Water Heater Recovery Rate
Recovery gph and AHRI first-hour rating from input rating, efficiency, and temperature rise.
Example
You enter
- Heater type gas_atmospheric
- Gas input rating (BTU/hr) 40000
- Recovery efficiency (blank = default) 0.8
- Incoming water temp (°F) 50
- Set-point temp (°F) 120
- Tank size (gal) 40
You get
- Q useful (Btu/hr) 32000
- Recovery rate 54.9 gph
- First-hour rating 82.9 gph
Details, formula, and sources
gph = input BTU/hr x efficiency / (8.33 x delta-T). Per DOE 10 CFR 430 / AHRI 1300.
gph = (input BTU/hr × recovery efficiency) / (8.33 × delta-T). Electric input = kW × 3412 BTU/hr. First-hour rating = recovery gph + 0.70 × stored gallons (DOE FHR convention). 8.33 BTU per gallon per degree F is first-principles water properties.
DOE 10 CFR 430 water-heater efficiency test procedure; AHRI 1300 (residential water heaters).
Free at energy.gov/eere for DOE test procedures and ahri.org for the AHRI directory / TOC.
Estimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
Field names used by the API: heater_type, input_btu_hr, efficiency, incoming_F, setpoint_F, tank_gal, q_useful_btu_hr, recovery_gph, first_hour_gph
- Recovery efficiency 0.98 electric / 0.80 atmospheric gas / 0.94 condensing gas unless the user enters the nameplate valueDOE 10 CFR 430 / AHRI 1300 typical
- First-hour usable storage 70% of nominal tank volume per the DOE first-hour-rating conventionDOE 10 CFR 430 FHR test
- Water energy constant 8.33 BTU per gallon per degree Ffirst-principles water properties