How the calculation works
Hours = (gallons × 8.34 × temperature rise) ÷ heater output, with an allowance added for the heat the pool loses while it is warming up.
That allowance is not a rounding error. An uncovered pool sheds heat from its surface continuously, so roughly a third of the heater's work goes to replacing what has already escaped. With a cover it is closer to a twentieth.
Worked example
A 20,000 gallon pool going from 68°F to 82°F needs about 2.33 million BTU. A 250,000 BTU heater on an uncovered pool delivers that in roughly 12 to 13 hours. Put a cover on and the same job takes about 10.
Assumptions
- The heater delivers its rated output continuously.
- A 35% allowance for heat loss uncovered, 5% covered.
- The pump runs throughout, since a heater only works while water is flowing through it.
Limitations
- Cold, windy or dry weather increases losses well beyond the standard allowance — heating in spring takes much longer than the same job in July.
- Heat pump output falls as air temperature drops, so a heat pump rated at 110,000 BTU on a warm day delivers far less on a cold morning.
- Solar heating depends entirely on sunshine and cannot be predicted from a rated output at all.
Questions people ask
How long does it take to heat a pool?
For a typical 20,000 gallon pool and a 250,000 BTU gas heater, roughly half a day for a 14 degree rise. A heat pump doing the same job usually takes two to three days.
Should I turn the heater off when I am not swimming?
If you swim most days, holding the temperature costs less than reheating from cold each time. If you swim at weekends only, letting it drop and reheating is usually cheaper — especially with a cover on in between.
Why is my pool heating so slowly?
Most often no cover, cold nights, or a heat pump working in air that is too cool for it. Low flow through the heater will also do it — check the filter pressure and the pump speed.