How Swimming Pool Heat Pumps Work

The unit draws in outdoor air using a fan. That air passes over an evaporator coil containing refrigerant gas at a very low temperature. Even when outside air feels cool to the touch, it still contains usable heat energy. The refrigerant absorbs this heat and evaporates into a gas.

The compressor then compresses that refrigerant gas, which causes its temperature to rise considerably. This hot, pressurised gas passes through a heat exchanger, where pool water circulates around it. Heat transfers from the refrigerant to the water. The refrigerant cools back down, returns to a liquid, and the cycle begins again.

The efficiency of a heat pump is expressed as its COP (Coefficient of Performance). A COP of 5 means the unit delivers 5 kW of heat for every 1 kW of electricity it uses. Standard heat pumps typically achieve COPs of between 5 and 7. Modern inverter models can reach up to 14 when running at reduced load.

In practical terms, a heat pump costs roughly half the amount of natural gas, a third of the cost of oil or propane, and a fifth of the cost of electric resistance heating to achieve the same result.

Older heat pumps operate on a simple on/off basis: they run at full power until the target temperature is reached, then stop. Inverter-driven heat pumps can vary the speed of the fan and compressor continuously. This means they can modulate their output to match exactly what the pool needs at any given moment.

Once the pool has reached its target temperature, an inverter unit reduces its output to around 25–50% of maximum. Running at this lower output is where the highest COPs are achieved — the unit consumes very little electricity but maintains a perfectly steady water temperature.

Inverter units also operate more quietly, run with less vibration, and have fewer abrupt on/off cycles, which reduces wear on the compressor over time.

The component that transfers heat from the refrigerant to the pool water is the heat exchanger. In quality heat pumps, this is made from titanium, which is highly resistant to the corrosive effects of chlorinated pool water. A titanium heat exchanger will last considerably longer than one made from copper or standard steel — so it is an important feature to check when comparing models.

All air-source heat pumps extract heat from the surrounding air. As the air temperature falls, there is less heat available to extract, so the output of the heat pump falls too. This is why correct sizing matters. A heat pump chosen based on warm summer conditions may not cope adequately on cool spring or autumn days.

When sizing a heat pump, we take into account the lowest air temperature the unit will be expected to operate in, and recommend a model whose output at that temperature is still sufficient to heat the pool. If you want to swim from April through October in the UK, the unit needs to be able to perform adequately at 8–10°C air temperature — not just in July.