Most people research pool heating backwards. They pick a heat pump because it is the option everyone mentions first, then find out later that a solar cover would have cut their bill more than any heater upgrade. The honest starting point for how to heat a swimming pool is not which appliance to buy. It is where your heat is actually going before you buy anything.
A typical uncovered outdoor pool loses most of its heat through evaporation, not through the pool walls or the surrounding air. That single fact changes the order most guides give you. Retention comes first. Heat source comes second. Below is every method used in the UK: air source heat pumps, gas, oil, and solar thermal, plus the covers and insulation that decide whether any of them are worth running.
Before comparing heaters, it helps to know where the heat goes. On an uncovered pool, evaporation typically accounts for around 70% of heat loss, day and night, with radiation to the night sky and conduction through the pool shell and surrounding ground making up most of the rest. Wind makes evaporation worse; still air and a cover make it far more manageable.
This is why a cover alone, with no heater at all, can add 3-4°C to a pool's temperature and extend the swimming season without running any heating equipment. Retention is not a footnote to heating. For most UK pools, it is the first and cheapest heating decision you make.
A pool heat pump does not generate heat directly. It extracts heat from the surrounding air and transfers it into the pool water, the same principle as a fridge running in reverse. Because it is moving heat rather than creating it, a heat pump typically returns five to seven times more heat energy than the electricity it consumes in warm summer air, which is why running costs sit well below gas or oil for most of the season. That ratio drops as the air temperature falls, so the same unit is markedly less efficient heating a pool in April than in July.
The trade-off is speed and weather dependence. Heat pumps are slow by comparison: they warm a pool gradually, often over one to three days from cold, and they lose efficiency as the air temperature drops, which makes them a strong fit for extending a UK swimming season from late spring to early autumn rather than heating through winter. Sizing matters more than brand: an undersized unit run constantly will cost more and heat less effectively than a correctly sized one run for a few hours a day.
For most UK outdoor pools used from May to September, a heat pump paired with a cover is the combination that shows up most often in running-cost comparisons, precisely because the cover stops the heat pump fighting evaporation losses all day.
Two practical points get missed. First, an ASHP produces low-grade heat compared with a gas or oil boiler, so heat is lost along the pipe run between the unit and the pool. Site it as close to the pool as the property allows rather than tucking it round the side of the house to keep it out of sight. Second, check your electrical supply has the spare capacity before you commit: a domestic pool ASHP usually needs a dedicated circuit, and larger units can require three-phase power your consumer unit may not currently have.
Noise is worth a mention too. Older compressor-driven units could be intrusive next to a terrace or a neighbour's garden. Modern soft-start inverter models are considerably quieter and rarely cause a problem, but it's still worth checking the decibel rating before you buy, not after it's installed six feet from where you sit outside.
A ground source heat pump draws on buried ground loops that sit at a near-constant temperature year-round, rather than fluctuating with the air the way an ASHP does. That stability is a genuine advantage for a house that needs heating through the coldest months.
For a pool, it rarely pays off. Ground works, trenches or boreholes, push installation costs well above an equivalent ASHP, and a UK outdoor pool is mostly used during the months an ASHP is already running at its most efficient. We don't recommend ground source heat pumps for pool heating specifically: the extra cost buys you winter stability most pool owners won't use, since few people are swimming outdoors in December regardless of how warm the water is. Ground source makes more sense as part of whole-house heating, where the pool is a secondary load on a system sized for the house, not the other way round.
Gas pool heaters burn natural gas (mains gas) or LPG to heat water directly, and they are the fastest method on this list. A gas heater can typically raise pool temperature several degrees within hours rather than days, which makes it the right choice for a one-off event, a holiday let with short guest turnarounds, or any pool that needs to go from cold to swimmable on a fixed date.
That speed comes at a running cost. Because gas heating creates heat rather than moving it, it uses far more energy per degree of warming than a heat pump, and with current UK energy prices, that translates to meaningfully higher day-to-day running costs for continuous use. Gas suits pools that are heated occasionally and heated fast. It is a poor fit for keeping a pool at a constant temperature all season, where the running cost gap against a heat pump becomes significant.
Oil-fired pool heaters exist for one reason: properties without a mains gas connection. Rural pools that cannot get LPG delivered economically, or that already run oil-fired central heating, sometimes extend the same fuel supply to the pool. Performance is broadly similar to a gas heater, fast heat-up, high running cost per degree, with the added requirement of on-site fuel storage and delivery.
Oil is rarely the first choice for a new installation today. Of every method on this page, off-grid gas and oil are the most expensive to run, since both create heat by burning fuel rather than moving it the way a heat pump does. Oil is usually the practical answer for an existing off-grid property that already has the infrastructure in place, rather than something worth installing from scratch when gas or a heat pump are both available.
Solar thermal panels, sometimes called solar pool heating or just solar heating, circulate pool water through tubing exposed to direct sunlight, warming it before returning it to the pool. Once installed, running costs are close to zero, since the pump you already use for filtration is usually enough to circulate water through the panels. The catch is dependency on weather and daylight hours, which makes solar thermal a supplement to another heat source in the UK climate rather than a standalone system for consistent temperatures.
Don't confuse solar thermal panels with a solar cover, covered next. They're different products doing different jobs. Panels heat the water actively; a cover just stops the heat you already have from escaping. Plenty of pools use both, and mixing the two up is a mistake we see homeowners make constantly when they start researching this.
A solar or thermal cover, typically a bubble-film or foam layer floated directly on the water, works by reducing evaporation, which as covered above accounts for around 70% of heat loss on an uncovered pool. Used consistently, a cover alone can add 3-4°C to water temperature with no heater running at all. Left on overnight and during the day when the pool is not in use, a cover can noticeably reduce how much heating a pool needs to maintain the same temperature, and it does this at a fraction of the cost of any heater on this list.
