Water Heater Recovery Calculator
Calculate recovery rate, time to reheat, and first hour rating
BTU Input
Check rating plate on water heater
Temperature Settings
Recovery Results
Recovery Rate
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Temperature Rise
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BTU Output (Usable)
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Efficiency
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First Hour Rating
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Tip: First Hour Rating = (Tank Capacity x 0.70) + (Recovery Rate x 1 hour)
Recovery Formula
GPH = BTU Output / (8.33 x Temperature Rise)
BTU Output = BTU Input x Efficiency
8.33 = Weight of water (lbs/gallon)
Temperature Rise = Set Point - Inlet Temp
Typical Recovery Rates
| Fuel Type | BTU Input | GPH @ 90F Rise |
|---|---|---|
| Gas (40K BTU) | 40,000 | 35-40 GPH |
| Gas (50K BTU) | 50,000 | 43-50 GPH |
| Electric (4.5kW) | 15,400 | 18-21 GPH |
| Electric (5.5kW) | 18,800 | 21-25 GPH |
| Oil (70K BTU) | 70,000 | 55-65 GPH |
| Heat Pump | ~12,000 | 15-18 GPH |
Understanding Water Heater Recovery
Recovery Rate
The number of gallons of water the heater can raise by a specific temperature (usually 90F) in one hour. Higher BTU input = faster recovery.
First Hour Rating (FHR)
The total gallons of hot water a fully heated tank can supply in one hour. This combines stored hot water plus recovery during use.
Efficiency Impact
Heat pump water heaters are most efficient but have slower recovery. Gas heaters recover fastest. Electric is in between.