Air Source Heat Pumps: A Complete Guide

Thinking about installing an air source heat pump? Or wondering how much an air-source heat pump costs?

Air source heat pumps are a highly efficient way to heat your home. They can work by themselves. Or partner up with your existing central heating system to provide hot water and affordable year-round heat.

What is an air source heat pump?

Most home heating systems either burn fuel, or convert electricity into heat. But heat pumps are different, as they don’t generate heat.

Instead, they move existing heat energy from outside into your home. This makes them more efficient. Since they deliver more heat energy than the electrical energy they consume. So a heat-pump system typically costs less to run than a traditional heating system too.

diagram of how a heat pump works

How does an air source heat pump work?

An air source heat pump works much like a fridge operating in reverse[1] to heat your home. Outside air is blown over a network of tubes filled with a refrigerant. This warms up the refrigerant, and it turns from a liquid into a gas.

This gas then passes through a compressor, which increases the pressure. Compression also adds more heat – similar to how the air hose warms up when you top up the air pressure in your tyres.

These compressed, hot gases now pass into a heat exchanger, surrounded by cool air or water. The refrigerant transfers its heat to this cool air or water, making it warm. And this is circulated around your home to provide heating. Meanwhile, the refrigerant condenses back into a cool liquid and starts the cycle all over again!

Getting the most from an air source heat pump

Air source heat pumps available on the Green Homes Grant

Using an air source heat pump for heating

Heat pumps work best when there’s less of a difference between the inside and outside temperatures[2]. In much the same way that your fridge has to work harder in hot weather. Think of it like riding a bike up a hill. The lower the bottom of the hill, and the higher the brow, the more work you’ll have to do to get there!

But this makes them a good match for underfloor heating[3] systems. Floors cover a much bigger area than radiators. So they don’t need to get as hot to provide the same amount of heat.

Air source heat pumps also have a lower output than a gas or oil-fired boiler. This means they can’t deliver heat as quickly. Instead they’re best used to heat your home up slowly over a longer period.

Returning to our hill analogy, it’s the difference between riding a bike up a gentle slope over a couple of hours; or using a polluting motorbike to speed up the hill!

What if I have radiators?

If you have radiators in your home, and you’re switching from a gas-fired heating system to an air source heat pump, you’ll probably need to buy some bigger radiators to keep your house toasty[4].

Using an air source heat pump for hot water

Heat pumps can also be a great source of hot water – but the water will be cooler than from a boiler. Practically this means that If you’re running a bath, you’ll need more hot water and less from the cold tap. So you’ll likely need a bigger hot water tank to cover your needs too.

Air source heat pump pros and cons

Air source heat pump advantages

  • Highly efficient source of heat and hot water

  • Zero carbon if used with a renewable tariff

  • Can closely match the running costs of other home heating systems

  • Installation cost offset by Renewable Heat Incentive payments

  • Low-maintenance with a long service life

  • Some systems can provide cooling in the summer

 

Air source heat pump disadvantages

  • Lower output temperature than conventional boilers – you may need to update your insulation and invest in bigger radiators too

  • May work best in older homes as part of a hybrid system with a conventional boiler

  • Need outdoor space, and can be noisy

  • Expensive to install and works best with a water tank

  • Don’t work as efficiently in extremely cold weather

 

Will my air source heat pump fit in the boiler cupboard?

Typically, air source heat pumps have a single ‘monobloc’ cabinet that sits outside your home. Some come with an indoor compressor unit[5] too – but it depends on what type you go for (see below).

It looks a bit like an air conditioning unit, but sits on the floor outside your house. So you’ll need to have some outdoor space preferably in a sunny area – for it to be located.  You will also need to consider space for a water tank inside your home.  If you don't have room for a water tank then a hybrid heat pump might be the way to go.

Air Source Heat Pump installer

Are there different types of air source heat pump installations?

Yes! Most air source heat pump installations in the UK are what’s known as ‘air to water’ types[6]. In these systems, the heat is transferred into a conventional ‘wet’ heating system. And this set-up gives you hot water and central heating.

A minority of UK air source heat pumps are ‘air to air’. And, as their name suggests, these transfer the heat into air, which is distributed around the home. They don’t provide hot water, though.

Because air source heat pumps give out less heat than a boiler, they’re not always suitable as a straightforward boiler replacement. Especially because there are still a lot of older, poorly insulated homes in the UK.

But heat pumps can often be combined with an existing central heating system and boiler. In these hybrid air-source heat pump systems, the heat pump provides a ‘base load’ for the day-to-day heating and hot water. With the boiler only firing up to provide hotter water. Or a quick heat boost during a particularly cold snap.

Hybrid systems can be a great way to reduce costs and carbon emissions in older properties. Particularly if you’re not on a gas supply, so you’re reliant on irregular and expensive deliveries of fuels, like LPG or heating oil.

Air source heat pump cost

An air source heat pump costs more than a new gas or oil-fired central heating system. The typical air source heat pump cost ranges from £4,000-8,000, depending on the pump brand and its heat output[7].
 

Air source heat pump installation costs

On top of that, you’ll need to pay for air source heat pump installation costs too. This could bring the total to between £5,000 and £10,000[8]. If you’re fitting new underfloor heating or an air distribution system, you’ll need to factor in these costs too. And potentially improved insulation, since air source heat pumps aren’t very effective at heating draughty or poorly-insulated homes.

How much does an air source heat pump cost to run?

