What size heat pump does my pool need?
As a starting point, allow about 1 kW of heating output for every 3.5 to 4 m³ of water. That is the ratio in AstralPool's tables for keeping the water at 26 °C with the air at 15 °C and the pool under a thermal cover. Without a cover, on a windy site or if you want to swim in winter, you may need twice that or more. iStarTec sizes and installs pool heat pumps with AstralPool equipment, connected after the filter and before the salt chlorinator.
- Rule of thumb: about 1 kW per 3.5 to 4 m³ (water at 26 °C, air at 15 °C, with a cover)
- Spec sheet: compare output and COP at 15 °C air, not just at 24 or 27 °C
- Saltwater pool: titanium heat exchanger, with the salt cell after the heat pump
- Installation: outdoors, close to the plant room, with a three-valve bypass
- Cover: cuts heating energy by 50 to 70% (US Department of Energy)
- Electrics: the circuit to the pool control panel is run by the electrician on the job
Contents
How much longer you can swim
The Algarve gets plenty of sunny spring and autumn days with pleasant air, but pool water is slow to respond. Tens of cubic metres of water lose overnight the heat they picked up during the day. Without heating, the water only stays comfortable in the hottest months.
For most villas, the realistic goal is a longer season: swimming from spring and carrying on well into autumn. Heating an outdoor pool in the middle of winter is possible, but the output you need and the energy it uses go up a lot, and it is rarely worth it.
If you let the villa to holidaymakers, a heated pool is a strong selling point for bookings in April, May or October, months when a cold pool simply goes unused.
How to work out the output
Start with the volume: length × width × average depth. A 10 × 4 m pool with an average depth of 1.4 m holds 56 m³. Then use the table as a starting point.
| Pool volume | Starting output |
|---|---|
| 20 m³ | 5 to 6 kW |
| 30 m³ | 8 to 9 kW |
| 40 m³ | 10 to 12 kW |
| 50 m³ | 12 to 15 kW |
| 60 m³ | 15 to 17 kW |
| 80 m³ | 20 to 25 kW |
Output measured with the air at 15 °C, to keep the water at 26 °C with the pool under a thermal cover. Figures based on AstralPool's recommended-volume tables. They are a starting point, not a calculation.
The rule that comes out of the table is simple: about 1 kW of output for every 3.5 to 4 m³ of water. For the 12.50 × 5.50 m pool we quoted for in Albufeira, holding about 80 m³, the starting point would be a 20 to 25 kW unit.
Four things push the output you need upwards:
- No cover. Evaporation then takes most of the heat. You may need twice the output or more.
- Wind and shade. A pool exposed to the wind, or without direct sun, loses more heat.
- Target temperature. Holding 28 °C takes more than holding 26 °C.
- A longer season. The colder the air in the months you want to swim, the more output you need.
A unit that is too small runs all day and still does not reach temperature in April or October, which is exactly when you need it. One that is too big costs more and gives you nothing back. Inverter models, which vary the compressor speed, cope better with a bit of headroom: once the pool is warm, they just run slowly.
How to read the spec sheet
Pool heat pump spec sheets give the output and the efficiency (COP) under more than one set of conditions. The first figure shown is nearly always the most flattering one, with the air at 24 or 27 °C. On spring and autumn mornings in the Algarve, the air is well below that.
To compare units, use the figures measured with the air at 15 °C. In AstralPool's current catalogues, the COP at 15 °C ranges from about 3.7 to 5: the unit delivers 3.7 to 5 kWh of heat for every kWh of electricity it uses.
- Output at 15 °C: this is the figure that matters for a longer season.
- COP at 15 °C: the higher it is, the less electricity the unit uses.
- Inverter or on/off: an inverter is quieter and more efficient at part load; an on/off unit is simpler.
- Noise: compare dB(A) figures measured at the same distance (1 m or 10 m). The difference between the two is large.
- Titanium heat exchanger: essential in a saltwater pool.
Where to put the unit
A heat pump takes its heat from the air. It needs open space in front and behind, and it must not draw back in the cold air it has just blown out. So it should not be shut inside the plant room or tucked into a corner where the air has nowhere to go.
- Close to the plant room, so the pipe runs are short.
- Away from bedroom windows, yours and the neighbours'.
- With the cold air blowing away from the terrace and the sun loungers.
- On a firm, level base, with room around it for servicing.
- With a drain for the condensate. The unit produces a fair amount of water when it runs, and a lot of people mistake it for a leak.
- Near the sea, with protection against salt air, which attacks the metal parts of outdoor units.
How it connects to the pool circuit
The order of the equipment in the plant room matters:
- Filtration pump and filter. The water reaches the heat pump already filtered.
- Heat pump. It heats the filtered water.
- Salt cell or chlorine dosing, last in line, so concentrated chlorine never passes through the heat pump's exchanger.
- Back to the pool through the return inlets.
The heat pump is connected through a three-valve bypass. That lets you set how much water flows through the unit and isolate it for servicing without shutting the pool down. The heat pump only heats while the filtration pump is running, so the filtration schedule has to match the heating schedule. Some models control the filtration themselves.
The power supply to the pool control panel, on its own circuit with RCD protection, is done by the electrician on the job. From the panel onwards, the connection and commissioning are ours.
Building or renovating a pool?
We do all the pool plumbing and size the heat pump together with the pump and filter, using AstralPool equipment. Building the pool itself (excavation, shell, waterproofing and finishes) is the builder's job. We can also supply just the equipment, already sized, when another team is doing the installation.
A cover is worth more than extra output
The biggest heat loss from an outdoor pool is evaporation. According to the US Department of Energy, covering the pool when it is not in use is the most effective way to cut heating, reducing the energy used by 50 to 70%. A covered pool also loses less water and fewer chemicals.
In practice, a pool that is covered at night needs a smaller unit and uses far less energy to hold the same temperature. A bubble cover is the simplest option. Roll-up or automatic covers take more work to install, but they go on in a minute, and it is using the cover every day that makes the difference.
Using the heat pump day to day
- Heat during the warmest hours of the day. The warmer the air, the more efficiently the unit runs. If you have solar PV panels, make the most of the sunny hours.
- Cover the pool at night and whenever nobody is using it.
- Switch the unit on a few days before the season starts. Heating tens of cubic metres from cold takes days, not hours.
- Keep the temperature steady through the season. Letting the pool cool down and heating it up again means making up days of heat.
- In a holiday let, schedule the heating ahead of your guests' arrival.
- At the end of the season, close the bypass and follow the manufacturer's shutdown instructions.
Heat pump or solar for the pool?
Dedicated solar pool collectors heat the water well in summer, when it is already pleasant. In spring and autumn, the months when you actually need heating, they produce less and depend on the weather. A heat pump is more predictable: it gives you the temperature you set, sun or no sun.
The two can be combined. In a villa that will also have solar thermal or a heat pump for hot water, it is worth looking at everything together, with one team.
New pool or existing pool
On a new pool, the best time to decide is before the concrete is poured, when the pipework and the plant room are being laid out. Even if you only buy the heat pump later, we leave the connections and the bypass ready. Adding them afterwards means more work.
An existing pool can take a heat pump too. It needs an outdoor spot near the plant room, a power supply and a connection to the circuit after the filter. On the technical visit we check all three and tell you what output makes sense for your pool.