Heat pumps for swimming pools: a guide for business-owned pools
This guide explains heat pumps for swimming pools from the point of view of a business that owns one, covering how they work, how to size one, what they cost to run against gas and what to ask for in a quote.
How an air-source pool heat pump works
Air-source, water-to-refrigerant units are the common type of pool heat pump. A fan draws outdoor air across an evaporator coil, where the refrigerant absorbs heat. A compressor raises the refrigerant’s temperature and pressure, and the hot refrigerant passes through a heat exchanger, commonly titanium because of the water chemistry, where it gives its heat to pool water pumped through the unit. The refrigerant then expands and returns to the coil to repeat the cycle. The electricity drives the compressor, fan and controls; most of the heat delivered comes from the air.
The ratio of heat delivered to electricity used is the COP. It is not fixed. It rises when the outdoor air is mild and the water set-point is modest, and it falls when the air is cold or damp or when you ask for a higher water temperature. A unit that performs well in a warm spell can perform noticeably less well in a cold week. That is why a seasonal figure is a better basis for cost estimates than a single headline COP; the SCOP guide explains the difference.
Pool water is also a different duty from space heating. The set-point is far lower than a radiator flow temperature, which is helpful for efficiency, but the load is continuous because the water is constantly losing heat. Pool water chemistry also matters: ask the manufacturer which heat exchanger materials suit your water treatment.
Seasonal or year-round: the first decision
Whether you heat the pool for part of the year or all year changes both the machine and the economics.
- Seasonal outdoor pools (holiday parks, some hotels, school or club lidos) are heated for the warm months only. Air temperatures are mild during that period, so a heat pump spends most of its time in an efficient range. The running hours are fewer, so the saving or cost difference is smaller in absolute terms.
- Year-round indoor pools (hotel leisure suites, gyms, school and club pools, care and retirement settings) need heat in every season. A heat pump on the outdoor air must then cope with the coldest weeks, when COP is lowest and defrosting is needed. Some designs pair it with a boiler or other heat source for the coldest periods; see the hybrid retrofit page for how a retained boiler fits alongside a heat pump.
Whichever applies, decide the target water temperature first and write it down. Different uses (lane swimming, teaching, leisure, therapy) are run at different temperatures, and each degree you hold costs energy. Confirm the temperature appropriate to your pool and users with your pool operator or pool-water guidance (for example from the Pool Water Treatment Advisory Group) rather than taking a figure from this page.
Where a pool loses heat, and why a cover matters
A heat pump replaces heat the pool has lost, so understanding the losses is the start of sizing.
- Evaporation from the surface is usually the largest loss on an open pool, and it increases with warmer water, moving air and activity on the surface.
- Conduction through the pool walls, floor and pipework to the ground and structure.
- Radiation and convection from the surface to the air and, for outdoor pools, to the sky.
- Top-up and backwash water, because fresh water has to be heated from mains temperature.
A pool cover reduces evaporation and surface heat loss, so a cover that is actually used every closed hour reduces the load the heat pump has to meet. A cover that is available but left off saves nothing. This site has not published a percentage for the saving, because it depends on the pool, the cover and how it is used; ask the supplier to state their assumption and check it against your own operating hours. The evaporated moisture in an indoor hall is also a ventilation matter, which this guide does not cover.
Sizing logic for a business pool
Sizing a heat pump for a swimming pool balances two separate heat demands: holding the water at temperature against continuous losses, and heating up from cold within a time you can accept. The inputs are:
- Pool volume in cubic metres (length × width × average depth).
- Surface area, which drives evaporation and surface losses.
- Target water temperature and the coldest water you will start from.
- Heat-up time that you are willing to wait after a drain, a long closure or a season start.
- Cover use and whether the pool is indoors or outdoors.
- Design air temperature for the coldest weeks you intend to heat, because output falls as the air gets colder.
The physics behind the heat-up figure is exact: one cubic metre of water weighs 1,000 kg and has a specific heat of 4.18 kJ per kg per kelvin, which is 4,180 kJ, or 1.16 kWh, per cubic metre per degree.
Assumptions (illustrative, not measured): a rectangular pool 12 m × 6 m with an average depth of 1.2 m, heated from 20°C to 28°C, with a 48-hour heat-up target.
Volume = 12 × 6 × 1.2 = 86.4 m³. Surface area = 12 × 6 = 72 m².
Energy to raise the water 8 K = 86.4 × 1.16 × 8 = about 802 kWh of heat.
Spread over 48 hours that is 802 ÷ 48 = about 16.7 kW of heat, before any losses during the heat-up.
Now add an assumed continuous loss of 10 kW (a placeholder for evaporation, conduction and top-up water; the real figure depends on the pool, cover and air conditions and has to come from a proper calculation). Heat output needed during the heat-up = 16.7 + 10 = about 26.7 kW. Once the pool is up to temperature, only the loss (10 kW in this illustration) has to be replaced, so a longer heat-up time lets a smaller unit do the job.
