How to heat a pool fast
One division decides the answer. Everything else is either a small optimization or a myth.
Quick picks
Ranked by how much each one shortens the wait, which is not the same as ranked by cost.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | No photo Fastest, if speed is the goal Raypak Digital Natural Gas Pool Heater Hours instead of most of a day, at about two and a half times the running cost. | Heating fast and occasionally, rather than holding a temperature | |
| 2 | No photo Makes everything faster Sun2Solar 1200 Series Crystal Clear Solar Cover A heater fighting overnight loss is a heater doing the same work twice. | Most pools, most budgets — the default recommendation | |
| 3 | No photo Slower, much cheaper Hayward HeatPro Pool Heat Pump If you can start it the night before, the speed penalty stops mattering. | Lowest running cost per degree when you want real temperature control |
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
The one division that decides it
Heating speed is not complicated. It is the energy you need divided by the energy your heater delivers per hour, and everything else is at the margins.
| Energy needed = gallons x 8.33 x ΔT | |
| Hours = energy needed ÷ delivered BTU per hour | |
| 20,000 gal x 8.33 x 10°F = 1,666,000 BTU | worked example |
| ÷ (400,000 BTU/hr gas heater x 82%) | = about 5.1 hours |
| ÷ 140,000 BTU/hr heat pump (output-rated) | = about 11.9 hours |
Same pool, same 10 degrees: five hours or twelve, depending entirely on the heater
These are ideal figures that assume no heat is escaping while you heat. In reality some is, which is why covering the pool while it heats genuinely shortens the job.
The calculator does this with your numbers and handles the rating trap below automatically.
The rating trap
What genuinely speeds it up
- A bigger heater. Unsatisfying but true: the division above is the whole story, and the denominator is the only term you can change dramatically.
- Covering the pool while it heats. This is the free one. Your heater is fighting the heat loss that is happening at the same time, and DOE puts about 70% of that loss down to evaporation. A cover on while heating means more of the heater’s output ends up as temperature rather than replacing losses. It matters most when it is windy or the air is cold.
- Starting the night before. Not faster in hours, but it means the pool is warm when you want it, which is the actual goal. A heat pump’s twelve hours is not a problem if it runs overnight.
- Aiming lower. Halving the temperature rise halves the time exactly. Five degrees instead of ten is half the wait and half the cost.
What does not help
- Turning the thermostat higher than your target. The heater is already at full output. A thermostat is a stop signal, not a throttle.
- Turning the pump to maximum. Flow rate has to be within the heater’s specified range, and beyond that more flow does not add heat. Too much flow can actually reduce transfer efficiency.
- Draining and refilling with hot water. People ask. Your water heater delivers a trivial fraction of a pool’s volume and does it at electric-resistance or gas cost, with no efficiency advantage.
The honest trade
If speed genuinely matters to you — a weekend pool, guests arriving, rental turnover — gas is the right answer even though it costs about 2.6 times what a heat pump costs per delivered BTU. Buy the speed if you need the speed. Just buy it knowingly.
If you can plan a day ahead, the heat pump’s twelve hours costs you nothing but foresight and saves you the running-cost multiple every time you use it. The full head-to-head works through where the crossover sits.
Every pick, in detail
What each one publishes, what is good and bad about it, and — the section this category usually omits — what we could not judge without owning one.
Raypak Digital Natural Gas Pool Heater
The right answer when speed matters more than running cost — a weekend pool you want warm on Friday afternoon rather than one you hold at temperature all season. At the assumptions we publish, gas costs roughly two and a half times what a heat pump costs per delivered BTU, and it will get there in a fifth of the time. Buy the speed if you need the speed.
Best for Heating fast and occasionally, rather than holding a temperature
| Type | Natural gas or propane fired, digital controlThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Rating convention | Rated by BTU/hr INPUTA 400,000 BTU gas heater burns 400,000 BTU an hour and delivers roughly 82% of it. Do not compare that number directly against a heat pump's output rating. | Published standard |
| Cost per million BTU delivered | About $28.36 at our published assumptionsUS average residential natural gas at $24.09/Mcf (EIA, June 2026) and an assumed 82% thermal efficiency. | Our arithmetic |
- Fast — hours rather than most of a day on a full-size pool
- Works the same in cold air, unlike a heat pump
- Lower purchase price than a heat pump of equivalent real output
- Roughly two and a half times a heat pump's cost per delivered BTU
- Needs a gas line sized for it, which is often the real cost
- Propane instead of natural gas raises the running cost by about a quarter again
- The efficiency this specific unit achieves in service
- Heat exchanger life against your water chemistry
- Whether your existing gas supply can feed it without an upgrade
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Sun2Solar 1200 Series Crystal Clear Solar Cover
The value pick, and the one whose logic actually holds up. A clear cover transmits more sunlight into the body of the pool instead of absorbing it at the surface, and 12 mil is the point where lifespan stops being the limiting factor. Bob Vila measured this and the 16-mil blue at the same 4 to 5 degrees in an hour, so you are paying less for the same measured result and giving up some years of service life.
