Water Heater Recovery Calculator

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

Inputs

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

Results

Recovery Rate

54.88 GPH

First Hour Rating

89.88 gal

≈ 4 wheelbarrows

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.

How the result changes with Set Temperature

Set Temperature	Recovery Rate	First Hour Rating
108	66.23 GPH	101.23 gal
128	49.25 GPH	84.25 gal
152	37.66 GPH	72.66 gal
172	31.49 GPH	66.49 gal

What each input means

Tank Capacity: Total capacity of the water heater tank in gallons. IPC 501.3 and NFPA 54 regulate water heater installation; ASHRAE 90.1 sets minimum efficiency standards. First Hour Rating (FHR) = tank capacity + recovery per hour.
BTU Input: Rated BTU input of the water heater burner or element.
Thermal Efficiency: Combustion or heating efficiency. Standard gas: ~80%, high-efficiency: 90%+, electric: ~98%.
Inlet Water Temperature: Temperature of incoming cold water. Varies by region and season (40–70°F typical).
Set Temperature: Thermostat set temperature. ASHRAE 188 and CDC recommend 120°F for scalding prevention in residential; 140°F minimum for Legionella control in commercial systems (with tempering valve to 120°F at fixtures).

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Tank Capacity = 50, BTU Input = 40000, Thermal Efficiency = 80, Inlet Water Temperature = 50 = 5 input(s) provided
Calculate Recovery Rate
Recovery Rate
54.88 = 54.88
Calculate First Hour Rating
First Hour Rating
89.88 = 89.88
Calculate Time to Full Recovery
Time to Full Recovery
0.91 = 0.91
Calculate Standby Loss
Standby Loss
1.37 = 1.37

Engine last updated July 28, 2026. Checked against 1 independently-derived test — how we verify calculators.

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