The short answer
A 20,000 gallon pool warmed 20°F in 24 hours needs 139,000 BTU/h of heat: a 200,000 BTU gas pool heater (rated by input, at the 82% federal minimum efficiency) or a 140,000 BTU heat pump. Wind and evaporation slow it down, so the larger size and a cover help.
Gas heater size by pool
Gas heater input to warm the pool 20°F in about a day, at 82% efficiency.
| Pool | Output needed | Gas heater | Heat pump |
|---|---|---|---|
| 10,000 gal | 69,500 | 150,000 | 85,000 |
| 15,000 gal | 104,250 | 150,000 | 110,000 |
| 20,000 gal | 139,000 | 200,000 | 140,000 |
| 25,000 gal | 173,750 | 250,000 | 140,000+ |
| 30,000 gal | 208,500 | 300,000 | 140,000+ |
How it works
The formula
output (BTU/h) = heat ÷ hours
gas input = output ÷ 0.82
Assumptions
- A BTU warms 1 lb of water by 1°F, and a gallon of water weighs about 8.34 lb.
- Gas efficiency: 82%, the federal minimum thermal efficiency for gas pool heaters made since April 16, 2013 (10 CFR 430.32(k)); a 2023 rule raises it from May 30, 2028. Gas pool heaters are sold by input: 150k, 200k, 250k, 300k, 350k, 400k BTU/h are common.
- Heat pumps are sold by output, typically 50k, 65k, 85k, 100k, 110k, 125k, 140k BTU/h, rated at warm, humid air; they put out less on cool nights.
- Heat lost from the pool while it warms (evaporation, wind, the ground) isn't counted, so real heat-up takes longer.
Worked example: a 20,000 gallon pool
Warm it 20°F in 24 hours
- 20,000 × 8.34 × 20 = 3,336,000 BTU
- ÷ 24 h = 139,000 BTU/h output
- ÷ 0.82 = 169,512 BTU/h input → 200,000 BTU gas heater
Too big or too small
- Too small: slow and never quite there. On a cool, windy night a small heater may lose as much heat as it adds.
- Bigger is fine for gas. A larger gas heater warms faster and runs fewer hours; fuel use is about the same.
- Use a cover. It cuts the heat lost to evaporation, which is most of a pool's heat loss.
Questions people ask
What size pool heater do I need?
Work out the heat to warm the water: gallons × 8.34 × the temperature rise. A 20,000 gallon pool warmed 20°F needs 3,336,000 BTU; to do it in 24 hours takes 139,000 BTU/h of output, so a 200,000 BTU gas heater (input, at 82% efficiency) or a 140,000 BTU heat pump.
How long does a pool heater take to heat a pool?
Heat needed ÷ heater output. A 200,000 BTU gas heater puts out about 164,000 BTU/h, so 20,000 gallons by 20°F takes about 20.3 hours, longer on a windy night because of heat lost from the surface.
Gas or heat pump?
Gas heats fast whatever the weather, so it suits occasional use and spas. A heat pump costs much less to run but heats slowly and loses output as the air cools; it suits pools kept warm all season in mild climates.
Does a pool cover matter?
A lot. Most of a pool's heat loss is evaporation from the surface, and a cover stops most of it, so the heater runs far less.