Tankless Geek topic guide
Tankless Water Heater Sizing
Size a tankless water heater from simultaneous fixture demand and required temperature rise, not bathroom count alone.
Updated
Direct answer
Add the flow rates of fixtures that may run together, then subtract the incoming-water temperature from the desired outlet temperature. Choose a model whose manufacturer performance chart meets both the required GPM and temperature rise.
Peak flow
List the fixtures likely to run at the same time and add their measured or rated flow. Low-flow fixtures can materially change the total.
Temperature rise
A unit produces less flow when incoming water is colder. A headline maximum GPM usually reflects a mild rise and should not be treated as winter performance.
Verify the exact model
Use the manufacturer performance chart for the exact model and voltage or fuel configuration. Add capacity only when the installation can supply the electrical load or gas input.
Questions people ask
Can I size by number of bathrooms?
Bathroom count is only a rough screen. Simultaneous fixture flow and temperature rise determine the required output.
Why does winter flow drop?
Colder inlet water requires more heat for each gallon, so the heater reaches its maximum input at a lower flow.