The simple answer is: yes, heat pumps work in cold weather. That’s the whole point of a heating system.
After all, they’re used commonly in Scandinavian countries, where winters are cold enough to freeze a moose. The UK is simple by comparison.
However… there are some caveats. Heat pumps can struggle in the cold, even if they’re technically ‘working’. And there’s a lot of misleading information out there, often based on guesswork, often fuelled by self-interest.
Here’s the honest answer, based on actual data.
Heat pump efficiency in cold weather
Heat pumps do work in cold weather, but their efficiency is greatly reduced.
Heating efficiency is measured using ‘COP’, or coefficient of performance. This measures how much energy (as heat) you get out for each unit of energy (as electricity) you put in. Over the whole year, this is averaged as ‘SCOP’ or Seasonal Coefficient of Performance.
Heat pumps typically have a SCOP of 3-4.
However, this quoted value is just an average, and is sometimes a little bit optimistic. When the temperature drops below 0°C, the COP will usually drop below 3, and keep dropping.
This graph is taken from this research in 2023, which took 550 heat pumps and 2760 individual measurements. They found that between -10°C and 5°C, the average heat pump COP in the UK was 2.5.
This efficiency drop has a knock-on effect on cost. At a COP of 4, a heat pump costs about the same as a gas boiler to run (due to electricity being roughly 4x the cost of gas, and a gas boiler having a COP of roughly 1). When the COP drops to 2.5, the cost of running a heat pump will be 60% more than running a gas boiler.
So this is the real answer to the question ‘do heat pumps work in the cold’: yes, but it’ll cost you about 60% more than gas.
Can heat pumps cope with extreme cold?
In the vast majority of conditions, heat pumps will work (albeit at a higher cost in lower temperatures). But what about in really extreme cold? Such as -10°C or below?
In reality, this is not a common problem in the UK, where average January temperatures are around 4°C. But, of course, this is about extremes – on the coldest day of the year, or in windswept bits of Scotland.
Here there are two key things to make sure your heat pump works in freezing weather:
- Correct sizing of your heat pump in kW
- Using a cold climate heat pump
The first thing is obvious – your heat pump needs to be big enough to deal with the heat loss on the coldest day. In fact, this is (or should be) accounted for during installation. I’ve written more about it here, but in essence an installer will ‘size’ your system based on a difference between an indoor and outdoor temperature of 24.2°C. In other words, if you keep your thermostat at 20°C, the heat pump will be adequately sized for outside temperatures of -4.2°C. Usually you’ll have some tolerance, so even below that your heat pump should work fine.
For temperatures under -10°C, many people turn to a cold climate heat pump. These are common in parts of Canada and Scandinavia, and can be rated to -30°C. They’re not without downsides – at very low temperatures, heat pumps need to run regular defrost cycles, and they are noisy too. You can read a lively discussion about cold climate heat pumps here.
In summary: yes, heat pumps will work in cold weather, and can work in freezing weather too. That said, in very cold climates I saw people would often have some sort of backup – gas boiler or immersion heater – just in case. It is sort of cheating, but quite sensible too if you’re expecting blizzards but want the benefits of a heat pump the rest of the time.
