Air Source Heat Pump
Air source heat pumps (ASHPs) absorb heat from the outside air to heat your home and hot water. They can still extract heat when air temperatures are as low as -15°C. If you have large garden space outside, you could consider a ground source heat pump. ASHPs need electricity to run, but because they are extracting renewable heat from the environment, the heat output is greater than the electricity input.
How do air source heat pumps work?
Heat from the air is absorbed at low temperature into a fluid. This fluid passes through a compressor, increasing the temperature, and transfers that higher temperature heat to the heating and hot water circuits of the house.
There are two main types of ASHP: air-to-water and air-to-air. Choosing an air-to-water or an air-to-air system will determine the type of heat distribution system you need.
Air to Water
Air-to-water heat pumps absorb heat from the outside air, and transfer the heat to water.
An air-to-water system distributes heat via your wet central heating system. Heat pumps work much more efficiently at a lower temperature than a standard boiler system would. This makes them more suitable for underfloor heating systems or larger radiators, which give out heat at lower temperatures over longer periods of time.
Air-to-water heat pumps are the most common model in the UK.
Air to Air
Air-to-air heat pumps require a warm air circulation system to move the warm air around your home. They will not provide you with hot water as well. Air-to-air heat pumps are not eligible for the UK Government’s Renewable Heat Incentive (RHI) scheme.
The benefits of air source heat pumps
- lower fuel bills are likely, especially if you are replacing conventional electric heating.
- potential income through the UK government’s Renewable Heat Incentive (RHI). (air-to-water)
- lower carbon emissions.
- no fuel deliveries needed.
- can heat your home as well as your water.
- minimal maintenance required.
- can be easier to install than a ground source heat pump.
During winter, an ASHP may need to be on constantly to heat your home efficiently. You will also notice that radiators won't feel as hot to the touch as they might do when you are using a gas or oil boiler.
Unlike gas and oil boilers, heat pumps deliver heat at lower temperatures over much longer periods. If you are installing an ASHP to replace a gas or oil boiler, you should consider whether you can also upgrade your insulation to get the most out of your ASHP. You might also consider fitting larger radiators or underfloor heating.
Is an Air Source heat pump suitable for me?
To tell if an air source heat pump is right for you, there are a few key questions to consider:
- Do you have somewhere to put it? You'll need a place outside your home where a unit can be fitted to a wall or placed on the ground. It will need plenty of space around it to get a good flow of air. A sunny wall is ideal.
- What type of heating system will you use? Air source heat pumps perform particularly well with underfloor heating systems or warm air heating because they operate at low temperatures. Homes without an existing central heating system will require one to be installed for an air source heat pump to work.
- Is your home well insulated? Since air source heat pumps work best when producing heat at a lower temperature than traditional boilers, it's important that your home is well insulated and draught-proofed to minimise heat-loss.
- What fuel will you be replacing? The system is more likely to pay for itself if it's replacing an expensive system like electric heating. You’re unlikely to save much on your heating bill if you’re switching from mains gas.
Are there different heat pumps for other types of property?
If your home doesn’t meet all the criteria for a heat pump to operate at optimum efficiency, it may still be possible to heat your home effectively using heat pump technology. One option is to fit a hybrid heat pump, where a heat pump and a conventional gas or oil boiler work in tandem.
With these systems, most of the time the heat pump will provide all your heating needs with the gas or oil boiler switched off. However, on occasions when the heat pump is not able to provide enough heat on its own, such as when outdoor temperatures are very low and your heating demand is high, the fossil fuel boiler turns on.
The system can either be set up so that the heat pump then switches off, allowing the boiler to provide all your heat, or it can be set up for both systems to run at the same time. This will depend on the cost-effectiveness and design of your particular system.
Another approach is to fit a high temperature heat pump. This is just like any other heat pump, but it has been designed to work effectively with a higher temperature output. It won’t achieve the same efficiencies as an equivalent system operating with a low temperature heating circuit, but it will be able to provide sufficient heat at a high enough temperature to keep you warm.
Costs, savings and financial support
Installing a typical system costs around £9,000 to £11,000. Running costs will vary depending on a number of factors including the size of your home, how well insulated it is and what room temperatures you are aiming to achieve.
How much you can save will depend on what system you use now, as well as what you are replacing it with. Your savings will be affected by:
- Your heat distribution system. If you have the opportunity, underfloor heating can be more efficient than radiators because the water doesn’t need to be so hot. If underfloor heating isn’t possible, use the largest radiators you can. Your installer should be able to advise on this.
- Your fuel costs. You will still have to pay fuel bills with a heat pump because it is powered by electricity, but you are likely to save money on your fuel bills (depending on the type of heating you are replacing).
- Your old heating system. If your old heating system was inefficient, you are more likely to see lower running costs with a new heat pump.
- Water heating. If the heat pump is providing hot water then this could limit the overall efficiency. You might want to consider solar water heating to provide hot water in the summer and help keep your heat pump efficiency up.
- Using controls. Learn how to control the system so you can get the most out of it. You will probably need to set the heating to come on for longer hours, but you might be able to set the thermostat lower and still feel comfortable. Your installer should explain to you how to control the system so you can use it most effectively.
Potential annual savings of installing a standard air source heat pump in an average sized, four-bedroom detached home:
England, Scotland and Wales
Figures are based fuel prices as of April 2020.
See the domestic Renewable Heat Incentive page for the latest information and proposed changes to the RHI scheme.
Heat pump systems typically come with a warranty of two to three years. Workmanship warranties for heat pumps can last for up to 10 years, for example, through QANW (Quality Assured National Warranties).
Many manufacturers also offer options for warranty extensions for a fee.
With regular scheduled maintenance, you can expect an ASHP to operate for 20 years or more.
Every year, you should check that the air inlet grill and evaporator are free from leaves or other debris. Remove any plants that start to grow near the heat pump. You may also be advised by your installer to check the central heating pressure gauge in your house from time to time. If so, you should be shown how to do this.
Ask your installer for written details of any other maintenance checks you should undertake to ensure everything is working properly. Consult with your supplier for exact maintenance requirements before you commit to installing a heat pump.
A professional should service the heat pump every two to three years.
Anti-freeze can be used to prevent the heat pump from freezing in cold winter weather.
Levels of anti-freeze and its concentration are some of the things that a professional installer will check when they come to service your heat pump. Though unusual for a domestic installation, some heat pumps have external refrigeration pipes, and these will need to be serviced annually by a refrigeration engineer.