A pool heating heat pump warms pool water with heat taken from the outdoor air and holds it at the set temperature. Because pool water is usually wanted at 26–28 °C, the heat pump works across a small temperature difference here, which is to say under the conditions in which it is most efficient. The right capacity depends less on the pool's volume than on its surface area, on whether it is outdoor or indoor, on whether a cover is used and on the months in which the pool will be used.
The AD range has no dedicated pool model. We size pool heating separately for each project and build the solution, as needed, from the residential series, the W series commercial outdoor units or the Mega central system. Among the applications of heat pumps, the pool is one whose calculation needs the most detail; this article explains the logic behind it.
Where does a pool lose its heat?
The real cost of heating a pool is not warming the water once but holding that temperature every day. Heat escapes in four ways:
- Evaporation: Every kilogram of water that evaporates from the surface carries away about 2,400 kJ of heat. In outdoor pools this is usually the largest loss; wind and dry air speed up evaporation.
- Radiation and convection: The water surface gives off heat to the sky and to the air above it, especially on clear, cloudless nights.
- Conduction: The walls and floor of the pool pass heat to the surrounding ground; in uninsulated pools this share grows.
- Fresh water: Cold mains water added because of filter backwashing and evaporation also has to be heated.
Since evaporation and radiation happen at the surface, the main input to the capacity calculation is the pool's surface area. A shallow and a deep pool with the same surface need roughly the same amount of heat per day; the deep pool simply takes longer for the first heat-up.
Outdoor and indoor pools
With an outdoor pool the aim is usually to extend the season: opening the pool early in spring and using it until late autumn, for instance. Here the weather sets the heat demand, and on windy, cool nights the losses climb quickly. Capacity is chosen for the coolest month in which the pool will be used. A villa pool on the Mediterranean or Aegean coast needs noticeably less than the same pool in the Marmara region.
With an indoor pool the water is heated all year, but most of the work happens in the pool hall. The hall air is usually kept 1–2 °C warmer than the water to limit evaporation. The evaporated moisture has to be removed from the hall, otherwise condensation forms on the glazing and the ceiling. An indoor pool project therefore calculates water heating, hall heating and dehumidification together; the capacity assigned to the pool water is only part of the total.
The cover: the cheapest capacity
A cover that closes the surface while the pool is not in use stops evaporation almost completely and greatly reduces night-time losses. An outdoor pool with a cover can be heated by a noticeably smaller heat pump than an uncovered pool held at the same temperature. In an indoor pool the cover also lowers the moisture load of the hall.
That is why the cover is one of the first things we ask about when preparing a quote. If a cover is planned, capacity is chosen accordingly; a system sized for an uncovered pool ends up larger than necessary once a cover is used.
Water temperature and running hours
The target water temperature depends on what the pool is used for. These ranges are common:
- Sports and training pools: 25–28 °C
- Villa, hotel and family pools: 27–29 °C
- Children's pools and therapy pools: 30–34 °C
As the water gets warmer, the difference between the surface and the air grows and evaporation speeds up. Raising the target by a degree or two therefore increases the daily heat demand out of proportion; keeping the temperature at the value that is really needed is the easiest saving.
With an outdoor pool, running hours also affect efficiency. An air-source heat pump delivers the same heat for less electricity the milder the outdoor air is. Running the heat pump during the warmest hours of the day and cutting losses with the cover at night is the way to heat the same pool with lower consumption. In an indoor pool, the heat pump runs all year with a steadier load for the hall air and the pool water.
First heat-up and daily heat demand
Choosing capacity answers two questions: how many hours the pool will take to reach the set temperature at the start of the season, and how the daily loss will be covered during the season. The first question can be answered with a simple calculation.
Example: let us assume we want to heat an outdoor pool with a 10 × 5 m surface and an average depth of 1.4 m, that is about 70 m³ of water, from 16 °C to 27 °C. The heat required follows from Q = m · c · ΔT: 70,000 kg × 4.19 kJ/kg·K × 11 K ≈ 3,226,000 kJ, or about 900 kWh. A heat pump with 20 kW of heating capacity would deliver this heat in about 45 hours if there were no losses at all. Because the surface keeps losing heat during the heat-up, the real time is longer; a cover shortens it. If we take an average COP of 5 under these conditions, 900 kWh of heat costs about 180 kWh of electricity. We explain in detail what the COP depends on in our article on heat pump electricity consumption.
The second question, the daily loss, depends on the surface area, how exposed the pool is to wind, the cover and the night-time temperatures of the region. Through the season the heat pump covers this loss by running for part of the day; the long first heat-up happens only once or twice a year. Rather than enlarging the capacity just for the first heat-up, accepting that it takes a few days is often the more economical choice.
How is the pool circuit connected?
Because pool water is chlorinated or salty, it does not enter the heat pump's water circuit directly. A corrosion-resistant heat exchanger is placed in between, usually a plate type in titanium or in a stainless steel suited to pool water. The heat pump feeds one side of the exchanger and the pool's filter circuit the other; the warmed water returns to the pool after the filter.
One advantage of this arrangement is that the pool can work together with the home's heating system. If the heat pump already heats the house, the pool is added to the system as a separate circuit and the controls are set to give priority to the house. In that case capacity is calculated for the days on which the pool and the house call for heat at the same time.
Which product do we use?
- Villa pools: The climate series chosen for the house, for example the Vaha series in coastal regions, can heat the pool through an additional circuit. Once the pool load is added, the next capacity up or a separate unit may be needed; the calculation shows which.
- Hotel, estate and sports centre pools: W series outdoor units with 28–56 kW capacity run together and are scaled to the combined load of the pool and the building.
- Large facilities: If several pools, halls and buildings have to be served from one plant, the 200–1,200 kW Mega central system is sized for the project.
The Vaha and W series produce water up to 55 °C. The 26–28 °C a pool needs lies far below that limit, so the unit runs on the pool circuit without strain. You can browse every model on our heat pump products page.
What we need for a quote
To calculate the capacity for your pool, this information is enough: the pool's dimensions and depth, whether it is outdoor or indoor, whether a cover will be used, the required water temperature, the months of use, the province where the pool is and how exposed it is to wind, and whether the pool will work together with the home's heating system. Send it through our quote form and we will size your pool for the project and suggest the right product.
