Work out water heater recovery rate instantly with clear inputs, formula shown and shareable results.
Recovery rate is the heat delivered to the water divided by the energy needed per gallon, which is 8.33 BTU per degree Fahrenheit. First hour rating adds roughly 70 percent of the stored volume to the hourly recovery, reflecting the usable hot water in the tank before incoming cold dilutes the draw.
Recovery rate
gal/h = input BTU/h x efficiency / (8.33 x temperature rise)
First hour rating
FHR ≈ recovery gal/h + 0.7 x tank gallons
gal/h = input BTU/h x efficiency / (8.33 x temperature rise). Recovery rate is the heat delivered to the water divided by the energy needed per gallon, which is 8.33 BTU per degree Fahrenheit.
Incoming mains water is colder, so the required temperature rise is larger and recovery in gallons per hour falls in direct proportion.
This calculator takes 4 inputs: Burner or element input, Recovery efficiency, Temperature rise, Tank capacity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.