What size pool heater for a 20,000-gallon pool?
A 20,000-gallon pool with 512 square feet of surface, heated from 65°F to 82°F in a month averaging 65°F, needs 2,835,600 BTU (28.4 therms of heat) plus whatever it loses on the way. Covering a 24-hour heat-up and those losses at the same time takes about 222,600 BTU/h of heat output: a 300,000 BTU/h gas heater.
How to work it out
- Heat: 20,000 gal × 8.34 lb per gal × 17°F (65°F → 82°F) = 2,835,600 BTU
- Your heater: 150,000 BTU/h gas input × 82% = 123,000 BTU/h into the water
- Heat-up: 2,835,600 BTU ÷ 123,000 BTU/h = 23.1 hours with no losses
- DOE sizing: 512 sq ft × 17°F rise (82°F − 65°F air) × 12 = 104,400 BTU/h
- Heat-up in 24 hours: 2,835,600 BTU ÷ 24 = 118,100 BTU/h
- Losses + heat-up: 104,400 BTU/h + 118,100 BTU/h = 222,600 BTU/h ÷ 82% = 271,500 BTU/h gas input → 300,000 BTU/h gas heater
Twenty thousand gallons is a typical inground pool: a 16 × 32 ft rectangle with a deep end of about 7 ft, or an 18 × 36 with a shallow sport floor. Here the surface is 512 sq ft, the season runs into months averaging 65°F, and the goal is 82°F.
Adjust for your project
- Heat output it has to deliver
- 222,600 BTU/h of heat output (271,500 BTU/h gas input at 82%)
- Heat-loss allowance (DOE: area × rise × 12)
- 104,400 BTU/h
- Output to heat up in the hours you want
- 118,100 BTU/h
- Heat to warm the water
- 2,835,600 BTU (28.4 therms of heat)
- Heat-up time with your heater (no losses)
- 23.1 hours
- Heat-up time allowing for losses (rough)
- 153 hours
- Warming rate with your heater (no losses)
- 0.74°F per hour
- The “allowing for losses” time subtracts the DOE formula’s heat-loss allowance from what the heater delivers. It is rough — wind, sun, a cover and night temperatures all move it.
- The no-loss heat-up time ignores losses. A pool loses heat the whole time it warms up, mostly through evaporation, so the real time is longer, more so on windy, cool nights. Keep a solar cover on during the heat-up and it gets much closer to the figure above.
20,000-gallon pool (512 sq ft, 65°F → 82°F) by coldest-month air temperature
| Average air | Heat output it has to deliver | Heater size to look for | Heat-up time allowing for losses (rough) |
|---|---|---|---|
| 50°F | 314,800 BTU/h of heat output (383,900 BTU/h gas input at 82%) | 400,000 BTU/h gas heater | — |
| 55°F | 284,000 BTU/h of heat output (346,400 BTU/h gas input at 82%) | 400,000 BTU/h gas heater | — |
| 60°F | 253,300 BTU/h of heat output (308,900 BTU/h gas input at 82%) | 400,000 BTU/h gas heater | — |
| 65°F | 222,600 BTU/h of heat output (271,500 BTU/h gas input at 82%) | 300,000 BTU/h gas heater | 153 hours |
| 70°F | 191,900 BTU/h of heat output (234,000 BTU/h gas input at 82%) | 250,000 BTU/h gas heater | 58 hours |
How many BTU a 20,000-gallon pool takes
Each degree Fahrenheit across 20,000 gallons takes 166,800 BTU, because the water weighs about 166,800 lb. A 17-degree spring warm-up from 65°F to 82°F is therefore about 2.84 million BTU, or 28 therms of heat delivered to the water — more once you count the gas the heater burns to make it.
- The losses. The DOE formula, 512 sq ft × 17°F × 12, gives about 104,400 BTU/h. Used here as the pool’s heat-loss allowance in a month averaging 65°F, it is heat the heater spends just standing still.