The example shows the shape of the calculation, not a unit size. A real design uses calculated losses, the pool hall or site conditions and the manufacturer’s output at your design air temperature. Compare it with the general approach in the commercial heat pump sizing guide.
What goes wrong when a unit is the wrong size
Undersized. The unit runs continuously and still cannot reach the set-point in cold weather, so water temperature drifts down, or back-up electric heating fills the gap at much higher unit cost. Heat-up after a closure takes far longer than planned.
Oversized. The unit reaches the set-point quickly then switches on and off in short bursts (short-cycling), which wastes efficiency and wears components. It also costs more to buy.
Neither failure shows on a datasheet. Ask what design air temperature and water temperature the quoted output is based on, and how the unit behaves below that. Modular or multi-unit arrangements can let part of the capacity switch off at part load.
Running cost against gas: the break-even method
Cost per kWh of heat equals energy price divided by efficiency. For a gas boiler that is its seasonal efficiency; for a heat pump it is the SCOP. Setting them equal gives the break-even SCOP: electricity price divided by (gas price divided by boiler efficiency). The running-costs guide works through the method in more detail.
DESNZ Quarterly Energy Prices for Q1 2026 (provisional, published 30 June 2026, excluding VAT, including Climate Change Levy) give medium-sized businesses an average of 25.0p per kWh for electricity and 4.55p per kWh for gas. Assuming an 88% boiler efficiency, gas heat costs 4.55 ÷ 0.88 = about 5.17p per kWh of heat. Break-even SCOP = 25.0 ÷ 5.17 = about 4.8.
Take the 10 kW assumed loss from the sizing example, running for 30 days: 10 × 24 × 30 = 7,200 kWh of heat. Gas: 7,200 × 5.17p = about £372. Heat pump at an assumed SCOP of 4.0: 25.0 ÷ 4.0 = 6.25p per kWh of heat, so 7,200 × 6.25p = £450. At an assumed SCOP of 5.0: 25.0 ÷ 5.0 = 5.0p per kWh of heat, so 7,200 × 5.0p = £360.
On those inputs the heat pump costs more than gas below a SCOP of about 4.8 and slightly less above it. A pool’s low set-point helps SCOP compared with space heating, but cold-month operation pulls the seasonal average down. Your own tariff, a time-of-use electricity rate, on-site generation, or a gas price different from the DESNZ average all change the result, and a small site may be far from the DESNZ medium band. Enter your own prices in the running-cost calculator before drawing a conclusion.
Noise, siting and planning
An air-source unit has a fan and compressor outdoors. Next to a hotel bedroom wing, a care-home garden or a neighbour’s fence, noise may decide the location. For commercial premises the permitted-development rights for air-source heat pumps are written for dwellinghouses and blocks of flats, so check with your local planning authority, which may ask for a noise assessment to BS 4142. The noise and planning guide sets out the questions to ask. Also leave room for airflow, condensate drainage and service access, and check the electrical supply capacity early with your distribution network operator.
Funding: what applies to pool heating
The Boiler Upgrade Scheme in England and Wales is a grant to replace fossil-fuel heating systems in homes and in buildings used for business. Pool heating is not a space or hot water heating system for a building, and this site has not verified that BUS covers a heat pump used only to heat pool water. The GOV.UK scheme pages do not mention swimming pools either way, so ask your installer to confirm eligibility in writing before you rely on a grant. Read the BUS for business premises guide for the size limits and rules that do apply. Note that a heat pump already in receipt of another public grant is not BUS-eligible.
The Public Sector Decarbonisation Scheme closed to new applications in November 2024 and no further phase has been announced, and the Industrial Energy Transformation Fund has closed with no successor. See the grants and funding page and check GOV.UK for any new round. For the tax treatment of the spend, confirm with your accountant whether a pool-heating installation is classed as plant or as an integral feature of a building.
What to ask for in a quote
- The pool volume, surface area, target temperature, heat-up time and cover assumptions the quote is based on.
- Heat output and COP at your stated design air and water temperatures, and the output at the coldest air temperature you intend to heat in.
- The estimated seasonal efficiency and the assumptions behind it, not just a headline COP.
- The heat exchanger material and compatibility with your pool water treatment.
- Refrigerant type and who handles it, and the sound power or pressure level at a stated distance.
- What back-up or top-up heating is included, and how controls prioritise between sources.
- Metering of electricity input and heat output so you can check performance, and servicing requirements.
- Installer credentials, which you should check on the public registers such as mcscertified.com where relevant.
Send your details through the enquiry form and we reply by email. This site is not an installer.
Decision checklist
- Is the pool seasonal or year-round, and what is its target water temperature?
- Do you have a calculated heat-loss figure, and is a cover used every closed hour?
- Have you compared heat-up time against the unit size?
- Have you worked out the break-even SCOP with your own tariffs?
- Is the unit’s noise acceptable at the nearest sensitive boundary, and has planning been checked?
- Is electrical supply capacity confirmed?
- Is grant eligibility, if relied on, confirmed in writing?
- Are metering and service terms in the specification?