Water Heater Recovery Rate Calculator

Work out water heater recovery rate instantly with clear inputs, formula shown and shareable results.

About Water Heater Recovery Rate Calculator

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.

Formulas

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

Worked example

With the default values below, Water Heater Recovery Rate Calculator returns Recovery rate of 48.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Burner or element inputminimum 1 BTU/h: 40,000 BTU/h
Recovery efficiencyaccepts 20–100 %: 80 %
Temperature riseminimum 1 °F: 80 °F
Tank capacityminimum 1 gal: 50 gal

Results

Recovery rate: 48.0 gal/h
First hour rating: 83.0 gal
Recovery in litres: 181.8 L/h
Time to reheat full tank: 62.5 min

Step by step

1Values used
2Recovery rate
3First hour rating
4Recovery rate= 48.0 gal/h
5First hour rating= 83.0 gal
6Recovery in litres= 181.8 L/h
7Time to reheat full tank= 62.5 min

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Water Heater Recovery Rate figure worked out?

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.

Why does winter performance drop?

Incoming mains water is colder, so the required temperature rise is larger and recovery in gallons per hour falls in direct proportion.

What values do I need to enter?

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.