How efficient are air source heat pumps?

06 December 2021 | Aimee Tweedale

You might have heard that the UK government recently announced a new grant to help homeowners install heat pumps from 20221

Why are heat pumps so hot right now? Because unlike the gas and oil boilers that many of us use to heat our homes at the moment, they’re powered by electricity rather than fossil fuels. This makes them a greener way to keep warm.

But as they’re such new technology, you might have some questions about heat pumps. In particular, one that comes up a lot is whether air source heat pumps – which take heat from the outside air – can work in winter. You might be pleasantly surprised by the answer!

Keep reading for everything you ever wanted to know about air source heat pump efficiency, and the best temperature for a heat pump to work. 

The best weather for an air source heat pump

The best climate for air source heat pumps is warm or moderate. This doesn’t mean they can’t work in other climates, but they’re most efficient when the temperature outside is warm. 

The optimum temperature for an air source heat pump to work most effectively is around 7C. At that temperature or higher, your heat pump should work very efficiently2.

Why air source heat pumps lose efficiency in cold weather

Air source heat pumps take warmth from the air and turn it into warmth for your home. Naturally, it follows that if the air surrounding your heat pump is cold, it will have to work a bit harder to extract that warmth!

Ground source heat pumps are slightly more efficient in cold weather. This is because the pipework is buried underground, where there is always a source of warmth.

The pipes of an air source heat pump are exposed to the air, which makes it a bit harder for the pump to heat up when it’s cold outside. There’s also a risk of snow and ice around the pipes. 

There are lots of different factors to consider when you’re choosing which central heating system is right for you. Read our guide to the pros and cons of air source heat pumps, and whether an air source heat pump, ground source heat pump, or boiler is right for you. 

At what temperature does a heat pump stop working?

The minimum temperature an air source heat pump needs to work is lower than you might expect. 

Different heat pumps made by different manufacturers will vary. But most, heat pumps can still work even when it’s as cold as -15C outside3

If the temperature gets even lower, your heat pump won’t just stop working completely. But the colder it is, the less efficient it will be. This means that if it’s -20C outside, your heat pump might struggle to heat your home to your usual warmth. But it will still provide some heat. 

Remember, it’s very unlikely to get that cold! The coldest temperature ever recorded in England is -26.1C, and that was in the 1980s4. For comparison, the average temperature in London in January is 6C5.

Find out more about how air source heat pumps work

How to use your air source heat pump efficiently

There are a few smart things you can do to make sure your heat pump works as efficiently as possible. 

  • Insulate your home. This is an absolute must! Air source heat pumps work their best in homes that are well-insulated against the cold. Check out our guides to roof insulation, loft insulation, and cavity wall insulation
  • Set it, then forget it. Heat pumps work super efficiently when you leave them running consistently. This is because they use a lot of energy when they boot up, as they try to quickly heat up to the right temperature. It’s far more efficient to leave them running at a moderate, comfortable temperature all the time. (Want to adjust the temperature in different rooms of your home? Check out our guide to smart thermostats.)
  • Pair it with the right kind of heating system. Air source heat pumps work best when they’re left on continuously, at low temperatures. This means they work best with large heat emitters like underfloor heating, or oversized radiators, which can spread warmth through a whole room with relatively low temperatures.
  • Don’t use “auto” mode. Most heat pumps come with an automatic setting, which lets the pump change its own temperature settings. But this doesn’t always have efficient results. You can probably use the pump more efficiently if you programme it to a set temperature yourself. 
  • Make sure the pump is clear of any obstructions. This includes clearing snow and ice from the pipes and vents. 

How efficient is an air source heat pump?

People often talk about the energy efficiency of air source heat pumps in terms of their CoP. CoP stands for Coefficient of Performance. The CoP figure tells you how much energy output (heat) the pump can generate per unit of energy input (electricity). 

For example, if a heat pump has a CoP of 3, it can generate 3 units of heat for every 1 unit of electricity. This makes it 300% efficient. 

The CoP of an air source heat pump will depend on how warm it is outside. During winter, it’s likely to be around 2.5. But during the warmer months, your heat pump could have a CoP of 3 or 4 (300% or 400% efficient). It could even go higher6

Compare this to a modern gas boiler, which is usually around 85%-95% efficient7. Air source heat pumps clearly come out on top!

How much energy does a heat pump use?

How much power you’ll need to heat your home with an air source heat pump depends on:

  • Your make and model of heat pump
  • The size of the pump
  • The size of your heating system (such as your radiators)
  • How efficient your pump is
  • The temperature outside
  • How big your home is

And more!

But, while we can’t tell you exactly how much power yours will use, we can give you an example. 

The average UK home uses about 12,000 kWh per year for heating. If you have a heat pump with an average CoP of 4, then that means you need around 3,000 kWh of electricity to power your heating for the year. This is because heat pumps use less electricity to create the same amount of heat. 

Find out more about the costs and savings of air source heat pumps.

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