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) 5.4415
- delta T 60 F
- Inlet temperature 50 F
- Target outlet (°F) 110
Checked against published worked examples from: Georgia Department of Public Health, Environmental Health.
Details, formula, and sources
Formula or tableDelivered heat (BTU/hr) = 8.33 × ΔT × GPM × 60 = rated input × thermal efficiency. Solved for GPM, required input, or temperature rise.
Source and editionASHRAE Handbook (HVAC Systems and Equipment) by name; manufacturer ratings on the heater nameplate.
Free accessASHRAE Handbook licensed; manufacturer data sheets free at each manufacturer site.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field nameskbtu_input, climate_zone, target_outlet_F, gpm, delta_T_F, inlet_F
- Specific heat × density factor 8.33 (BTU per gal-°F)physical fact for water
- Default efficiency 0.82 (the former DOE Energy Factor minimum for gas tankless; the current rating is a Uniform Energy Factor, 0.81 minimum) unless the user supplies the unit's ownDOE 10 CFR 430