Tankless Water Heater GPM
Achievable flow from burner kBTU and inlet/outlet temperatures.
Example
You enter
- Burner input (kBTU/hr) 199
- Climate zone 5A_Chicago_IL
- Target outlet (°F) 110
You get
- Hot-water flow (gpm) 6.636
- delta T 60 F
- Inlet temperature 50 F
- Target outlet (°F) 110
Details, formula, and sources
Required heater output (BTU/hr) = 8.34 × ΔT × GPM × 60. Solved for GPM given the heater's rated input × thermal efficiency.
ASHRAE Handbook (HVAC Systems and Equipment) by name; manufacturer ratings on the heater nameplate.
ASHRAE Handbook licensed; manufacturer data sheets free at each manufacturer site.
Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
Field names used by the API: kbtu_input, climate_zone, target_outlet_F, gpm, delta_T_F, inlet_F
- Specific heat × density factor 8.34 (BTU per gal-°F)physical fact for water
- Default thermal efficiency 0.82 unless user supplies a tested EFDOE federal minimum for gas tankless