- The heat-up. 2,835,600 BTU over 24 hours is another 118,150 BTU/h. The heater has to supply both at once: about 222,600 BTU/h of output.
- Gas. 222,600 ÷ 0.82 = about 271,500 BTU/h of gas input, so a 300,000 BTU/h heater is the next common size.
- Undersized heaters crawl. A 150,000 BTU/h gas heater puts about 123,000 BTU/h into the water — only about 18,600 BTU/h more than the losses — so the warm-up would take days, not hours.
- Heat pump. Even the largest standard heat pump (125,000 BTU/h, 6 hp) is short of 222,600. A heat pump still works for this pool if you accept a slower first heat-up and keep a cover on, so most of its output goes into holding temperature.
Swimming into colder months raises the loss allowance fast: at an average of 55°F the rise is 27°F and the DOE formula alone asks for about 166,000 BTU/h. For a smaller pool and a single 10-degree heat-up, compare how long to heat a pool 10 degrees. To price either option, the pool heating cost calculator uses the same numbers; what a heat pump pool heater costs per month runs a 125,000 BTU/h unit through a month.
Different size? Use the full Pool Heater Size & Heat-Up Time Calculator.
What you’ll need
- Gas pool heaterPremium price
Heats fastest and works in any weather, at the highest running cost. Sized in BTU per hour.
See options on Amazon (opens in a new tab) - Heat pump pool heaterPremium price
Much cheaper to run than gas in mild weather, but slower, and output drops as the air gets cooler.
See options on Amazon (opens in a new tab) - Floating pool thermometerBudget price
You need the real water temperature for heater sizing, heating cost and the saturation index.
See options on Amazon (opens in a new tab)
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Frequently asked questions
What size heat pump for a 20,000-gallon pool?
A one-day heat-up from 65°F to 82°F with 65°F average air would need 222,600 BTU/h or more of heat pump output. A 125,000 BTU/h heat pump gets there in roughly 138 hours allowing for losses (22.7 hours if no heat escaped), so plan on a cover and a slower start.
How long does a 250,000 BTU heater take to heat 20,000 gallons?
About 28 hours from 65°F to 82°F at 82% efficiency, allowing for losses in 65°F weather, or 13.8 hours if no heat escaped.
What size gas heater for 20,000 gallons if I swim when it averages 55°F?
About a 400,000 BTU/h gas heater: the DOE formula asks for 165,900 BTU/h of output when the air averages 55°F.
Other common sizes
- How long to heat a pool 10 degrees — 12.5 hours
Sources
- US DOE Energy Saver — Gas swimming pool heaters (archived) (opens in a new tab) — Sizing: pool area × temperature rise × 12, based on a 1° to 1-1/4°F rise per hour and a 3-1/2 mph average wind; ×1.5 for a 1-1/2°F rise, ×2 for 2°F. Gas heaters from 75,000 to 450,000 Btu; some reach 89%–95% efficiency.
- US DOE Energy Saver — Heat pump swimming pool heaters (archived) (opens in a new tab) — COPs usually 3.0 to 7.0, rated at 80°F air, 63% humidity and 80°F water; efficient above about 45–50°F outside; standard sizes 3.5 hp/75,000 Btu, 5 hp/100,000 Btu and 6 hp/125,000 Btu; same ×12 sizing formula.
- ENERGY STAR — Pool pumps (pool heating notes) (opens in a new tab) — The federal minimum efficiency for gas pool heaters is 82%; a cover saves 50–70% of heating energy.
- US DOE Energy Saver — Swimming pool covers (archived) (opens in a new tab) — Evaporation is by far the largest source of energy loss; each pound of 80°F water that evaporates takes 1,048 Btu out of the pool.
- US DOE Energy Saver — Managing swimming pool temperature (archived) (opens in a new tab) — Typical setting 78–82°F; each degree warmer costs 10%–30% more energy.
- US EIA — British thermal units (Btu) (opens in a new tab) — 1 kWh = 3,412 Btu; 1 therm = 100,000 Btu.