Using Air Source Heat Pumps in Cold Climates

Air source heat pumps are a heating system that can work with standard indoor heating apparatus to help provide warmth for a home through the colder months.

An air source heat pump unit sits outside a home and works by extracting heat stored naturally within the air.

However, as outdoor temperatures drop it can become harder for an air source heat pump to extract heat as effectively, and so in colder climates it can be important to consider how efficiently a heat pump will operate.

Air source heat pumps can still typically work below freezing temperatures but in extremely cold climates an air source heat pump system may not be an effective heating solution on its own.

For the coldest of climates it can be possible to provide a hybrid heat pump system where a typical air source heat pump setup is provided alongside a traditional heating appliance such as a boiler or furnace.

Furthermore, technology is continuously advancing to allow a next generation of cold climate heat pumps (CCHPs) that can provide more effective and efficient heating in freezing temperatures.

Our home is served by an air source heat pump system that is the sole provider of heating and hot water.

Air Source Heat Pump
Our air source heat pump that operates in our milder climate where temperatures don’t typically reach too low for the heat pump to work ineffectively

We live in a milder climate (the UK) and so temperatures are unlikely to fall below what our heat pump can effectively work within. We therefore don’t have a heat pump that is designed for cold climates or have any backup heating system.

However, more importance is placed on the effectiveness of a heat pump in other locations that experience much colder temperatures.

We discuss throughout this article what problems air source heat pumps can need to overcome in colder climates and how a heat pump can still be effective in cold weather with the right setup.

Air Source Heat Pump Problems In Cold Weather

Air source heat pumps (ASHPs) work by extracting heat stored naturally within outside air using an external ASHP unit and moving that heat indoors through refrigerant in pipes for use in an indoor heating system.

Air source heat pumps can provide an alternative to traditional heating appliances (such as gas boilers) where fuel is combusted, and heat is provided. A benefit to these more conventional heating systems is that heat can be generated no matter the outside temperature.

On the other hand, air source heat pumps rely on there being sufficient heat energy within outside air in order to extract effectively.

The main issue with heat pumps can be that as temperatures fall there’s less heat energy within the air and so an ASHP will have to work harder and for longer to extract the same amount of heat.

In colder climates an air source heat pump system can therefore have a harder time heating a home compared to using one in a milder climate.

Air Source Heat Pump Front
An air source heat pump will need to work harder in colder temperatures because there’s less heat energy available within the air

Ice can accumulate on the coils of an outdoor air source heat pump and heating won’t be provided while a heat pump is in its defrost cycle.

Furthermore, current standard air source heat pumps will typically only work down to a certain temperature before the manufacturer can’t guarantee their effective operation.

The temperature in which a heat pump can stop working efficiently can vary between models but as an example our heat pump can work down to -20°C (-4°F).

Air Source Heat Pump Operating Temperatures
Although very cold with less heat available, our heat pump can still work down to minus temperatures

At such low temperatures, heat output from a heat pump can drop off. For example, the graph below shows how the maximum outlet water temperature from our ASHP drops down below 0°C to the minimum operating temperature of -20°C.

Maximum heat output from out ASHP drops off as temperatures fall below freezing

In colder temperatures a heat pump will be using more electricity and ultimately working less efficiently overall. See our article on air source heat pump efficiency for more information.

However, air source heat pumps will always need to work harder in colder climates because lower outdoor air temperatures mean less heat energy available.

Air source heat pumps can still work effectively in cold climates but there are ways to still be able to use a heat pump in very cold climates, and advancing technology means that better solutions are being provided.

How Air Source Heat Pumps Can Work In Cold Climates

Air source heat pumps can work in colder weather by:

  • Working harder and operating for longer periods of time to provide the same desired indoor temperatures.
  • Using a compressor to maximize the heat generated from outside air.
  • Potentially working alongside another form of heating system as part of a hybrid heat pump (HHP) setup.

An external air source heat pump unit won’t typically work all of the time under normal operating conditions but in colder climates a heat pump can need to work harder and for longer.

Lower air temperature means less heat energy available and so an ASHP can be operating for longer periods of time to help bring up indoor temperatures to the same desired level.

Similarly, a heat pump can be working harder by increasing fan speed to force more air through the unit and therefore help capture more heat.

Air Source Heat Pump Fan
In colder temperatures, fans will have to work harder to force more air through the heat pump

Air source heat pumps also use a compressor to increase the pressure of the refrigerant, which in turn increases temperature, and allows them to maximize the temperature gains from extracting heat from air. Our ASHP has a compressor built into the outdoor unit behind an access panel.

See our article on how air source heat pumps can work in winter for more in-depth information.

While all of the above can help an air source heat pump to work in a cold climate, if temperatures are below the minimum operating temperature for that heat pump, then an ASHP may not be able to work effectively at all.

In the coldest of climates it can be more common to install a hybrid heat pump system where another form of heating appliance can be installed alongside a heat pump. That way, if temperatures fall too low the backup heating system can kick in.

See our article on hybrid air source heat pumps for more information.

Cold Climate Heat Pump Technology

Advances in technology are helping heat pumps to work more effectively in colder climates.

These cold climate heat pumps (CCHPs) are more efficient in freezing temperatures and can help to reduce electricity bills that can typically be higher when using a heat pump in colder temperatures.

Such technology can include variable compressors instead of fixed-speed, and using refrigerant that has an even lower boiling point.

Air Source Heat Pump Behind
Lower boiling point refrigerant can help air source heat pumps to capture heat at lower temperatures

The Department Of Energy (DOE) provided the most recent announcement on cold climate heat pump technology breakthrough.

Tips For Owning An Air Source Heat Pump In A Cold Climate

When using an air source heat pump in a cold climate they should be installed as to help prevent airflow through the unit from being compromised and also maintained to allow for effective operation.

  • Ensure that a home is properly insulated: Heat pumps will be more effective at bring up internal temperature when heat isn’t lost from a home too quickly.
  • Clean the unit’s filters regularly: Cleaning the filters will help the system run more efficiently and prevent dust and dirt from building up inside.
  • Keep the area around your heat pump clear of snow and ice: Snow and ice can block airflow and reduce a heat pump’s efficiency.
Text from our heat pump manual regarding how snow fall can be dealt with

For more information see our articles on air source heat pump installation requirements and air source heat pump maintenance requirements.

Further Reading

Parts Of An Air Source Heat Pump Explained

How An Air Source Heat Pump Works

Pros & Cons Of Air Source Heat Pumps

Can An Air Source Heat Pump Be Wall Mounted?

Air Source Heat Pump Enclosures

Air Source Heat Pump COP Explained

Air To Air vs Air To Water Heat Pumps