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Pool Cover Heat Loss

Where a pool's heat goes

A warm pool loses heat in four main ways. Evaporation turns water into vapour, and every pound evaporated carries off roughly 1,000 BTU. Convection carries heat away as wind moves air across the surface. Radiation sends heat to the cold night sky, especially on clear nights. Conduction moves a small share into the ground or through walls. Of these, evaporation commonly dominates, which is why the U.S. Department of Energy and pool industry sources point to covers as the first efficiency step for heated pools.

How a cover helps

A cover floating on or sitting directly above the water traps vapour, so evaporation nearly stops. It also reduces convection by keeping wind off the surface and slows radiation. Insulated or bubble covers add a layer of trapped air. The result is that the pool loses far less heat overnight, so the heater runs fewer hours the next day. Our sizing calculators reflect this with a cover setting.

How much can you save?

Savings depend on climate, wind, humidity, the temperature gap and how consistently the cover is used. Industry and government guidance commonly cite savings in the range of 50 to 70 percent of heating energy for covered pools versus uncovered. Treat that as a broad planning range. A cover left in the shed saves nothing, so convenience matters: a reel or automatic cover that is easy to use often delivers more real savings than a better cover that stays rolled up.

Types of covers

• Solar (bubble) covers: light, low-cost, trap heat and let some sunlight warm the water. See the solar cover guide.
• Thermal or foam covers: thicker insulation, better overnight retention, heavier to handle.
• Liquid covers: a thin surface film that slows evaporation; weaker than solid covers and disrupted by swimmers and wind.
• Automatic safety covers: motorized track systems that are easy to use daily. Some are rated as safety covers; check standards.
• Winter covers: for closing the pool, not for daily heat retention.

Safety

Solar and thermal covers are not safety covers. People and pets can become trapped under a floating cover. Remove it completely before anyone swims, never let children play on it, and store it away from the pool edge. Only covers certified to a recognized safety standard should be treated as barriers, and even then follow the manufacturer's guidance and local fence rules.

Wind breaks and other steps

Fences, hedges or walls that reduce wind across the pool can lower convective and evaporative losses. Lowering the target temperature by a degree or two helps too. Turning off water features and waterfalls at night reduces evaporation, since splashing exposes more water to air. Running the heater during the warmest part of the day, when air temperatures are higher, can help heat pumps work more efficiently.

Worked example

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

Assumptions: 512 sq ft pool, water 82°F, average night air 60°F, average wind. Uncovered holding loss by the 12 BTU/h rule ≈ 512 × 22 × 12 ≈ 135,000 BTU/h. If a cover cuts that by half for 10 night hours, it saves about 675,000 BTU per night. Over 30 nights that is about 20 million BTU, or roughly 240 therms of gas input at 84% efficiency.

Chemistry under a cover

Covers reduce sunlight on the water, which slows chlorine loss, and they cut evaporation, which concentrates fewer minerals. You may need less sanitizer, but test regularly and follow chemical labels. Remove the cover after shock treatments as the product label directs to let gases vent.

Choosing and caring for a cover

Measure the pool carefully, including steps and ledges. Trim a solar cover to fit with a small overlap. A reel makes daily use easier. Clean the cover with plain water, store it shaded when off the pool, and replace it when it becomes brittle or torn. A damaged cover loses much of its value.

Using your cover with the calculators

Our pool heater size calculator and pool heat pump size calculator include a cover option. Choose "cover every night" only if that is a realistic habit. Then compare running cost with and without a cover in the operating cost calculator by halving or reducing monthly run hours.

When the cover should come off

Remove the cover completely before swimming, during and after heavy shock dosing as labels direct, and in high heat if the water is getting uncomfortably warm. Do not leave a solar cover on an unheated, closed pool for months in direct sun without checking the maker's guidance; some materials degrade quickly when trapped heat builds up between cover and water. Fold or reel it neatly so it does not crease and crack.

Covers and indoor pools

Indoor pools lose heat to room air and add humidity that the building's ventilation must remove. A cover reduces evaporation there too, which can lower dehumidification load as well as water heating. Indoor pool systems are engineered; changes should go through the building's HVAC professional.

Measuring your own savings

For a simple test, note pool temperature at sunset and sunrise for several nights without the cover, then several nights with it, keeping the heater off. The overnight drop with the cover is typically much smaller. That difference, multiplied by gallons × 8.34, gives a rough BTU saving per night for your pool and climate.

FAQ

Is a solar cover enough?

For many outdoor pools it is the best value first step; thermal covers hold more heat overnight.

Should I leave the cover on during the day?

If no one is swimming, yes; a solar cover still lets some sun through.

Does a liquid cover work as well?

Usually not; it helps a little, but solid covers block much more evaporation.

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