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How Long To Heat A Pool

The basic formula

Water's heat capacity makes the maths simple. One BTU raises one pound of water by 1°F, and one US gallon weighs about 8.34 lb. So the energy to warm a pool is gallons × 8.34 × temperature rise. Divide that by the net BTU per hour reaching the water and you have hours. Net heat is the heater's output minus what the pool loses while heating.

Gas heaters

A gas heater's label shows input BTU/h. Output is input × efficiency. A 400,000 BTU/h heater at 84 percent delivers about 336,000 BTU/h; spread across 20,000 gallons, that is 336,000 ÷ 166,800 ≈ 2°F per hour before losses. Smaller heaters or bigger pools rise more slowly. On a calm day with a cover on, real results can be close to the calculation; on a windy night with no cover, much slower.

Heat pumps

Heat pumps are rated by output, often 50,000 to 140,000 BTU/h for residential pool units at favourable air conditions, less in cool air. A 100,000 BTU/h output unit on a 20,000-gallon pool gives about 0.6°F per hour before losses. A 15°F rise could take a day or more of continuous running even in good weather, and several days in cool weather. That is why heat pump owners usually keep the pool at temperature through the season.

Solar

Solar adds heat only in sunshine and at modest rates. Expect gradual gains across several sunny days, especially with a cover.

What slows heating

• Surface loss: evaporation and wind can take a large share of heater output; a cover helps most.
• Cool or clear nights: radiation to the sky increases losses.
• Short pump runtime: heaters only heat while water flows.
• Low gas pressure or undersized gas line: a gas heater may not reach rated input.
• Scale or dirty filters: reduce flow and heat transfer.
• Cold air (heat pumps): lower output and COP.

Using our calculator

The pool heating time calculator takes gallons, start and target temperatures, heater rating, efficiency and a loss allowance. For a heat pump, enter output at the current air temperature and 100 percent efficiency. Get gallons from the pool volume calculator.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

Assumptions: 18,000 gallons, 64°F to 80°F, 250,000 BTU/h input gas heater at 84%, 25% loss allowance. Energy ≈ 18,000 × 8.34 × 16 ≈ 2.4 million BTU. Net ≈ 250,000 × 0.84 × 0.75 ≈ 157,500 BTU/h. Time ≈ 15.3 hours. The same pool with a heat pump delivering 90,000 BTU/h and a 25% loss allowance: net ≈ 67,500 BTU/h, so about 36 hours.

Planning tips

Start heating a day or two before you need the pool, especially with a heat pump. Keep the cover on during heat-up. Run the pump continuously while heating. Set realistic targets: lifting 15°F in cool spring air is a big job for any heater. If you heat only for weekends with gas, compare the cost of reheating each time against holding a lower temperature midweek using the operating cost calculator.

Reheating vs maintaining

For heat pumps, maintaining is generally more practical, since recovery is slow. For gas, it depends: letting the pool cool midweek saves the holding loss, but you pay to reheat. In cool, windy weather without a cover, holding can cost more than reheating; with a good cover, holding losses shrink. Run both scenarios for your pool.

When heating takes far too long

If real heating is much slower than your estimate, check that the pump is running, the filter is clean, valves are set to send water through the heater and the cover is on. Then compare inlet and outlet water temperature at the heater if you have a thermometer; a very small difference suggests the heater is not delivering. Further diagnosis, especially of gas pressure, burners or refrigerant, belongs to a qualified technician. See pool heater not heating.

Seasonal start-up

The first heat of the season is the largest. Water may sit in the 50s or low 60s after opening, and air is still cool. Plan the start-up for a mild stretch of weather, keep the cover on and run the pump around the clock until you reach temperature. Heat pumps in particular may struggle in early spring; a few warm days make a big difference.

Rate of rise in plain numbers

A useful mental shortcut: degrees per hour ≈ net BTU/h ÷ (gallons × 8.34). For a 15,000-gallon pool, each 125,000 BTU/h of net heat gives about 1°F per hour. For 30,000 gallons, the same heat gives half a degree.

Heating a pool after a long vacation

If the pool cooled while you were away, the reheat time depends on how far it dropped. A drop of 8 to 10°F in a week without heat is common in mild weather, more in cool weather. Instead of letting the pool fall that far, some owners leave a lower set point running while away, especially with a cover on. With a heat pump that is usually cheaper than a large reheat. With gas, compare the two approaches using the operating cost calculator before you leave.

FAQ

Can I heat a pool in one day?

With an adequately sized gas heater, a calm day and a cover, often yes for moderate rises.

Should I run the heater overnight?

Heat pumps work better in daytime warmth; gas heaters work either way but lose more to cold night air without a cover.

Does a bigger pump speed heating?

Only up to the heater's rated flow range; too much flow can reduce efficiency or trigger bypass.

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