.
Can an electric tankless heater run a whole house?
Sometimes, in warm climates with low flow fixtures. In the worked example on this page, 6.5 gpm at a 70°F rise needs about 66.7 kW, which most home electrical services cannot supply. DOE notes electric demand heaters typically manage a 70°F rise at about 2 gpm.
Is the BTU number the same as the rating on the box?
Gas tankless heaters are usually sold by maximum input BTU per hour. Output is lower because some heat goes out the vent, which is why the calculator divides by efficiency.

Sources

  1. Sizing a New Water Heater, U.S. Department of Energy (PDF copy)
  2. Showerheads, U.S. EPA WaterSense
  3. Bathroom Faucets, U.S. EPA WaterSense