Pool heating basics: heat pumps, solar and season length

Two different jobs: heating up and staying warm

Start with the physics; it makes every later decision easy. It takes 1.16 kWh of heat to raise one cubic metre of water by one degree. An 8 × 4 m pool at 1.2–1.6 m holds roughly 45 m³, so taking it from a chilly 18 °C to a comfortable 27 °C needs about 470 kWh. That is the spring heat-up: a one-off, and not the expensive part of the year.

The expensive part is staying warm. Every day the pool gives heat back — mostly through evaporation, the rest through cold night air and wind — and the heater exists to replace it. Over a season an uncovered pool can spend more energy standing still than it ever spent warming up. This is why an honest heating article spends its first page on covers.

Two consequences follow. A heater is sized to keep up, not to sprint — the first heat-up of the year takes a day or more, whatever you buy. And the cheapest degree is the one you never lost.

The heat pump is the default answer

An air-source pool heat pump does not make heat, it moves it: energy out of the outside air, into the water, at four to six times the efficiency of a direct electric heater. In a temperate climate that has made it the standard for private pools, and it is the only common heater whose running cost does not make you ration the season.

Sizing rule of thumb: 0.3–0.4 kW of heating capacity per cubic metre of covered water. For that 45 m³ family pool the rule lands at 14–18 kW, and a 12 kW machine still works if you always cover and are patient in April. Uncovered, or if you want it warm the day after you decide, take the top of the range.

Read the rating conditions before you compare prices. Headline outputs are quoted at warm air — A27/W26 on the datasheet — and the same machine gives far less at 10 °C, which is precisely April and October. Ask for the output at A15, and prefer an inverter unit: it modulates instead of cycling, runs quieter and stays efficient at part load.

Where it goes, and how loud it is

A heat pump is a large fan in a box that blows cold air. It needs free air at the intake and the discharge — usually 30–50 cm to the sides and a clear metre in front — and it must never be shut into a tight cupboard where it re-breathes its own exhaust. Efficiency falls immediately when it does.

Noise is what people underestimate. Expect 50–60 dB at a metre, for hours at a time, on the evenings you are outside. Not under a bedroom window — yours or the neighbour's — and not beside the loungers. A few extra metres of pipe are cheap; moving the machine later is not. It also only fires when water flows through it, so the filter pump has to run whenever you want heat.

In a temperate climate a cover buys more degrees than the next size of heat pump — and it buys them every night, for free.

Run it as a thermostat, not as a kettle

Heat pumps are slow by design. Our 45 m³ pool needs about 52 kWh per degree, and a 14 kW machine delivers that in roughly four hours. Nine degrees is therefore a day and a half of continuous running — fine once in May, miserable every Friday.

So hold a setpoint rather than chase one. Choose 26–27 °C, cover the pool every night, and let the machine replace what the night took. The weekend strategy is tempting — a cool pool does lose less heat than a warm one — but no domestic machine delivers nine degrees on a Friday evening. What works is a small drop: let a covered pool fall two or three degrees mid-week and pull it back on Thursday.

One setting is worth more than the brand: if you have solar panels, run the heat pump at midday. A covered pool heated on the house's own electricity between eleven and four is about as cheap as heated water gets.

Temperature is a budget dial

Every degree costs, and the cost is not linear in pleasure. 24 °C is sporty and fine for lengths; 26–28 °C is the family band where children stay in the water instead of on the towel; 30 °C is a resort experience with a resort bill. As a rule of thumb each extra degree adds roughly a tenth to the heating energy, because losses grow with the gap between water and air.

The high end has costs that are not financial. Chlorine burns off faster, algae wake up, and the pool stops being refreshing on the hot day you built it for. If somebody needs warm water for medical reasons, design for it from the start — it changes the size of the machine, not just the number on the dial.

Solar: free heat with footnotes

Solar absorber mats and panels are the cheapest heat there is once they are on the roof. The pump pushes pool water through black tubing, and a controller diverts flow whenever the collector is warmer than the pool. Plan on 50–100% of the pool's surface area in collector, roughly south-facing and unshaded, plus a pump with enough head to get water up there.

The footnote is timing. Solar delivers when the sun shines, which is exactly when the pool needs it least, and it cannot promise a temperature — a cloudy week is a cooling week. It is excellent as a season-stretcher in April and September and as a way to cut the July bill. It replaces the heat pump only if your climate is generous and your expectations are flexible.

The largest solar collector you own is the pool itself. An unshaded position, a dark interior finish and a windbreak on the prevailing side are worth real degrees and cost nothing to run. All three are design decisions, and none is cheap to retrofit.

What actually buys you a season

In a temperate European garden an unheated, uncovered pool is genuinely comfortable for about six weeks around midsummer. Add a cover and it becomes eight to ten. Add a properly sized heat pump used with the cover, and May to late September is realistic — five months, most of it at 26–27 °C.

The order matters more than the budget. The cover removes the loss, the heat pump replaces what is left, and solar discounts the bill. Money spent out of that order buys less season per euro: a heat pump on an uncovered pool is a machine heating the sky.

The step beyond is an enclosure or a low roof: a different kind of purchase that buys shoulder months, keeps the water clean and cuts the bill again. Worth pricing if swimming in April is a real ambition.

Other routes, and what the builder will ask

A boiler heat exchanger — gas, oil or pellet — is fast, and makes sense if the boiler already exists and you want the pool warm by tomorrow. Its running cost makes it a tool for short bursts, not a season-holder. Direct electric resistance heating is only sensible for a plunge pool or a spa, where the volume is small. Ground-source or district heat are fine if they already exist with spare capacity.

Whatever you choose, decide before the plant room is built. The builder will want the volume, the target temperature, whether there is a cover, the electrical supply (larger units want three phase), the pipe run to the pool, and a spot where the machine will annoy nobody. Open your size in the designer and you have the volume and water surface to hand over — those two numbers drive every other line of the quote.

What goes wrong in practice

1. Sizing from the headline kilowatts

   That number is measured in warm air. In April the same machine delivers a fraction of it, which is when you actually needed it. Compare outputs quoted at 10–15 °C.

2. The heat pump on the terrace

   Fifty to sixty decibels for six hours, two metres from the loungers or under a bedroom window, is a mistake you live with for a decade. Choose the spot before the model.

3. Heating an uncovered pool

   Roughly double the energy for the same temperature, and the shoulder season still slips away. The cover is not an accessory to the heater; it is half of it.

4. Weekend blasting

   Expecting nine degrees by Friday night from a machine that gives one degree every four hours. Hold a temperature all week, or accept cold water.

5. Chasing 30 °C

   Steeply more expensive, harder on chlorine, friendlier to algae — and the water stops being refreshing on exactly the day you wanted it.