Air source heat pumps are a more sustainable alternative to electric storage heaters and oil- or gas-fired boilers. They provide hot water and heat your home using energy from the air outside rather than requiring any fuel input, offering greater energy efficiency than conventional boilers and helping to reduce carbon emissions.
This guide explains everything you need to know about air source heat pumps. Along the way, we’ll cover all of the key points including what they are, how they work, the different types, their pros and cons, the costs, the grants available, and some other frequently asked questions.
What is an air source heat pump?
An air source heat pump absorbs energy from the outside air and uses it to provide hot water and heat a home. They can operate even with outside temperatures as low as -15°C. The system consists of two units: an external unit that sits against the outside wall of a property and takes in heat from the environment, and an internal module that houses the parts required to transfer heat to the central heating system.
How does an air source heat pump work?
An air source heat pump provides heating and hot water using the following process:
- To extract energy from the surrounding environment, the external part of an air source heat pump draws air in through fans (this requires some electricity input).
- Once inside, the air passes over a network of tubes filled with a liquid known as a refrigerant. As air comes into contact with the tubes, the refrigerant heats up and evaporates (turning from a liquid into a gas).
- The gas is then passed through a compressor to increase its pressure and heat it further – this also requires some electricity, but more heat energy comes out than goes in.
- Next, the compressed, hot refrigerant gas travels into a heat exchanger, where it transfers its energy to cool air or water.
- Once the air or water has warmed up, it is transported to the central heating system and circulated around the home (some is stored in the hot water tank for later use).
- The refrigerant condenses, returning to its cool liquid form and moving back to the external unit to be heated once again.
Types of air source heat pump
Air source heat pumps come in two types: air-to-water and air-to-air. The differences between the two types are explained below.
The vast majority of air source heat pumps in the UK are air-to-water. In this setup, water is heated up in the heat exchanger before passing on to a wet central heating system (i.e. one which uses water in pipes to heat a home). An air-to-water heat pump will provide a home with hot water as well as heating it.
Less common in the UK, air-to-air systems heat up air in the heat exchanger. This type of air source heat pump requires a warm air circulation system rather than wet central heating. As such, it can’t be used to provide hot water. On the other hand, air-to-air heat pumps can be reversed to offer air conditioning during the warmer months.
Heating a home with an air source heat pump
With an air source heat pump, the air or water in the central heating system will be warmed, but won’t reach temperatures that are as high as you’d get with an electric storage heater or a conventional oil- or gas-powered boiler.
For this reason, air source heat pumps work best with large radiators and underfloor heating systems that can heat a home slowly over a longer period of time. The low-temperature heating also means that a house needs to be well insulated to be heated effectively by an air source heat pump.
Advantages and disadvantages of air source heat pumps
When evaluating whether an air source heat pump is the right choice for a property, it’s important to understand the pros and cons of this type of heating system.
Air source heat pumps offer the following advantages:
- Air source heat pumps have lower carbon emissions than conventional heating appliances (when powered by renewable energy sources, they can have zero carbon emissions);
- They provide a highly energy-efficient method of heating a home and providing hot water;
- The cost of an air source heat pump is partly covered by the Renewable Heat Incentive grant (more on this below);
- Running costs are roughly equivalent to other home heating systems and cheaper than older systems;
- Little maintenance is required and they last a long time;
- Air source heat pumps are easier to install than ground source heat pumps;
- Air-to-water pumps can provide both heating and hot water.
At the same time, if you’re considering installing an air source heat pump, you should be aware of these disadvantages:
- They take up outdoor space and can be noisy (both of these factors vary with different models, which will depend on the size of the home, its heating system, and how well insulated it is);
- Air source heat pumps are more expensive to install than a conventional boiler;
- As they require electricity to operate the fans and compress the refrigerant, they cannot be considered to be truly carbon neutral (unless you power them using renewables);
- Air source heat pumps lose some of their energy efficiency in very cold conditions;
- These systems heat homes and produce hot water at lower temperatures than conventional boilers, meaning that larger radiators, underfloor heating systems, and good insulation are required to make the most of them.
The cost of air source heat pumps
Installing an air source heat pump is usually more expensive than a new oil- or gas-fired boiler. It’s useful to break the costs down into the installation (including parts and labour) and running costs.
The installation process is the costly part. To install an air source heat pump in the UK, the total amount would usually be around £7,000 to £11,000. The components should cost between £4,000 and £8,000 depending on the size of system your home requires, and the rest of the cost is for the installation itself.
There may be additional costs incurred due to the need to adapt the home. For example, you might need to add underfloor heating and switch out radiators for larger ones – in some cases, you may need to improve the insulation in the home.
Air source heat pumps are an energy-efficient way to heat a home under most weather conditions in the UK. With this in mind, the running costs aren’t too high despite the need to compress and heat the refrigerant using electricity.
For an average four-bedroom detached house, the difference in running costs by comparison to the current heating varies considerably depending on which type of system it has:
- Air source heat pumps are around £920 to £1300 cheaper to run per year than old LPG boilers or electric storage heaters;
- They are between £315 and £560 less expensive to run each year than old gas boilers, new electric storage heaters, and new LPG boilers;
- Running an air source heat pump costs roughly the same as a new gas or oil boiler (in some cases slightly more).
Air source heat pump grants
Despite the high installation costs, it is worth knowing that grants are available in the UK for houses that are heated using air source heat pumps. The Renewable Heat Incentive (RHI) provides payments of up to £1,300 per year for seven years after the installation of an air-to-water heat pump (air-to-air systems are not eligible). If a participating installer is used, these payments can go directly to them to reduce the upfront cost.
How efficient are air source heat pumps?
To understand the energy efficiency of an air source heat pump, you need to know its Seasonal Performance Factor (SPF) – this will vary between different units and depending on the outside temperature. A typical model has an SPF of around 3.2 when the air around it is at 7°C: for every kWh of electricity required to operate the fans and compress the refrigerant, it will produce 3.2kWh of heat.
In the average UK weather conditions, air source heat pumps are highly energy efficient. Despite fluctuations throughout the year as temperatures drop, they are an efficient method of heating a home and produce lower carbon emissions than conventional boilers and electric storage heaters.
Are air source heat pumps noisy?
The noise levels produced by air source heat pumps vary depending on the size, the quality of the components and installation, and the outside temperature. In general, they shouldn’t make more than a low whirring sound if working effectively. Modern models are designed to make even less sound, with thicker cases and quieter fans.
Measured from next to the fans, air source heat pumps can produce a sound of between 40 and 60 decibels. To put this into perspective, an average conversation at home is around 50 decibels, conversation in an office is typically around 60 decibels, and a dishwasher generates a noise of around 80 decibels.
Is planning permission required?
In the UK, the need for planning permission or otherwise depends on the level of noise that your air source heat pump will generate: to avoid planning permission, a pump must produce no greater than 42 decibels of noise measured from 1m outside of the window of the closest neighbouring property. The manufacturers and installers should be able to advise on whether this will be the case prior to installation depending on how far away neighbouring houses are.
There are other conditions to be met, although all of these will apply in most cases:
- The air source heat pump must be at least 1m away from the property boundary;
- The volume of its compressor unit must not be greater than 0.6 cubic metres;
- Only one installation of an air source heat pump is allowed (and not if there is an existing wind turbine).
Visit the Planning Portal’s guide to planning permission for air source heat pumps to learn more. Always discuss the installation with the local planning authority before making any plans.
This guide has covered all of the important information about air source heat pumps, from the way they work to the costs of installing and running them.