Determining Your Water Flow Rate for Tankless Water Heaters

Selecting tankless hot water heaters isn't very difficult. Select one based on the maximum amount of hot water to meet your peak demand.

Selecting Tankless Water Heaters

Selecting tankless hot water heaters isn't very difficult. Select one based on the maximum amount of hot water to meet your peak demand. Use the following assumptions on flow for various appliances to find the size of unit that is right for your purposes: Faucets: 1.0 gallons (3.78 liters) to 2.5 gallons (9.46 liters) per minute.

Low-flow showerheads: 1.0 gallons (3.78 liters) to 2 gallons (7.57 liters) per minute. Older standard shower heads: 2.5 gallons (9.46 liters) to 3.0 gallons (11.36 liters) per minute.

Clothes washers and dishwashers: 1 gallon (3.79 liters) to 2 gallons (7.57 liters) per minute.

Unless you know otherwise, assume that the incoming temperature is 55° F (12.78° C). You will want your water heated to 110° F (43.33° C) for most uses. To determine how much of a temperature rise you need, subtract the incoming temperature from the desired output temperature. In this example, the needed rise is 55° F (12.78° C).

List the number of hot water devices you expect to have open at any one time, and add up their flow rates. This is the desired flow rate for the heater. Selecting a tankless water heater is nearly impossible without knowing the flow rate you need.

As an example, assume the following conditions: One hot faucet open with a flow rate of 1.0 gallons (3.79 liters) per minute. One person bathing using a shower head with a flow rate of 2.5 gallons (9.46 liters) per minute. Add the two flow rates together. If the inlet water temperature is 55° F (12.78° C), the needed flow rate through the heater would need to be no greater than 3.5 gallons (13.25 liters) per minute. Faster flow rates or cooler inlet temperatures will reduce the temperature at the most distant faucet.