Sizing Residential Tankless Water Heaters

  January 21, 2020      Bradford White


When sizing for a tankless water heater there are several pieces of information that are needed to make the product selection:

  • Coldest incoming water temperatures (typically 40°F).
  • Desired set point temperature (average approx. 110-120°F).
  • Flow rates (GPM--gallons per minute) for all fixtures that could potentially be running at the same time.*

* Remember to be realistic. While the application may have two showers, five sinks, a dishwasher and a washing machine in a house it does not mean they are all going to be running at one time.


  • Total the flow rates of the fixtures that will likely run simultaneously.
  • Determine deltaT.
  • Refer to the GPM flow Rate portion of the spec sheet of the product. If the desired deltaT is not shown, refer to the recovery formula below.

Take the known GPH recovery and multiply by known temperature rise. Divide this by your desired temperature rise.

Example for sizing a tankless water heater (refer to the chart below):

1) 2 showers (2.5 GPM each) + 1 lavatory sink (0.5 GPM) = 5.5 GPM

2) 110°F - 40°F = 70°F deltaT

3) 5.2 (GPM) x 75 (°F) = 390

4) 390/70 (°F) = 5.57 GPM at a 70°F deltaT

In this example, one Model B would satisfy the demand.