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The average air temperature throughout the UK is around 7°C. Air source heat pumps work by converting the solar energy stored in the surrounding air into useful heat. The heat is picked up out of the surrounding atmosphere and is transferred to either an air or water based heating system. Air is an inexhaustible source of energy and therefore a sustainable solution for the future.

Air source heat pumps look similar to a large fan. They draw in the surrounding air over the evaporator where the heat is extracted/used. With the heat removed, colder air is then vented away from the unit. An air source heat pump is slightly less efficient than a ground source mainly because of the fluctuating temperature in the atmosphere, compared to the more stable conditions in the ground. However, the installation of these units is less expensive. As with all heat pumps, air source models are most efficient at producing low temperatures for distribution systems such as underfloor heating.

Their efficiency is helped by a higher ambient temperature, however, an air source heat pump will also work in temperatures below 0°C and are capable of operating to temperatures as low as -20°C, although the colder the temperature the less efficient the heat pump becomes. The efficiency of an air source heat pump is rated as COP (Coefficient of Performance). The COP is calculated by dividing the useful heat output by the energy input which is usually rated at around 3.

This means for every 1kW of electrical input, 3kW of thermal output is achieved; essentially meaning the heat pump is 300% efficient. They are known to have a COP as high as 4 or 5, similar to a ground source heat pump but this often depends upon how the efficiency is being measured. COP’s with air source heat pumps are measured under standard conditions of a set air temperature to a set flow temperature. These are typically A2 or A7/W35 meaning that the COP has been calculated when the incoming air is 2°C or 7°C and the flow out to the heating system is 35°C (typical of a wet based underfloor system).Although air source heat pumps require a good volume of air flow across the heat exchanger they can be located indoors as well as outdoors.

Location of the outdoor units is fairly critical because they are quite large intrusive looking objects and they will make a little noise. They should, however, be located as close to the building as possible to limit the distance the ‘warm pipes’ have to travel. Air source heat pumps carry all the benefits of a ground source heat pump and although they are slightly less efficient, a major advantage of an air source heat pump over a ground source heat pump is that they are more suited to smaller properties or where ground space is limited. With this in mind general installation costs are less, with savings on collector pipes and excavation work associated with ground source heat pumps. Inverter driven air source heat pumps are now available which can ramp up output depending upon demand; this helps with efficiency and will eliminate the requirement of a buffer vessel. Please ask CA Heat Pumps for more details.

There are two designs of air source heat pumps, being either an air to water or air to air system. Air to water heat pumps work by converting the available energy in the surrounding air into heat. If the heat is then transferred to water the ‘heat energy’ can be used as a conventional heating system i.e. to heat underfloor or radiators and provide domestic hot water. Air to air source heat pumps operate in the same manner as air to water heat pumps but without being plumbed into a wet based heating system, they circulate warm air internally to provide a comfortable ambient temperature inside the home. Air to air heat pumps are more suited where space is extremely limited because their only requirement is an external wall making them ideal for apartments or smaller homes. These systems also offer an added benefit of cooling and air purification. These models of heat pumps can heat properties of up to 100m2.

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