Heat-up time by heater size and temperature rise
Ideal minimum hours to raise a 20,000-gallon pool (efficiency 0.82) by a given temperature, from hours = gallons × 8.34 × ΔT ÷ (BTU/hr × eff). Real times run longer because of surface loss.
| Temp rise | 100k BTU | 200k BTU | 400k BTU |
|---|---|---|---|
| +5 °F | 10.2 h | 5.1 h | 2.5 h |
| +10 °F | 20.3 h | 10.2 h | 5.1 h |
| +15 °F | 30.5 h | 15.3 h | 7.6 h |
| +20 °F | 40.7 h | 20.3 h | 10.2 h |
20,000-gallon pool, efficiency 0.82, ideal minimum (surface loss adds time).