Storage Water-Heater Sizing (First-Hour Rating)

The first-hour rating (FHR) is the usable storage plus one hour of recovery, with recovery = input x efficiency / (8.33 x rise), checked against the peak-hour draw. The manufacturer's rated FHR on the EnergyGuide label governs the final selection; this is a sizing check, not the rating.

recovery_gph = input_btuh x efficiency/100 / (8.33 x rise_F); first-hour rating FHR_gph = tank_gal x usable_fraction + recovery_gph; the verdict compares FHR to the peak-hour demand.

First-principles recovery Q = 8.33 x gph x delta-T with the DOE/AHRI first-hour-rating definition (usable storage plus one hour of recovery), by name, not reproduced.

The recovery relation is public physics; the DOE/AHRI first-hour-rating test value on the EnergyGuide label and the manufacturer govern the rating.

Estimate. AHJ and licensed professional govern.

Field names used by the API: tank_gal, input_btuh, efficiency_pct, rise_F, usable_fraction, peak_hour_gal, recovery_gph, fhr_gph, short_by_gph

• Constants 8.33 lb/gal water and the default 0.70 usable-storage fraction (before the stored water drops below the set point) are editablefirst principles / DOE FHR method
• FHR definition first-hour rating = usable storage + one hour of recovery; match it to the peak-hour draw, not to tank gallons aloneDOE/AHRI first-hour-rating method
• Check, not rating this is a sizing check; the manufacturer's tested FHR on the yellow EnergyGuide label is the governing valuemanufacturer EnergyGuide label