Determining the Required Instant Flow Water Heater Temperature Rise

The second step of sizing your Instant Flow Water Heater is to determine its required temperature rise.

Temperature Rise is the difference between the incoming cold water temperature to the desired hot water temperature. For personal use (bathrooms and sinks) you will want your hot water at 120÷F (49÷C), unless you use a dishwasher without an internal heater, in which case you will want your hot water at 140÷F (60÷C).

If you live in a cold climate, your incoming water is colder. This means that the temperature rise you need is bigger, hence a bigger tank less water heater.


Average ground water temperatures in the USA is between 50÷F (10÷C) and 55÷F (13÷C) winter to summer, so for a basic calculation you may assume that your input water (ground) is 50÷F (10÷C). Your actual input water may be slightly colder (not likely) or very much warmer (likely in warm climates).
If your ground water is colder – you will get less output. If your ground water is warmer – you will get more output.

You should consult the image below to determine the temperature of the incoming cold water.

US climate zone map for temperature rise calculation

To determine the required temperature rise, simply subtract the incoming water temperature from the desired output temperature.

For example, if you live in Illinois than your incoming water temperature is 42÷F (5÷C). Assuming that you need your hot water for a washing machine – you will need it at 140÷F (60÷C).

Your required temperature rise is 98÷F (55÷C)

Most instant flow water heater systems are rated for a variety of inlet temperatures. Typically, a 70÷F (39÷C) water temperature rise is possible at a flow rate of 5 GPM through a gas tankless hot water heater, and 2 GPM through electric ones.

Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet.

Tankless water heater efficiency, also known as Energy Factor (EF) has direct impact on your annual hot water bill and operating costs and should be accounted for as well.