Since tankless units only heat water as it is needed, the first step is to determine the combined flow rate (in gallons per minute) of all fixtures running at peak demand. For example, let’s say you expect to simultaneously run a hot water faucet with a flow rate of 1 gallon per minute and 2 shower heads with flow rates of 2.5 gallons per minute. The flow rate through the tankless water heater would need to be at least 6 gallons per minute.

Flow rate capabilities vary by unit and depend on how much the heater must increase the temperature of your water. So for step 2, you must determine your required temperature rise. To determine temperature rise, subtract your incoming water temperature from your desired output temperature. Incoming water temperature differs by location and changes throughout the year. Going by the lower temperature in your range can help ensure your unit will not be undersized during the coldest parts of the year.

For example, let’s assume that the incoming water temperature is 55 degrees, and, for most uses, you’ll want your water heated to around 110°F. In this example, you’d need an ondemand water heater that produces a temperature rise of 55°F at your required flow.

With these requirements, you should look for a heater capable of providing flow rates of at least 6 gallons per minute at a 55degree temperature rise. Most manufacturers provide charts that can tell you which heater meets your needs. You can find these charts in the manuals tab on our product pages. According to Takagi, a TKjr model would not work at our example measurements, but a Tk4 model would.

Other points:
Gas tankless water heaters are able to produce a larger temperature rise per gpm than electric models.
Most demand water heaters are rated for a variety of inlet temperatures.
Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet.