Air source heat pumps taster

Air source heat pumps explained

  • Discover how air source heat pumps work
  • Air source heat pump costs and payback times revealed
  • Pros and cons of installing an air source heat pump
  • Things to consider when getting a heat pump

An air source heat pump (ASHP) is usually placed outside at the side or back of a property, and takes heat from the air and boosts it to a higher temperature using a heat pump. This heat is then used to heat radiators, underfloor heating systems or even warm air convectors and hot water in your home. 

The pump needs electricity to run, but the idea is that it uses less electrical energy than the heat it produces.

Ground source heat pumps are also available. They draw heat from the ground via a network of water pipes buried underground, usually in your garden. See our separate guide to ground source heat pumps for more details.

Air source heat pump costs and savings

ASHPs are cheaper than ground source heat pumps. The Energy Saving Trust (EST) estimates that the cost of installing a typical ASHP system ranges between £7,000 and £14,000. 

The payback period (the time it takes for the cost of the system to be recouped in energy savings) depends on how efficiently your system works, the type of system you're replacing, whether you can get money with the RHI and how you'll be using the heat generated from the pump. 

The EST says that an average performing air source heat pump in an average four-bedroom detached home could save:

  • between £545 to £880 a year if replacing oil
  • between £550 and £1,060 a year if replacing electric heating. 

It also estimated that he RHI would pay an extra £805 to £1,280 a year.

Here are some general rules of thumb to bear in mind:

  • Like ground source heat pumps, air-to-water ASHPs work better with underfloor heating systems. If underfloor heating is not possible, large radiators should be used. This is because the heat generated by the heat pump is not as high as that produced by a conventional boiler, so a larger surface area is needed to achieve similar temperatures in your home.
  • Air-to-water heat pumps could be better suited to new-build properties than retrofit - this is because costs could be reduced if the heat pump is included as part of the building specification, rather than having to retrofit underfloor heating later on.
  • Heat pumps can save you more on your heating bills if you're replacing an electric, oil, LPG or coal system, rather than gas.
  • A well-insulated house is essential to best optimise the heat generated by your ASHP - otherwise the heat the pump is generating escapes more easily.
  • Once in place, the heat pump should require little maintenance.
  • Heat pumps qualify for the Renewable Heat Incentive (RHI).
Air source heat pumps in content 3

Air source heat pumps need electricity to run

 

Installing an air source heat pump

If you're considering installing an air source heat pump you can head to Which? Local to find an installer recommended by other Which? members.

ASHPs look similar to air-conditioning units and are less disruptive to install than ground source heat pumps, as they do not require any digging in your garden. 

An ASHP works a bit like a refrigerator in reverse. The process consists of an evaporator, a compressor and a condenser. It absorbs heat from the outside air and the heat pump compressor then increases the temperature of that heat further to create useful heat.

There are two main types of ASHP:

  • Air-to-water systems take heat from the outside air and feed it into your wet central heating system. As the heat produced is cooler than that from a conventional boiler, you may need to install larger radiators or underfloor heating in your home to make the most of it.
  • Air-to-air systems take heat from the outside air and feed it into your home through fans. This type of system cannot produce hot water.

In the summer, the ASHP can be operated in reverse, like an air-conditioning unit, to provide cool air for your home.

Pros of air source heat pumps

  • Air source heat pumps can generate less CO2 than conventional heating systems.
  • They are cheaper than ground source heat pumps and easier to install, particularly for retrofit, although their efficiency can be lower than with ground source heat pumps.
  • ASHPs can provide heating and hot water.
  • They require very little maintenance.
  • They can be used for air conditioning in the summer.
  • You need to use electricity to power the pump which circulates the liquid in the outside loop, but for every unit of electricity used by the pump, you get between two and three units of heat – making this an efficient way to heat a building.
  • Cheaper Economy 7 electricity tariffs can be used to lower the cost of electricity to power the heat pump and special heat pump tariffs may be available from some electricity suppliers – alternatively consider solar photovoltaic panels or a wind turbine (if you are in a suitable area) for a greener source of electricity.

Cons of air source heat pumps

  • You'll need enough space in your garden for the external condenser unit (comparable in size to an air-conditioning unit). Condenser units can be noisy and also blow out colder air to the immediate environment.
  • You still need to use electricity to drive the pump, so an air source heat pump can't be considered completely zero-carbon unless this is provided by a renewable source, such as solar power or a wind turbine.

How green are air source heat pumps?

An air source heat pump system can help to lower your carbon footprint as it uses a renewable, natural source of heat – air. The amount of CO2 you'll save depends on the fuel you are replacing. For example, it will be higher if you are replacing electric heating than natural gas.

A heat pump also requires a supplementary source of power, usually electricity, to power the heat pump, so there will still be some resulting CO2 emissions.

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