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Supply Water Temperature Impact on Tankless Sizing


ave ground water temps

Did You Know?

Average supply water temperature of your area can affect what size tankless water heater you get. In addition to the number and type of fixtures you are going to run at the same time, you will also have to consider the temperature of your groundwater.

The colder the groundwater gets the less hot water can be produced by a unit for a given gallons per minute (GPM) flow rate. A tankless water heater in Texas would have to be rated 33% to 50% larger in Maine to serve the same number and types of fixtures.

This is because the tankless unit has to warm the incoming cold water more in colder climates than warmer areas of the country.

The difference between the the temperature of the hot water exiting the heater and the cold water entering the unit is called the temperature rise. If you want a shower up to 110°F and you live in Florida with groundwater at 72°F, then you need a 38°F temperature rise (110-72=38).

A tankless water heater is sized by rating its temperature rise at a given GPM. A unit could be rated at a 33°F Temperature Rise at 2.0 GPM. Based on manufacturer’s data, this same unit could also provide a 65°F Temperature Rise at 1.0 GPM.

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