Pool Heat Pump: Selection and Sizing
A pool heat pump is only as economical as its sizing. By considering heating output, cover, efficiency and siting together from the outset, you lower operating costs and avoid expensive purchasing mistakes.
Heating Output by Pool Volume and Target Season
The heating output required depends not on pool volume alone, but on the interplay of water quantity, target temperature, season length, location and heat losses. The longer the season, the higher the desired temperature and the more wind-exposed the pool, the more output is needed. For a typical outdoor pool with a target temperature of around 28 °C in Central Europe, the following reference values provide an initial guide.
| Pool volume | Core season June–August (covered) | Extended season May–September (covered) | Without a cover |
|---|---|---|---|
| up to 20 m³ | 5–7 kW | 7–9 kW | one to two output classes higher |
| 20–35 m³ | 7–9 kW | 9–12 kW | one to two output classes higher |
| 35–50 m³ | 9–12 kW | 12–15 kW | size significantly larger |
| 50–70 m³ | 12–15 kW | 15–18 kW | size significantly larger |
| over 70 m³ | individual sizing by a specialist company | ||
These figures are reference values for covered pools and do not replace a project-specific calculation. Wind exposure, pool depth, the desired heat-up time after winter and the circulation output actually available should all feed into the final sizing. If you want to return to temperature quickly after cool spells, choose the upper end of the respective range.
The Cover: The Strongest Lever in Sizing
The greatest heat loss from an outdoor pool occurs not at the pool floor but at the water surface – above all through evaporation, supplemented by radiation and convection in wind. A cover reduces these losses considerably and is therefore the single most economical measure of all. In practice, a consistently used cover can almost halve the heating-output demand.
For sizing, this means: plan the cover before the heat pump, not as a later accessory. If you calculate without a cover, you have to size noticeably larger, pay more for purchase and electricity, and still often fail to hold the target temperature permanently on cool nights. A solar cover and a slatted cover lie directly on the water surface and suppress evaporation most effectively; an enclosure with an air gap above the water, by contrast, slows evaporation somewhat less. The three options also differ markedly in comfort, handling and cost.
COP, Operating Limit and Bivalence Point
The COP (Coefficient of Performance) indicates how much heat the heat pump delivers per kilowatt-hour of electricity used. A COP of 5 means: 1 kWh of electricity becomes around 5 kWh of heat. Under favourable conditions – warm air, a moderate target temperature, high humidity – pool heat pumps achieve high COP values; with cool air and a high target temperature the value drops considerably.
When comparing data sheets, one point is decisive: COP figures apply only under the stated test conditions (such as air and water temperature, humidity). Two units can be compared fairly only if these conditions are identical. A high laboratory value in warm air says little about operation on a cool May evening.
As the outside temperature falls, both the output delivered and the COP decline. Below a device-dependent lower operating limit, the heat pump works uneconomically or shuts off. The term bivalence point, which comes from building heating, strictly speaking denotes the switchover point to a second heat generator and applies only to a limited extent to a monovalent pool heat pump; for the pool, the lower operating limit is the more apt term. For peak season alone this rarely matters. If you want to extend the season into the early- and late-season periods, you should look for a low lower operating point and a sufficient output reserve, so that the pool can still reheat even in cooler air.
Full-Inverter or On/Off Technology?
The most important technical difference lies in output control. On/Off units run at a fixed output and switch off when the target temperature is reached. Full-inverter units vary the speed of the compressor and fan steplessly and adapt the output to demand.
| Feature | On/Off | Full-Inverter |
|---|---|---|
| Output control | fixed, on/off operation | steplessly modulated |
| Part-load behaviour | many switching cycles | long, quiet continuous running |
| Efficiency (annual average) | lower | higher, as mostly in the favourable part-load range |
| Noise | louder, audible start cycles | usually quieter, silent mode common |
| Purchase cost | lower | higher |
For pools with a high water volume, long running times and a desire for low operating costs, full-inverter technology is generally the better choice: it runs more steadily, works more efficiently in the part-load range and is quieter – an advantage that pays off over a long running time per season. On/Off units remain an option on a tight budget and for short use.
