Water Heater Input for a Target Recovery
The inverse of the water-heater-recovery tile: the burner or element input needed to sustain a target recovery rate, input BTU/hr = recovery_gph x 8.33 x rise / efficiency (electric kW = input / 3412). A 54.9 gph recovery over a 70 F rise at 80% needs a 40,000 BTU/hr gas heater; a lighter 25.8 gph duty at 0.98 is a 4.5 kW electric element. Answers 'what input do I need' instead of the recovery from one rating. Steady recovery only - a tank credits stored volume for short peaks. Per DOE 10 CFR 430 / AHRI 1300.
Formula and source
input BTU/hr = (recovery_gph × 8.33 × delta-T) / recovery efficiency; electric kW = input / 3412. The inverse of gph = (input × efficiency) / (8.33 × delta-T).
DOE 10 CFR 430 water-heater efficiency test procedure; AHRI 1300 (residential water heaters).
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