What are Air source heat pumps (ASHPs)?
Air source heat pumps absorb heat from the outside air. This heat can then be used to heat radiators, underfloor heating systems, or warm air convectors and hot water in your home.
Different from a ground source heat pump, an air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside. It can obtain heat from the air even when the temperature is as low as -15° C. Air Source Heat Pumps will still have some impact on the environment as they need electricity to run, but the heat they extract from the air is constantly being renewed naturally.
The benefits of air source heat pumps
• Lower fuel bills, especially if you are replacing conventional electric heating
• potential income through the UK government’s Renewable Heat Incentive program
• lower home carbon emissions, depending on which fuel you are replacing
• no fuel deliveries needed
• can heat your home as well as your water
• minimal maintenance required
• easier to install than a ground source heat pump.
Unlike gas and oil boilers, heat pumps deliver heat at lower temperatures over much longer periods. During the winter they 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.
How do air source heat pumps work?
Heat from the air is absorbed at low temperature into a fluid. This fluid then passes through a compressor where its temperature is increased, and transfers its higher temperature heat to the heating and hot water circuits of the house.
There are two main types of air source heat pump systems:
Air 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 Air
An air-to-air system produces warm air which is circulated by fans to heat your home. They are unlikely to provide you with hot water as well.
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.
• Is your home well insulated? Since air source heat pumps work best when producing heat at a lower temperature than traditional boilers, it’s essential that your home is well insulated and draught-proofed for the heating system to be most efficient.
• What fuel will you be replacing? The system will pay for itself much more quickly if it’s replacing an electricity or coal heating system. Heat pumps may not be the best option for homes using mains gas.
• What type of heating system will you use? Air source heat pumps can perform better with underfloor heating systems or warm air heating than with radiator-based systems because of the lower water temperatures required.
• Is the system intended for a new development? Combining the installation with other building work can reduce the cost of installing the system.
What if there are times when the ASHP cannot provide enough heat?
In a well-insulated property, air-source heat pumps can provide all your heating needs by themselves. However, in older properties where it is not possible to insulate to a high enough standard for heat pumps to be fully effective on their own, you can have another heating system in place alongside the heat pump, usually a traditional gas or oil-fired boiler.
This set up is called a hybrid heat pump* or a bivalent system. With these systems, most of the time the heat pump will provide all your heating needs, and the gas or oil boiler will be 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 design of your particular system and which set-up is likely to be more cost-effective for you.
(* The term ‘hybrid heat pump’ can also refer to a solar pre-heat, heat pump, which is a different system.)
Costs, savings and financial support
Installing a typical system costs around £6,000 to £8,000. 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 will save on the fuel 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 control; 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.
Below is an indication of the potential annual savings that can be obtained by installing a standard air source heat pump in an average sized, four-bedroom detached home:
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. You can expect them to operate for 20 years or more, however they do require regular scheduled maintenance. A yearly check by you and a more detailed check by a professional installer every three to five years should be sufficient.
The installer should leave written details of any 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. You are likely to be advised to carry out a yearly check that the air inlet grill and evaporator are free of leaves or other debris.
Any plants that have started to grow near the heat pump unit will also need to be removed.
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.
To prevent the heat pump from freezing in cold winter weather anti-freeze is used. Levels of anti-freeze and its concentration is one of the things that a professional installer will check when he comes to service your heat pump. If your heat pump has external refrigeration pipes, (very unusual for a domestic system) these will need to be serviced annually by a refrigeration engineer.
Before starting, the developer must apply to the relevant planning authority for a determination as to whether the prior approval of the authority will be required for the siting and external appearance of the air source heat pump.
The application needs to be accompanied by a range of other information and several other conditions apply.