Siting, Airflow and Noise
A pool heat pump extracts heat from the ambient air and therefore needs free, unobstructed air intake. Maintain the minimum clearances to walls, hedges and other obstacles specified by the manufacturer, and prevent the cooled exhaust air from being drawn in again (air short-circuit). Also provide a condensate drain, because condensation forms at the evaporator during operation.
- Do not install in enclosed spaces without defined airflow.
- Do not place directly beneath bedroom windows or on the boundary with a neighbour.
- Set on a firm, level and vibration-decoupled base.
For noise, it is worth looking at the stated sound power level in dB(A) – and being aware that manufacturers sometimes quote the quieter sound pressure at a distance of several metres. A silent or night mode lowers the level during periods when quiet is needed. For the permissible noise level at the neighbour’s boundary, ambient noise limits apply that differ according to the type of area and the time of day; when in doubt, clarify the boundary clearance and siting with a specialist company and the responsible authority.
Integration into the Pipework and Frost Protection
The heat pump is connected into the filter circuit after the filter and before disinfection, usually via a bypass. Three ball valves – inlet, outlet and a bypass valve – make it possible to regulate the flow through the heat pump and maintain the minimum throughput stated on the data sheet. On the pressure side, depending on the system, d50 (DN40) or d63 (DN50) is used; the unit’s connection is often d50, so a transition has to be planned in.
Fit removable unions at the heat pump’s connections so that the unit can be dismantled for maintenance and winter without cutting. Before the first frost, the heat pump must be free of water: shut off the bypass, drain the unit via the unions and blow out the residual water in accordance with the manufacturer’s specification. The actual bypass installation – solvent cementing, insertion depth, curing time – is covered in detail by the pipework guide.
Bill of materials
Example – please verify with a specialist. Parts and quantities depend on the installation; sizes shown are examples.
| Component | Size | Art. No. | KLN No. | Qty |
|---|---|---|---|---|
| 2-way ball valve M1, solvent socket (PVC-U) | d63 | 128815A | 51511560 | 3 |
| Union Ocean, solvent socket / solvent socket | d50 | 170397 | 51512461 | 2 |
| Reducing bush (PVC-U) | d63 | 2 | ||
| 90° Elbow, solvent socket (PVC-U) | d63 | 70507 | 51513083 | 2 |
| PVC-U pressure pipe with socket | d63 | 76911 | 51515805 | as needed |
| Griffon UNI-100 PVC-U adhesive | – | 20023 | 51512566 | 1 |
| Griffon cleaner / degreaser | – | 1 | ||
| Heat pump (full-inverter recommended) | – | – | – | Select by the kW requirement from the sizing table; main component outside the catalog without a KLN number |
| Pool cover / slatted cover | – | – | – | Often halves the heating output required and should be planned in before the heat pump |
FAQ
How many kW does my heat pump need for a 40 m³ pool?
For a covered 40 m³ pool with a target temperature of around 28 °C, you are looking at roughly 9–12 kW in the core season and rather 12–15 kW for the extended season. Without a cover, you should size significantly larger.
Is a full-inverter unit worth it compared to On/Off?
With long running times per season, yes: a full-inverter unit runs more efficiently in the part-load range, more quietly and with fewer switching cycles. On/Off is only worth considering on a tight budget and for short use.
Can the heat pump run in winter?
It is designed for the swimming season, not for winter operation. Below its operating limit it works uneconomically; before frost the unit must be drained and taken out of service.
How loud is a pool heat pump?
That depends on the model; look at the sound power level in dB(A) and a silent mode. Compare only values that refer to the same measurement quantity and the same distance.
Where is the heat pump connected into the circuit?
In the filter circuit, after the filter and before disinfection, usually via a bypass with three ball valves for flow control.
This guide provides general technical orientation and does not replace professional planning. Safety-, electrical- and permit-relevant work must be carried out by qualified specialists.
