Recovery rate of a 40,000 BTU gas water heater at a 90°F rise
| Recovery rate | 40.5gal/hr |
|---|---|
| Time to reheat one showerabout 12 gallons of stored hot water | 18minutes |
| Heat delivered to the water40,000 BTU/hr input at 76% recovery efficiency | 30,400BTU/hr |
| Temperature risetank setpoint minus incoming water temperature | 90°F |
| Heaterthe standard 40- and 50-gallon gas heater | 40,000 BTU atmospheric gas |
Notes
- Capacity is what you have; recovery is what comes back. A 90°F rise at 30,400 BTU/hr delivered gives 40.5 gallons an hour — roughly 18 minutes to make good one shower. Two showers back to back are decided by this number, not by the size of the tank.
- Gas recovers fast enough that tank size matters less. At 40.5 GPH this heater replaces a shower in about 18 minutes, which is why a 40-gallon gas tank often outperforms a 50-gallon electric one in a busy house.
- Incoming water temperature moves this number all year. 90°F is the rise you asked for; in a northern winter the mains can arrive at 40°F and the rise becomes 80°F, cutting recovery proportionally. A heater that just copes in August is the same heater that runs out in February.
- Turning the thermostat up is not the fix people think it is. It stores more usable heat, so the first draw lasts longer, but recovery gets slower because the rise is larger. Above 120°F it also becomes a scald risk — the standard answer is a thermostatic mixing valve at the tank, which stores hot and delivers safe.
- The rating that combines both is first-hour rating, printed on the yellow EnergyGuide label: usable storage plus one hour of recovery. It is the honest single number for comparing two heaters, and it is the one the tank's gallon figure gets mistaken for.
40.5 gallons an hour
Recovery is a division, and every term in it is knowable:
30,400 BTU/hr delivered ÷ (8.33 × 90°F) = 40.5 GPH
8.33 is the weight of a gallon of water in pounds, which is also the BTU needed to raise it one degree. That is the whole formula, and it is why a heater's recovery changes with the season without anything being wrong.
What it means at the tap
About 18 minutes to put back the roughly 12 gallons of stored hot water one ten-minute shower uses. Two showers in a row are decided by this number; the gallon rating on the tank decides only the first one.
The number to compare heaters by
First-hour rating, printed on the yellow EnergyGuide label. It combines usable storage with one hour of recovery, which is the honest way to compare a fast-recovering small tank against a slow-recovering large one — and it is the number the tank's gallon figure keeps getting mistaken for.
Nearby sizes
Read more
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- Water Heater Replacement Cost by State — 50 Gallon Gas, All 51Like-for-like is the cheap path. Every change away from it adds a trade, and that is why two quotes on "a new water heater" can differ by three thousand dollars.
- Is a 40 Gallon Water Heater Enough for 2 Bathrooms?Two bathrooms is not a load. Two showers at 7:10am is a load, and the answer changes completely depending on what heats the water.
- T&P Valve Keeps Releasing Water — Thermal Expansion, Not a Bad ValveThe valve is the messenger. Replacing it silences the message for a few weeks and leaves the pressure exactly where it was.