A safety cover or hard automatic cover is a different thing entirely, built for child safety and debris control first, heat retention a distant second. If running cost is what you care about, using a solar or thermal cover consistently pays back faster than almost anything else on this page, and it's usually the step guides bury at the bottom instead of putting first.
Heat loss does not stop at the water surface. Uninsulated pipework running between the plant room and the pool, and an uninsulated plant room itself, both lose heat continuously, particularly on pools with a heat pump or boiler working to maintain temperature. Lagging exposed pipework and insulating plant room walls are inexpensive one-off jobs, usually completed during a build or retrofit, that reduce the ongoing load on whichever heat source you choose.
For an in-ground pool, insulation around the shell itself, particularly in exposed or elevated positions, reduces conductive heat loss into the surrounding ground and structure. Specify it before the shell goes in. Retrofitting it later is far harder and far more expensive, so raise it with your builder at the design stage, not after the build starts.
For indoor pools, this isn't optional. Since 2010, UK Building Regulations Part L require a new indoor pool basin to achieve a U-value no worse than 0.25 W/m²K, specifically to limit heat loss and running costs. Your builder should already be pricing this in, but it's worth confirming rather than assuming.
A high water table is a heat-loss factor that's easy to miss at the design stage. Groundwater sitting close to or against the pool shell draws heat away continuously, since it stays close to the local ground temperature rather than your pool's target temperature. If your site has a high water table, ask your builder how the insulation and drainage are specified to account for it, not just whether the shell will hold water.
| Method | Running Cost | Heat-Up Speed | Best For |
|---|---|---|---|
| Air source heat pump | Low | Slow (1–3 days) | Extending the UK season, May–Sept |
| Ground source heat pump | Low (once installed) | Slow, stable year-round | Whole-house heating with a pool as secondary load, not a dedicated pool investment |
| Gas | High | Fast (hours) | Occasional use, events, short-stay lets |
| Oil | High | Fast (hours) | Off-grid properties without mains gas |
| Solar thermal | Near zero | Slow, weather-dependent | Supplementing another heat source |
| Solar / thermal cover | Very low | N/A (retention, not heating) | Every pool, regardless of heat source |
For a full breakdown of what a heating system costs to install as part of a new build, including how heater choice interacts with pool size and filtration, see our swimming pool cost guide.
This is the question almost nobody raises before installation, and it is worth asking regardless of which heater you choose. Salt water pools generate chlorine through electrolysis, and the chloride in the water is well known in the pool industry, including guidance from the Pool Water Treatment Advisory Group (PWTAG), to be more corrosive to metal components than plain fresh water. Many heat pump manufacturers specify a titanium heat exchanger, or note adjusted warranty terms, specifically for salt-chlorinated pools, precisely because of this.
Origin Aqua's Mineral+Biome® system doesn't add salt to the water and doesn't produce chlorine at any stage. It works through a living microbial filtration process instead, so none of the corrosion risk above applies to a Mineral+Biome® pool the way it can to a salt system. We also supply and install air source heat pumps, so if you're specifying water treatment and heating together, whether for a new pool or a retrofit, we can size and quote the ASHP alongside Mineral+Biome® rather than you coordinating two separate suppliers.
Is a ground source heat pump worth it for a pool?
Usually not. Ground source heat pumps cost significantly more to install than air source, mainly due to the ground works, and a UK outdoor pool is mainly used during the months an ASHP is already running efficiently. Ground source makes more sense as part of whole-house heating than as a dedicated pool investment.
Origin Aqua is an independent pool water-quality advisor, not a pool builder. We supply and install air source heat pumps, but that doesn't bias which water treatment we'd recommend: chlorine, salt, or Mineral+Biome® is a separate decision from which heating system suits your pool, and we have no reason to link the two. If you are specifying a new pool or upgrading an existing one, water treatment is one of the decisions worth finalising before the build, alongside your heating choice, rather than after.
What's the cheapest way to heat a swimming pool in the UK?
A solar or thermal cover combined with an air source heat pump is typically the lowest running-cost combination for a UK outdoor pool. The cover cuts the roughly 70% of heat lost to evaporation, and the heat pump returns 5 to 7 units of heat per unit of electricity in summer, rather than burning fuel to generate heat the way gas or oil do.
How long does it take to heat a pool from cold?
A heat pump typically takes one to three days to raise water temperature from cold, depending on pool size and air temperature. Gas and oil heaters can achieve the same rise in a matter of hours, at a higher running cost.
Can you add heating to an existing unheated pool?
Yes. Heat pumps, gas heaters, and solar thermal panels can all be retrofitted to an existing pool without rebuilding it, provided there is suitable space and electrical or gas supply nearby. A cover can be added to any pool immediately, with no installation required.
Is it worth it to heat a pool?
For most UK outdoor pools, heating extends the usable season from a few warm weeks to several months, which is the main reason owners choose to heat at all. Whether the running cost is worth it depends heavily on whether a cover is used consistently, since that single factor changes the cost of any heating method significantly.
What is the black hose trick for heating a pool?
Running a length of black hose or pipe exposed to direct sun, with pool water passing through it before returning to the pool, uses solar heat absorbed by the black surface to warm the water slightly. It is a genuine low-cost technique, though the heat gain is modest compared with dedicated solar thermal panels or a cover.
Is it better to heat a pool at night or during the day?
Daytime, if you're using a heat pump. It's pulling heat from the air, so a warm afternoon costs less to run than a cold night. Put the cover on once the sun goes down and you keep most of what you paid to heat.
Have questions about how heating and water treatment work together on your pool? Get a free specification →