Plumbing
Tankless Water Heater Size Calculator
Size a tankless heater from simultaneous flow and the temperature rise your winter water needs.
Result
Tankless capacity falls in winter because the incoming water is colder and the required rise is larger. Size on the coldest month, not an annual average. Gas units usually need a larger gas line and a different vent; electric units often need service capacity a typical house does not have.
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About this calculator
Why does my tankless underperform in winter?
Because it is rated at a given flow and rise. When incoming water drops from 70 to 45 degrees the rise needed grows by half, and the unit simply cannot pass as much water.
Where does 500 come from?
Water weighs about 8.33 lb per gallon and takes 1 BTU per pound per degree, so a gallon per minute for an hour is 8.33 x 60, which is roughly 500 BTU per degree.
Is electric tankless realistic for a whole house?
Rarely, outside warm climates. Whole-house electric units often need 100 amps or more of dedicated capacity, which most homes do not have spare.
The formula
BTU/h = GPM x temperature rise x 500, divided by efficiency
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