HouseChalk
Gallons per minute (GPM)
Gallons per minute measures the flow rate of water through a fixture or system at a given temperature rise.
Why it matters
Tankless water heaters are sized by GPM. If your household needs exceed the unit's capacity, water temperature drops during simultaneous use.
Where people get this wrong
GPM for a tankless heater depends on the temperature rise needed. Colder incoming water means lower effective GPM from the same unit.
Real-world example
Your shower uses 2.5 GPM and the kitchen faucet uses 1.5 GPM. If both run at once, you need a tankless heater rated for at least 4 GPM at your local temperature rise.
Where this hits your build
This comes up during rough-in and final fixture installation. Accessibility for future repairs matters as much as the initial install.
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