How does a heat pump work in winter?

Living more economically can be beneficial to the environment and your wallet. Due to their efficiency and effectiveness, many people are turning to heat pumps to live more sustainably.

The Government is also supporting the development of heat pump technology and unveiled its Heat Pump Investment Roadmap in 2023.

Despite this, some concerns exist regarding how heat pumps perform in cold weather. After all, you'll rely on your heating system the most during the winter months.

Here, we explore how heat pumps work in cold weather and what you can do to stop your heat pump from freezing and ensure good levels of energy efficiency, even in cold climates.

How do heat pumps work?

There are two main types of heat pumps - air source heat pumps and ground source heat pumps - but, for the purposes of this overview, both operate in the same fundamental ways. 

Air source heat pumps work similarly to air conditioners. In fact, an air-to-air heat pump is actually an air conditioner operating in reverse. Using its refrigeration cycle to create heating instead of cooling. 

Heat pumps use a heat exchanger that extracts heat energy from the air or ground (depending on the type of device) and a compressor, which moves a refrigerant through the system. The hot refrigerant is then passed into another heat exchanger to heat water, which is then delivered into our homes via radiators or underfloor heating.

The way that heat pumps generate and transfer heat can make them more efficient and cheaper than electric heaters and boilers.

Air source heat pumps

Air source heat pumps absorb heat energy from the air to increase the temperature inside a home either by blowing warm air into the room, or heating water to use in radiators and underfloor heating, depending on the type of heat pump installed. The efficiency of air source pumps may vary depending on the season and the time of day.

Essentially, if it's mild outside, an air source heat pump will find it easier to transfer the warm energy into your home. Pretty straightforward, right?

Ground source heat pumps

A ground source heat pump takes heat from the ground and transfers it into radiators, underfloor heating and hot water cylinders.

Because they absorb underground temperatures, ground source heat pumps offer a more consistent temperature all year round, making them a more dependable system than air source heat pumps.

With this being said, both types of heat pump can reliably produce high temperatures.

How do heat pumps work in cold weather?

Heat pump manufacturers design their systems to work in cold weather, and the refrigerant liquid inside heat pump systems can absorb heat in freezing weather, even to temperatures as chilly as -20ºC.

As the air gets cooler outside, your air source heat pump's efficiency will drop, and it may use more electricity than it would on a warmer day to heat your home.

Although UK winters can get pretty nippy, the ground temperature rarely drops below 8ºC, so a ground source heat pump will still be able to provide heat energy for your home, even when the weather is at its coldest. 

The efficiency of heat pumps is measured using the coefficient of performance (CoP), which compares the input to the output. So, if your heat pump uses 0.5kW of electricity to produce 2kW of heat, the CoP is 4. The higher the CoP of your heat pump, the more efficient it is.

Does the cold weather affect heat pump efficiency?

Regardless of your heat pump type, it will work less efficiently in a cold climate. Air source heating systems, in particular, will find it difficult because the air temperature is typically lower than the ground temperature. A ground source heat pump may be a wiser investment if you live in a colder climate.

Another factor that will affect the performance of your heat pump, especially in cold weather, is the temperature you want your home to be. When it's chilly outside, you naturally want your home to be warmer, but this increases the strain on the pump, which can lower its efficiency and output.

At what temperature does an air source heat pump stop working?

The temperature at which an air source heat pump stops working will depend on the specific make and model. In the UK, we're fortunate that the air temperature rarely drops below -5ºC. While most pumps may work less effectively at this temperature, they can still draw warmth from the outdoors.

Air source heat pumps are typically designed to work to around -20ºC. At this temperature, the pump may struggle to extract enough heat from the air to warm your home.

If you live in an area where these temperatures may be possible, consider investing in a backup heating system, like electric heaters or radiators, that you could use to keep warm.
You should review the manufacturer's instructions and specifications to ascertain the precise temperature at which your heat pump may stop working efficiently.

Additionally, liaise with a professional installer as they may be able to recommend suitable systems.

Do you have to winter-proof your heat pump?

One of the first things you can do to winter-proof your heat pump is to ensure it's in an optimal position. Air source heat pumps must be located in a reasonable amount of space, allowing air to move freely.

Where possible, you should shelter your air source heat pump to protect it from snow and rain, as this can affect how efficiently it works in cold weather.

The most important thing you can do to maximise the efficiency of your air source heat pump is to ensure it's regularly maintained and looked after. Just as you should service your boiler each year in preparation for winter, you should have your heat pump inspected before the colder months, too.

Although it's unlikely, condensation can form and freeze on the outdoor component of the system, creating a layer of ice that will affect the pump's performance. To mitigate the risk of your heat pump freezing, you should have it inspected annually.

What to do if your heat pump is frozen?

A frozen heat pump can drastically decrease the appliance’s efficiency - increasing energy consumption while reducing the pump's output. Here are six things you can do if you notice the performance of your heat pump has declined:

1. Check the air filter (air-to-air heat pumps only)

A clogged or dirty filter can restrict the airflow, making the pump more susceptible to freezing. If the air filter is dirty or clogged, you should clean or replace it in line with the manufacturer's instructions.

2. Clear any debris from around the external unit 

In the colder months, snow, ice, and fallen leaves can all collect around the heat pump and cause blockages. If you notice any debris, clear it carefully, and if you feel confident enough, use lukewarm water to melt any ice and snow.

3. Turn off your system 

If you're worried about potential damage being done to the system, you should switch it off. Depending on the type of heat pump you own, this could be as simple as setting the thermostat off.

4. Wait for the ice and snow to thaw 

Although it could take a few hours, waiting for the pump to thaw naturally is the safest option.

5. Check the refrigerant levels

Low levels of refrigerant may be the cause of your freezing problems. If you think you may have a leak, contact a heating, ventilation and air conditioning (HVAC) technician to inspect your system as soon as possible. Do not try to fix a leak yourself.

6. Make sure your heat pump is regularly maintained

To prevent freezing issues, schedule regular maintenance. This will mean potential problems can be identified early, and all components can be kept in good working order.