How well does an Air Source heat pump work on cold days?

Answer by Dale Blundell (Atkins)

A good ASHP can still extract energy from the air at temperatures as low as -20°C.  However, the COP of an air source heat pump decreases with falling ambient air temperature, as this increases the temperature lift of the heat pump.  This means that on a cold day the efficiency will decrease, limiting the output power.  It is normal practice to use an electrical resistive heater to boost output on these occasions. 

Ground or water source heat pumps do not suffer from such a dramatic change in performance over the year, so the need for booster heaters is reduced.

Heat pumps consume a large amount of power, and will often represent the largest single electrical load of a house.  For low densities of heat pump uptake the electrical network will only be modestly impacted.  But where there is a high penetration of heat pumps, then on a cold day the low diversity of this combined load might overload the local distribution network.

To help alleviate this problem, thermal storage can be used to reduce peaks. This may take the form of high thermal mass screed used in underfloor heating, or hot water storage. Controls can also be set to start space heating earlier, which is possible in houses with high insulation, low draughts, and higher thermal mass. 

To avoid freezing of the working fluid at very low temperatures, it is important that the correct concentration of glycol is maintained.  This will be checked during a professional service. On extremely cold nights it is possible that where the glycol is too low, the working fluid will freeze.  Setting a minimum fallback temperature is good practice with all heating systems, and will avoid this problem.