Sizing a tankless water heater comes down to two numbers: the peak flow rate (in gallons per minute) you’ll ever need from running fixtures at the same time, and the temperature rise your unit has to produce — target output temperature minus your incoming water temperature, which depends heavily on your climate.
Check every fixture that might run at the same time:
Regional incoming-water-temperature ranges are commonly published estimates, not a live lookup by ZIP code — your actual groundwater temperature varies by season and depth.
Likely Causes, Ranked
- Peak simultaneous flow rate (GPM)
Add up the flow rate of every fixture that might realistically run at the same time — not your total fixture count, just your worst realistic overlap. - Incoming water temperature by climate
Colder-climate incoming groundwater runs roughly 35–50°F in winter; warmer climates run closer to 65–75°F — the colder your water starts, the bigger the unit has to be to hit the same output temperature.
FAQ
Why does the same tankless unit perform differently in different states?
Because incoming water temperature varies a lot by climate and season. A unit that easily keeps up with two showers in Florida may struggle with the same load in Minnesota in January, because it has to raise the water temperature much further.
Written by HotWater Decoded — water heater repair specialists. Technically reviewed by a licensed technician on our team.
- Typical fixture flow rate ranges (shower, kitchen faucet, dishwasher, washing machine) published across major tankless manufacturers’ sizing guides
- Regional incoming groundwater temperature ranges commonly cited in DOE and manufacturer sizing documentation
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