Air source heat pumps work by using the outside heat in the air to warm your home, but what happens when the temperature starts to drop or even go into minus figures? Find out how an air pump can continue to do its job even when the thermometer is on the way down in winter.

You may be reluctant to consider an air source heat pump as a viable alternative to your fossil fuel burning system. A common misconception is that since air source heat pumps draw heat from the air, they will cease to be of use once the external temperatures fall to low levels in the winter months.

Thankfully, air source heat pumps can continue to draw ambient heat, even with temperatures as low as -20 °C. Admittedly, they will be required to work much harder, and therefore require more electricity, but air source heat pumps will still function well in the colder months.

How does an air source heat pump work in winter?

So how does an air source heat pump work in winter? Well, largely they work in the same way as during the rest of the year, they just need to work that bit harder.
Heat pumps work like an air conditioner in reverse by absorbing heat from the air outside and using it to heat the home. Even though it may feel cold outside in winter, there is still some heat in the air that can be used to feed an air source heat pump system.

Heat is pulled by the rotating external fan from the surrounding air into a heat exchanger coil which contains a refrigerant fluid. The refrigerant fluid boils and evaporates, and becomes a gas which is then compressed to increase the temperature further. This heated gas can then be transferred into your central heating system.
Refrigerant is able to absorb heat at very low temperatures, including most of those that are reached in the UK during the coldest months of the year, when even in the most extreme locations, it very rarely goes below -10 °C.

Working harder in winter

Heat pumps require electricity to operate, but usually in small quantities that makes them a far more economical option than a gas or other fossil fuelled boilers. When temperatures begin to fall, more electricity will be required to extract heat from the air.

This additional electricity required will affect efficiency and mean that your heat pump won’t be as efficient in winter as it would be in the milder months. That said, a heat pump will still be likely to be more economical over the course of the year compared with a gas or oil boiler.

Efficiency of a heat pump is measured using Coefficient of Performance (CoP) which is calculated by comparing the amount of input to the amount of output. The higher the CoP, the better.

Some examples of CoP performance for air source heat pumps includes:

• Air source heat pump using 1 kW of electricity to produce 3 kW of heat  has a CoP is 3.
• Typically, within a temperature range of −3 °C to 10 °C, the CoP is around 3–3.5.
• In very mild weather, the COP of an air source heat pump can be up to 4.
• If the temperature drops to -8°C, CoP could drop to around 2.4.

It is highly unlikely that the temperature in the UK would drop so low that the heat pump would not produce any heat.

Keeping your air source heat pump working in winter

A properly maintained heat pump should continue to work effectively throughout the winter months. In order to remain efficient and running safely, it should be serviced annually.

Additionally, you should make sure that the airflow of your heat pump is clear and free of any debris which could impact on effectiveness and efficiency. Any maintenance is best carried out by a qualified heating engineer.

In some circumstances, an air source heat pump may be required to go into defrost settings. In cold weather, the piping and pump may become frozen, in which case your air source heat pump will usually automatically send energy towards thawing the ice that has formed. If this process does not start automatically, there is usually a defrosting process that you can start yourself, so consult the manual. This allows air source heat pumps to do their work, even in the cold conditions of winter.

Choosing your air source heat pump

An air source heat pump should continue to work for you in winter, and provide for your heating needs, as long as you get the right one. In the UK, we don’t suffer the extremes of temperature as in some other countries, so a quality air source heat pump will ensure your home is heated throughout winter.

Air source heat pumps have lower initial installation costs than other heat pumps and will be the most economic or practical choice for most households. You can also get an air source heat pump grant as part of a UK Government backed scheme and this will reduce the initial price of the unit. In rare instances, it may be necessary to have a backup source of heating for when your air source heat pump starts to decline in efficiency. A hybrid heating system that switches between a heat pump and a traditional boiler can switch intelligently between the two heating systems depending on which would perform most efficiently at that time.