Water Heater Recovery Calculator

Calculate water heater recovery rate, first hour rating, time to full recovery, and standby heat loss.

Tank Capacity

gal

Residential sizing: 1–2 people = 30 gal; 3–4 people = 40–50 gal; 5+ people = 50–80 gal

BTU Input

BTU/hr

Thermal Efficiency

Inlet Water Temperature

°F

Set Temperature

°F

Residential: 120°F (scalding prevention); Commercial: 140°F minimum per ASHRAE 188 (Legionella)

Tab to move between fields

Recovery Rate

54.88 GPH

First Hour Rating

89.88 gal

Time to Full Recovery0.91 hours

Standby Loss1.37 GPH equiv

How to Use This Calculator

Enter the tank capacity in gallons and the BTU input from the unit's nameplate.
Set the thermal efficiency (0.60–0.95 depending on energy factor rating).
Enter the cold inlet temperature and your hot water set temperature (120°F recommended).
Review Recovery Rate (GPH) and First Hour Rating — the First Hour Rating is what the EnergyGuide label shows.
Use Time to Full Recovery to understand how long after morning demand before the tank is ready again.

Ad Placeholder

Related Calculators

Size water supply piping based on fixture units, building height, available pressure, and pipe run length.

Size thermal expansion tanks for hot water systems based on system volume, temperature range, and pressure settings.

Ad Placeholder