Tankless Water Heater GPM
Size a tankless water heater using simultaneous hot-water flow at the required temperature rise. The largest GPM figure in a sales listing may describe warmer inlet water than your home has in winter.
GPM means gallons per minute. Temperature rise is the difference between the desired heater outlet temperature and incoming cold-water temperature. Both belong in the same sizing calculation.
Step 1: Establish temperature rise
Temperature rise = outlet temperature − inlet temperature.
An assumed 120°F outlet and 45°F inlet require a 75°F rise. A 120°F outlet and 70°F inlet require a 50°F rise. Use a measured or defensible winter inlet condition, not the temperature of water that has warmed while sitting in an indoor pipe.
Navien's NPE-240A2 specifications list 11.2 GPM at a 35°F rise and 5.2 GPM at a 75°F rise. This model example shows how the same appliance's output changes with conditions; it is not a recommendation or a universal tankless rating. Navien temperature-rise table.
Step 2: Identify concurrent heater-side flow
List only fixtures that realistically run together. Measure or verify their flows. A 2 GPM mixed shower does not necessarily require 2 GPM from the heater because cold water may be added at the fixture.
Using an illustrative 120°F hot supply, 45°F cold supply and 105°F mixed shower, the ideal hot fraction is (105 − 45) ÷ (120 − 45) = 0.8. Two 2 GPM showers would therefore draw approximately 3.2 GPM from the hot supply under those assumptions. Add other actual simultaneous hot draws separately.
Mixing calculations are estimates. Controls, pipe losses, changing inlet conditions and fixture behavior affect the real result. If a calculator conservatively treats all fixture flow as hot, state that assumption.
Step 3: Check the output curve and infrastructure
| Check | Reason |
|---|---|
| Output at design temperature rise | Establishes useful winter capacity |
| Minimum activation flow | Establishes behavior at small draws |
| Gas or electrical input | Establishes supply assessment |
| Water pressure and quality requirements | Establishes operating and maintenance conditions |
| Altitude and installation instructions | Identifies model-specific limitations |
An ideal thermal-load check is approximately 500 × GPM × rise in °F = Btu/hour delivered to water. At 4 GPM and 75°F rise, that is roughly 150,000 Btu/hour, or 44 kW of heat. Actual required input depends on appliance efficiency and design. This is a sizing sanity check, not a fuel-line or circuit-sizing method.
Verify the planning result
Use the tankless GPM calculator to organize flow and temperature inputs. Have qualified installers verify the exact output curve and supply capacity as part of installation.
Compare tankless versus tank and tankless versus heat pump if the conversion work is substantial. Review installed cost, check rebates for the specific model, and apply best water heater criteria after the candidate meets winter demand.