How do air source heat pumps work in winter?

How do air source heat pumps work in winter?
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    Due to their very nature, air source heat pumps work most efficiently in warmer weather. However, they are still remarkably efficient in very cold weather and can provide more than enough heat energy for the average home’s central heating system and hot water requirements.

    An air source heat pump absorbs heat from the air through a refrigerant fluid and passes it through a heat exchanger, creating heat energy. A heat pump can comfortably produce heat energy in the winter months, even when the outdoor temperature drops to -18°C, and sometimes even as low as -25°C.

    While it has been known to fall below -25°C in the UK, it’s only happened a handful of times in the past 30 or so years. So you shouldn’t encounter any problems with your heat pump system, even in exceptionally cold weather. However, you should expect an increase in your energy bills during the winter, as your heat pump will have to work harder to produce heat energy.

    Here is Cinergi’s expert guide on how air source heat pumps work in the winter. For more air source heat pump information, please see our FAQs page. To book a consultation or a heat pump service, please get in touch.

    Are air source heat pumps any good in winter?

    Air source heat pumps absorb heat from the air, so logic says that if the temperature falls below zero it won’t work – but this is a common misconception. Even in cold air, there is still sufficient heat for the heat pump to absorb and convert into energy.

    Heat pump technology has moved on significantly in recent years and they work extremely well in cold weather thanks to modern compressors. The pump can adjust to changes in the outdoor temperature, meaning that it speeds up energy consumption on cold days and slows down on warmer days. This ensures maximum efficiency on average over the course of the year.

    It’s true that air source heat pumps need to work a lot harder in colder climates. But while you will lose efficiency in the winter, your heat pump will still be, on average, around three to four times more efficient than a traditional gas boiler.

    What temperature do air source heat pumps stop working?

    Leading brands of air source heat pumps can work when the temperature drops to -25°C; more than adequate for homes in the UK. Most have a defrost mode that turns off the fan and converts the evaporator into a condenser, in order to warm the refrigerant and defrost the build up of ice.

    It is unlikely that you will notice a drop in temperature when your unit is in defrost mode, as the process will only take a few minutes.

    What is the best way to run a heat pump in winter?

    The most efficient way to keep your home warm with an air source heat pump is by combining it with large radiators, or better still, underfloor heating. To run at maximum efficiency, your home also needs to be well insulated.

    If you live in a particularly cold part of the country, then it may well be worth considering a ground source heat pump instead of an air source heat pump. Even when the temperature drops below zero, the ground is still around 8-13°C, providing more than enough energy to heat an average home. However, ground source heat pumps do require a reasonably sized plot of land in which to lay the pipes or drill boreholes.

    A hybrid system is another option for properties with very large heat losses. Many of our customers opt for an air source heat pump combined with a gas boiler, giving them a reliable back up system. Today’s intelligent technology is capable of switching between the two systems depending on what is the most efficient at the time.

    How do I protect my heat pump in the winter?

    It’s not wise to directly cover a heat pump system, as it restricts the airflow and compromises efficiency. You also run the risk of mould developing. But there are a few steps you can take to preserve your heat pump’s efficiency in the winter months.

    • Ensure the unit is free of debris, such as leaves and excess snow. Ideally, there should be clearance area of around 2-foot around the unit – top and sides – to ensure the air can circulate.
    • It is actually counter-intuitive to install a shelter around an air source heat pump. Because they work by extracting heat from the surrounding air, restricting access to this air can cause the ambient temperature around the heat pump to drop which then decreases its efficiency.
    • As with all heating systems, avoid frozen pipes by insulating them if necessary and be vigilant to the signs. All heat pump systems installed by Cinergi will be protected against freezing either by dosing the system with glycol (antifreeze) or by installing dump valves on the primary pipework.
    • Book a service with a reputable expert such as Cinergi, to ensure your air source heat pump is in tip-top condition to see you through the winter.

    How can Cinergi help?

    The best way to ensure your air source heat pump is running at maximum efficiency is to book in a service before the next cold snap.

    Cinergi is a close-knit, family team of accredited engineers who live and breathe air source heat pumps. Call us today to book a service and for impartial advice on how to get the most out of your heat pump system this winter.

    Looking to install a heat pump? Cinergi is unique in that we are accredited to install gas, LPG and oil boilers, as well as air source heat pumps. As such, we are in a unique position to offer the wide range of heating solutions for your particular property and budget. Book a discovery call today and one our team will be in touch as soon as possible.


    Air source heat pumps are designed to work in extreme temperatures, even as low as -25°C, meaning that they are still more efficient than traditional gas boilers for most UK homes.

    Though a bit of snow and ice shouldn’t stop your heat pump from working, it’s a good idea to clear the area around it regularly to maintain air circulation and efficiency. If you find that the compressor is frozen inside, it indicates a larger problem and you should call an engineer for help.


    How efficient are air source heat pumps in winter?

    Heat pump efficiency is measured in terms of COP (Coefficiency of Performance), ie. how much heat is generated per unit of electricity. So the higher the COP, the more efficient it is. On average, you can expect an air source heat pump to have a COP of 3.5.

    This may drop to as low as 2.5 COP in the winter, as the pump will be required to use more electricity to generate heat. It is, therefore, more useful to consider a heat pump’s Seasonal Coefficient of Performance (SCoP), which gives giving you an average value over the whole year.

    Should I turn my heat pump off in extreme cold?

    The temperature in the UK has only fallen below -25°C on three occasions in recorded history. So, given that air source heat pumps are effective in cold weather and temperatures as low as -25°C, you shouldn’t need to turn it off in the winter.

    Do heat pumps make the air dry in the house in winter?

    This depends on what air source heat pump you have; air to water heat pumps do not affect the internal environment at all (the humidity will be the same as running with your current heating system). Air to air heat pumps can reduce the humidity in the air as they work very similarly to air conditioning, however, air to air heat pumps are very rare.

    Do air source heat pumps pumps freeze up in winter?

    Air and ground source heat pumps are located outside and therefore exposed to the elements. It’s not a cause for concern if the exterior casing is covered in snow or ice, but if the compressor inside is frozen that indicates all is not well and you should call your supplier.

    All systems installed by Cinergi are fitted with a defrost mode which will kick in as necessary, but can be triggered manually.

    Should I cover my heat pump in winter?

    It’s natural for heat pumps to become covered in ice and snow in the winter and you shouldn’t cover them up. If you do, you will block the airflow and lose efficiency. You also run the risk of the unit becoming mouldy.

    Further reading on air source heat pumps:

    Picture of Dan Loveridge
    Dan Loveridge

    Dan is the managing director at Cinergi, a renewable energy company based in southern England specialising in air source heat pumps and EV charging solutions.

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