Air source heat pumps might require a big upfront investment. But the good news is that they are one of the most efficient ways to heat your home.

An air source heat pump produces more heat energy than it uses in electricity. So while a modern gas boiler is more than 90% efficient – and electric heaters are 100% – air-source heat pump efficiency can be three or four times higher[9.1][9.2]!

Their high efficiency means that air source heat pump running costs can be far lower than what you’d typically pay for your heating system. Here’s why…

It costs about 4.6p per kilowatt hour (kWh) to heat a home with gas. And it’s about 9-16p per kWh using standard electric heaters[10]. A typical air source heat pump might cost about 4.7p per kWh to run. But if you use cheap-rate Economy 7 electricity or an GoElec tariff this could be as low as 2.3p[11]!

Energy prices vary – and our homes and use patterns differ too. So it’s hard to give an exact figure for how much money you could save in a year. But, based on some industry-wide figures, we’ve put together this example:

A modern four-bedroom house might need about 19,000kWh of heat per year[12]. This would cost around £874 in gas[13]. Using an air source heat pump and electricity priced at 16p per kWh the cost would be almost identical[14]. However, even if you did only half of your heating overnight – using the cheapest cheap-rate electricity – you could save 25%[15]: about £215.

The savings increase when you consider hot water. On average, a four-person household uses about 160 litres a day[16]. This requires about another 2,800kWh per year[17]. Using a 90% efficient gas boiler that’s another £129. But if you got all your hot water from an air source heat pump powered by cheap-rate electricity, you might only pay £65[18].

hybrid heat pump being inspected

What about air source heat pump servicing?

Air source heat pumps use well-established technology, and may have a working life of 20 years or more[22]. But just like a boiler, you should get your air source heat pump professionally serviced – every two or three years – to make sure that it’s still working at its best.

If you have a unit where the compressor is inside your home, you should have the coolant pipes checked annually.

The good thing about air source heat pumps is that they don’t need a huge amount of user maintenance. The main thing is to be sure that there’s a plentiful supply of air. This means regularly checking for leaves or rubbish caught in the fan’s grilles. And pruning back any plants growing too close to the unit or its pipes.

In very cold weather, make sure you clear snow from the air source heat pump and check that the grilles aren’t blocked by ice[23].

In a hybrid system, your gas, oil or LPG boiler will need annual servicing. You should check the boiler pressure at least a couple of times a year. And bleed your radiators if you have them.

Are air source heat pumps the right fit for you?

Installing an air source heat pump is a big decision. So you need to weigh up whether it’s the right fit for your home.

Air source heat pumps offer a low-carbon alternative to gas or oil-fired central heating. And while they’re comparatively expensive to install, this may be offset by grants and lower running costs.

They’re best suited to well-insulated modern homes – working alongside technologies like underfloor heating. But they’re just as effective in older properties as part of a hybrid heat pump system to reduce costs and emissions. And it’s likely we’ll see many more installations as we shift to low-carbon heating systems to help Britain achieve Net Zero by 2050.

Sources:

[1] This is a technical explanation of the cycle
[2,4,5] https://ggbec.co.uk/air-source-heat-pumps-ashp-5-things-consider

[3] https://www.warmup.co.uk/blog/guide-to-underfloor-heating-heat-output

[6] https://energysavingtrust.org.uk/renewable-energy/heat/air-source-heat-pumps

[7] e.g. https://www.theunderfloorheatingstore.com/samsung-premium-heat-pump

[8] See here and here

[9]https://www.thegreenage.co.uk/cheaper-heat-home-gas-electricity/ and https://www.greenmatch.co.uk/blog/2014/08/the-running-costs-of-heat-pumps
[10] Taking the EST’s 4.17p/kWh figure for gas and multiplying by 90% efficiency. 9-16p reflects the cost of electricity on E7 and standard tariffs.

[11] Using EDF’s GoElectric figure of 8p/kWh, and dividing it by a 3.5 coefficient of performance.

[12] This site quotes 125kWh per square metre per year. They go with 200sq metres, but that seems big, so we’ve used 150sq metres and rounded up.

[13] 90% efficient gas cost of 4.6kWh multiplied by 19,000kWh.

[14] 16p/kWh divided by CoP of 3.5, multiplied by 19,000kWh.

[15] Again, calculated at 2.3p/kWh based on EDF’s GoElectric tariff. You save 25% because it’s half the cost of gas for half of the time.

[16] https://www.hotwater.org.uk/sizing-a-hot-water-cylinder/

[17] The formula is (4.2xlitresxtemp difference)/3,600. For 160 litres heated from 10-50 degrees that gives 7.5kWh per day.

[18] It’s 4.6p/kWh (90% efficient gas) vs 2.3p/kWh (GoElectric tariff).

[19] https://www.which.co.uk/reviews/renewable-heat-incentive/article/renewable-heat-incentive-rhi/what-is-the-renewable-heat-incentive#technologies

[20] https://www.which.co.uk/reviews/renewable-heat-incentive/article/renewable-heat-incentive-rhi/rhi-costs-and-earnings. They’re capped at 20,000kWh/year, which is currently worth £1,300.

[21] https://www.simpleenergyadvice.org.uk/pages/green-homes-grant

[22] https://energysavingtrust.org.uk/renewable-energy/heat/air-source-heat-pumps

[23] https://www.imsheatpumps.co.uk/blog/common-air-source-heating-problems/