Sizing an air source heat pump

An air source heat pump must be correctly sized to ensure that it can provide the required amount of heat output at the design conditions – typically when the outside temperature is at -3°C. To do this an installer must first accurately determine the heat loss from the property and then, using this information, correctly select the required size of heat pump based on the design outside air temperature.

The heat loss from the property must be determined from a full room by room heat loss calculation for the property concerned and not from:

*  The output of the previously used gas or oil fired boiler

*  An estimated heat requirement based on the floor area or building volume

*  The use of a simple heat loss calculator (e.g. Mears calculator)

*  The Energy saving Trust ‘Whole House Boiler Sizing’ simplified calculation method

The declared heat outputs for all ASHP’s are based on the average UK outside air temperature of 7°C. However, as mentioned above, a suitable ASHP has to be selected that will deliver the required heat output at the design outside air temperature (typically -3°C) when there is less heat energy in the air for the ASHP to draw from. At -3°C, the output of the ASHP will be reduced (typically to around 80% of the declared output). It is this lower heat output value that must be used when selecting the size of ASHP to be installed. The installer should ensure that any proposed ASHP system has been correctly sized and the unit selected in this way before proceeding with the installation.