Recovery rate of a heat pump water heater at a 90°F rise
| Recovery rate | 13.7gal/hr |
|---|---|
| Time to reheat one showerabout 12 gallons of stored hot water | 53minutes |
| Heat delivered to the water10,236 BTU/hr input at 100% recovery efficiency | 10,236BTU/hr |
| Temperature risetank setpoint minus incoming water temperature | 90°F |
| A standard 40,000 BTU gas heater3× this unit at the same rise | 40.5gal/hr |
| Heaterabout 1 kW drawn at a COP near 3 — cheap to run, slow to recover | heat pump, compressor only |
Notes
- Capacity is what you have; recovery is what comes back. A 90°F rise at 10,236 BTU/hr delivered gives 13.7 gallons an hour — roughly 53 minutes to make good one shower. Two showers back to back are decided by this number, not by the size of the tank.
- This is why swapping gas for electric feels like a downgrade. The same 50-gallon tank recovers at 13.7 GPH on electricity against 40.5 GPH on an ordinary 40,000 BTU gas burner — 3 times slower. Nothing is faulty; the element simply cannot put heat in as fast as a flame. Going electric usually means going up a tank size to compensate.
- 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.
13.7 gallons an hour
Recovery is a division, and every term in it is knowable:
10,236 BTU/hr delivered ÷ (8.33 × 90°F) = 13.7 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 53 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
- 120 or 140 — The Temperature Setting Is a Genuine Trade-OffThe two pieces of standard advice contradict each other, and there is a fitting that resolves it.
- Recovery Rate — How Fast the Tank Refills With Hot WaterTwo households with the same tank have different experiences, and the difference is how fast it comes back.
- Two Showers at Once — Why It Empties a Tank So FastThe tank holds fifty gallons and it does not hold fifty gallons of usable hot water.
- Sediment — Why an Older Heater Gets Slower and NoisierThe popping noise from an old gas water heater is steam forming under a layer of mineral deposit.