Air Source Heat Pump

Air source heat pumps (ASHPs) extract heat from air which passes through a large heat exchanger known as an evaporator. As the name suggests, the evaporator changes the state of the refrigerant from liquid to gas and as it does so absorbs heat energy. When this gas is compressed it gets hot. The heated gas is passed through a second heat exchanger where heat can be taken away as hot air or as heated water.

Air source heat pumps will work at ambient temperatures as low as

-20 degrees although the efficiency and the maximum hot water temperature may be reduced below about -10 degrees.

These heat pumps have amazing efficiencies in warmer weather and so a viable and low cost alternative to other heat sources for swimming pool heating.

Air source units tend not to be very beautiful. There are important issues other then aesthitics which must be considered before purchasing. These include:

Efficiency at different temperatures. Often quoted Coefficient of Performances (COPs) are quoted at silly ambient temperatures like 30 degrees which we do not see often in the UK. Check the output capacity and efficiency through the entire operating range.

Noise level and frequency. Units with inverter drives, such as the Hitachi unit shown above has little low frequency noise but does have a distinctive variable pitch noise in operation. This is generally a good thing as the higher frequencies are much more easily absorbed by surroundings. Fixed speed compressors have a low freqyuency hum or drone which will easily be transmitted through building foundations and can be really annoying if precautions are not taken to acoustically separate the heat pump from the building.

Start current. When the compressor is required it can take a massive amount of power to start. This can cause lights to dim momentarilly and causes real headaches for the electricity supply companies who are required to keep supplies to minimum standards. An inverter drive compressor may require only 1 or 2 amps to start and is much more gentle both on the ears and on the power supply.

Size. An important aspect of the unit is the size of the air side heat exchanger. These are generally more efficient the bigger they are. This can result in some units looking quite massive at first sight. It is surprising how quickly they can get lost in even modest size gardens though, and of course they can always be painted to help them blend into the surroundings.


Site your heat pump as close to the house as you feel is reasonable as this reduces the amount of heat lost in the pipes. Do not mount it on a slab abbutting the house as this will transmit viration and noise into the house. Ensure that adequate provision is made for drainage and do not allow the condensate to drip over walkways as these will freeze in the winter and will present a serious hazard to pedestrians. Do not position the unit near sleeping areas. They tend to work hardest at night as that is when the heat load is highest so they oftwen run for long periods after dark.

Some ASHPs can be mounted inside a building and have extending ventilation pipes to take air to and from the outside. This reduces the visual impact but takes up quite a lot of space. These are not to be confused with Exhaust air heat pumps (EAHPs) which provide ventilation within the building and extract the heat from the air as it leaves.