Easy Pool Calc

How long does it take to heat a pool 10 degrees?

Short answer

Raising a 15,000-gallon pool from 70°F to 80°F takes 1,251,000 BTU (12.5 therms of heat). A 100,000 BTU/h heat pump delivers that in about 12.5 hours if no heat escaped. Allowing for losses in a month averaging 65°F, it is more like 66 hours with the pool uncovered, which is why a cover during the heat-up matters so much.

How to work it out

  1. Heat: 15,000 gal × 8.34 lb per gal × 10°F (70°F → 80°F) = 1,251,000 BTU
  2. Your heat pump: 100,000 BTU/h of heat output
  3. Heat-up: 1,251,000 BTU ÷ 100,000 BTU/h = 12.5 hours with no losses
  4. DOE sizing: 450 sq ft × 15°F rise (80°F − 65°F air) × 12 = 81,000 BTU/h
  5. Heat-up in 24 hours: 1,251,000 BTU ÷ 24 = 52,100 BTU/h
  6. Losses + heat-up: 81,000 BTU/h + 52,100 BTU/h = 133,100 BTU/h → 133,100 BTU/h or more of heat pump output

Ten degrees is the classic question: the water is 70°F, you want 80°F, and the heater is on. The answer depends on only two things, the gallons and the heat the heater actually delivers, and then on how much the pool loses while it works.

Adjust for your project

gal
Don’t know it? Work it out with the pool volume calculator or the above-ground pool calculator.
sq ft
Length × width for a rectangle. For round, oval or kidney pools the pool volume calculator shows the surface area along with the gallons.
°F
From a pool thermometer, at arm’s depth
°F
The DOE calls 78–82°F (26–28°C) the usual range; each degree warmer costs 10–30% more energy
°F
The monthly average of day and night (your area’s climate normals), not the afternoon high
Gas heaters are usually rated by gas input; heat pumps by heat output. 1 kW = 3,412 BTU/h
hours
The wait you would accept for the first heat-up. Shorter means a bigger heater; heat pumps usually take longer.
Heater size to look for133,100 BTU/h or more of heat pump output
Heat output it has to deliver
133,100 BTU/h of heat output
Heat-loss allowance (DOE: area × rise × 12)
81,000 BTU/h
Output to heat up in the hours you want
52,100 BTU/h
Heat to warm the water
1,251,000 BTU (12.5 therms of heat)
Heat-up time with your heater (no losses)
12.5 hours
Heat-up time allowing for losses (rough)
66 hours
Warming rate with your heater (no losses)
0.8°F per hour
  • That is more than the DOE’s standard heat pump sizes (75,000–125,000 BTU/h, 22–37 kW). Look at larger models, accept a slower heat-up (more hours), or run the numbers for a gas heater.
  • 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.
  • Heat pumps are rated at 80°F (27°C) air. In cooler air the output drops, and the DOE says they work efficiently only above about 45–50°F (7–10°C). The COP describes electricity in versus heat out; it doesn’t change the heat-up time.

For this job · affiliate link
Heat pump pool heaterHeat pumps are sold by heat output, so match the size above directly, and check the output figure at the cooler air temperature you’ll actually swim in.
View on Amazon (opens in a new tab)

Hours to heat 15,000 gallons by 10°F, by heater rating (heat pump)

Hours to heat 15,000 gallons by 10°F, by heater rating (heat pump)
HeaterHeat-up time with your heater (no losses)Warming rate with your heater (no losses)
75,000 BTU/h16.7 hours0.6°F per hour
100,000 BTU/h12.5 hours0.8°F per hour
125,000 BTU/h10 hours1°F per hour

The 10-degree arithmetic

Fifteen thousand gallons of water weighs about 125,100 lb, and one BTU warms one pound by one degree, so ten degrees is 1,251,000 BTU. Everything else is division.

15,000 gallons, 70°F to 80°F. Heat pumps at their rated output (80°F air); gas heaters rated by input at the 82% federal minimum efficiency.
HeaterHeat into the waterHours for 10°F (no losses)
75,000 BTU/h heat pump75,000 BTU/h16.7
100,000 BTU/h heat pump100,000 BTU/h12.5
125,000 BTU/h heat pump125,000 BTU/h10
150,000 BTU/h gas at 82%123,000 BTU/h10.2
250,000 BTU/h gas at 82%205,000 BTU/h6.1
400,000 BTU/h gas at 82%328,000 BTU/h3.8

Those are floors, not promises. The pool keeps losing heat while it warms, mostly through evaporation from the surface, so a heat pump that adds about eight-tenths of a degree an hour can lose a good share of that on a breezy night. A heat pump also produces less than its rating when the air is below the 80°F it was tested at. Covering the pool during the heat-up is the single biggest help. For a bigger pool sized from scratch, see what size pool heater for 20,000 gallons; to see what those hours cost, try the pool heating cost calculator.

Different size? Use the full Pool Heater Size & Heat-Up Time Calculator.

What you’ll need

As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.

Frequently asked questions

How long does a 250,000 BTU gas heater take to heat 15,000 gallons 10 degrees?

About 6.1 hours at 82% efficiency (1.64°F per hour) before losses, or roughly 10 hours allowing for losses.

How long to heat a pool 20 degrees with a 100,000 BTU heat pump?

About 25 hours for 15,000 gallons from 60°F to 80°F, before losses; the water needs 2,502,000 BTU (25 therms of heat). Twice the rise, twice the time.

How long to heat a 10,000-gallon pool 10 degrees with a heat pump?

About 8.3 hours with a 100,000 BTU/h heat pump; it needs 834,000 BTU (8.3 therms of heat).

Other common sizes

Sources