Best for Most pools, most budgets — the default recommendation
| Thickness | 12 milThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Color | Clear | Retail listing |
| Stated lifespan | 3 to 4 years for the 12-mil clear tierSwim University's published figure for 12-mil clear covers generally. | Retail listing |
| Bubble side | Bubbles face the water | Published standard |
4 to 5 degrees F in one hour
Claimed by Bob Vila, measured across all covers in their group. Conditions: One hour under 2pm sun on 89 to 92 degree F days. Source
- Clear transmits more light into the water instead of stopping it at the surface
- 12 mil is the sweet spot: real service life without 16-mil bulk
- Cheaper than the 16-mil tier with no measured heating penalty
- Shorter published lifespan tier than 16 mil
- A clear cover shows algae and debris more obviously, which some people dislike
- Whether clear genuinely outlasts or under-lasts blue in the same thickness
- Real-world tear resistance at a pool with rough edges
- How much the clear material clouds over a season of UV
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Hayward HeatPro Pool Heat Pump
If you are heating a pool with electricity, this is the shape of machine to do it with. A heat pump moves heat instead of making it, and at the assumptions we publish it delivers a million BTU into the water for about a fifth of what an electric resistance heater costs to deliver the same. It is slower than gas and it fades as the air gets colder, which is exactly when you want it — that trade is the whole decision.
Best for Lowest running cost per degree when you want real temperature control
| Type | Air-source heat pumpThe manufacturer publishes no retrievable specification sheet; figure is as listed at retail. | Retail listing |
|---|---|---|
| Rating convention | Rated by BTU/hr OUTPUTThis is the trap in the category. Gas heaters are rated by fuel INPUT, heat pumps by heat OUTPUT. A 140,000 BTU heat pump and a 140,000 BTU gas heater do not deliver the same heat. | Published standard |
| Cost per million BTU delivered | About $10.75 at our published assumptionsUS average residential electricity at 18.34 cents/kWh (EIA, June 2026) and an assumed COP of 5.0. Both are substitutable on our running-cost calculator. | Our arithmetic |
- Cheapest running cost per delivered BTU of any controllable heater
- No gas line, no combustion, no flue
- Holds a set temperature rather than depending on the weather
- Slow — expect most of a day to move a full-size pool ten degrees
- Efficiency falls with air temperature, so it is weakest in the shoulder season
- High purchase price against a gas heater of comparable output
- The COP this specific unit achieves at your conditions
- Compressor and control-board longevity
- Noise at the distance your deck actually sits from it
These need a unit in hand across a season. We do not have one, so we do not have an opinion worth printing on them.
#ad We earn a commission if you buy through this link, at no extra cost to you. It never changes the ranking. How this site is funded. Prices last checked September 2, 2026.
Questions people actually ask
How long does it take to heat a pool?
Does turning the thermostat up higher heat the pool faster?
Should I leave the cover on while the heater runs?
- Average retail price of electricity to US residential customers: 18.34 cents/kWh, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential natural gas price: $24.09 per thousand cubic feet, June 2026U.S. Energy Information Administration · read 2026-09-02
- US residential propane price: $2.674 per gallon, week ending 30 March 2026U.S. Energy Information Administration · read 2026-09-02
- Energy content conversion factors: 3,412 BTU per kWh, 1,036 BTU per cubic foot of natural gas, 91,452 BTU per gallon of propaneU.S. Energy Information Administration · read 2026-09-02
- U.S. Department of Energy pool-energy figures (evaporation share, BTU per pound evaporated, cover savings, recommended temperatures, wind effect)Recreonics, reproducing and attributing the U.S. Department of Energy · read 2026-09-02
- Pool heater roundup: the 8.33 BTU per gallon per °F figure, attributed on the page to Alicia Toedter of Leslie'sBob Vila · read 2026-09-02
Every URL above was fetched and returned content on the date shown. Where a figure is ours rather than somebody else’s, the arithmetic is on the page and on the methodology page. If you find an error in it, we would genuinely like to know.
Read next
How long does it take to heat a pool?
The heating-time formula with worked examples across pool sizes and heater types, and the input-versus-output rating difference that makes listings misleading.
What size pool heater do I need?
How to size a heater from your pool's actual energy requirement, the input-versus-output rating trap, and why the answer gets smaller once you own a cover.
Heat pump vs gas pool heater
The two options priced per delivered BTU and timed on the same pool, with the one question that decides which trade suits you.
Pool heating time calculator
Enter your gallons, the rise you want and your heater's rating. Handles the gas-versus-heat-pump rating trap and prints the whole calculation.
How to heat a pool
The complete walkthrough: what your pool actually needs in BTU, every heating option priced per delivered million BTU, and the order to try them in.