Air Source Heat Pumps

Although an air source heat pump replaces a boiler as the home’s heat source, the way it works is very different:

• An air source heat pump does not create heat. It simply moves it from one place to another through the vapour compression cycle (or refrigeration process) to make it more useable. Heat from the air gets absorbed into a fluid, which is then compressed, raising its temperature. The higher temperature is then transferred into the heating system
• Electricity is needed to power the pump. The efficiency, or the measure of the heat energy output per kW of electricity, is stated as the SCOP (Seasonal Coefficient of Performance). For example, a SCOP of 3.2 means that for every 1kW of electricity, 3.2kW is generated
• The air source heat pump gets its energy from the surrounding air, so as the air temperature drops, so does the efficiency. The bigger the difference between the outside air and the target temperature (either the indoor room temperature or domestic hot water), the lower the efficiency. It is therefore key to understand the heat load of the property and the performance characteristics of the heat pump
• For space heating, air source heat pumps work best with underfloor heating, but low-flow temperature radiators will work as well.

One of the first decisions to make when buying an air source heat pump is whether it will provide space heating or domestic hot water — or both.

The key here is that the ‘flow temperature’ (the temperature of the water in the heating system) is different:

• Space heating will usually require a flow temperature of around 35°C to 45°C for underfloor heating or low-temperature radiators
• Domestic hot water will, however, require a minimum flow temperature of 55°C.

On new homes that meet recent and current Building Regulations most air source heat pumps can do both, but this is not always the case.

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