Water Heater Recovery Rate

GPH = (BTU/hr × efficiency) / (8.33 × ΔT). Hot water output rate.

Inputs

Standard gas: 76-82%. High-efficiency condensing: 90-96%. Tankless: 80-94%.

Heat from cold (~50 °F) to setpoint (typically 120 °F): 70 °F rise.

Result

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How to use this calculator

  • Enter burner BTU rating.
  • Enter efficiency (label or AFUE).
  • ΔT = setpoint − incoming cold (typically 70 °F).

About this calculator

Water heater recovery rate (GPH = gallons per hour) measures continuous hot water output. Formula: GPH = (BTU/hr × efficiency) / (8.33 × ΔT), where 8.33 lb/gal is water mass. A 40,000 BTU gas heater at 80% efficiency raising water 70 °F: ~55 GPH ≈ 0.92 GPM. Tankless heaters quoted at peak GPM. Tank heaters: tank capacity gives buffer for short bursts (shower); recovery rate matters for long demands (back-to-back showers).

Frequently asked

BTU vs. wattage for electric?+
1 W = 3.412 BTU/hr. Electric water heater 4500 W = 15,354 BTU/hr (much less than gas).
Why 8.33?+
1 gallon water weighs 8.33 lb. To raise 1 lb water 1 °F = 1 BTU. So to raise 1 gallon: 8.33 BTU.
Tank vs tankless?+
Tank: 40-80 gal storage + recovery. Burst capacity. Tankless: no storage, infinite at design GPM. Different sizing.
Sizing guide?+
Family of 4: 50-80 gal tank + 40k BTU recovery. Sustained 2-shower demand: 6-7 GPM tankless or hybrid.
Why lower ΔT in summer?+
Incoming cold water warmer (~65 °F vs 50 °F winter). Less heat needed; recovery rate is higher.

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