Air source heat pumps (ASHPs) absorb heat from the outside air to heat the property and the hot water even in ambient air temperatures is as low as -15°C. They operate on electricity, but because they extract renewable heat from the outside, the heat output is greater than the electricity input.

How air source heat pumps work

Air source heat pumps use the same type of technology that keeps a fridge cold, but in reverse. A liquid refrigerant that has an extremely low boiling point is pumped on a loop between two heat exchangers. This refrigerant takes on heat energy from the outside ambient air temperature and turns into a gas as it warms up. This gas is then compressed back into a liquid, which increases its temperature further. 

The warm refrigerant then goes through a heat exchanger, which transfers the warmth to a separate body of water for circulating round the central heating system. During the final stage, the liquid refrigerant goes through an expansion valve reducing the pressure and temperature and the cycle repeats. They include an internal defrost cycle system ensuring operation in all seasons.

Coefficient of Performance or COP is why heat pumps are a useful technology, so if 1kWh of energy is put into a system, more than 1kWh of heat energy will be produced. Usually, a heat pump has a maximum CoP of 2 or 3, meaning for 1kWh of electricity you will get 2 or 3 kWh of heat. The ratio of ‘electricity in’ to ‘heat out’ will change over the course of the year according to the climate.

Heat pumps also work more efficiently when there are gradual rather than sudden temperature demands on the system and so need to be controlled differently from traditional central heating systems

Types of ASHP

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 are the most common model in the UK and work by absorbing heat from the air outside and transferring the heat to water. This system distributes heat via your wet central heating system and work much more efficiently at a lower temperature than a standard boiler system. 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
    Air-to-air heat pumps require a warm air circulation system to move the warm air around the property. They will not provide you with hot water as well and are not eligible for the UK Government’s Renewable Heat Incentive (RHI) scheme.
  • Other types of ASHP

If the property does not meet all the criteria for a heat pump to operate at optimum efficiency, it may still be possible to heat it 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 would kick in.
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 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 will not 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.

The benefits of air source heat pumps

  • Lower fuel bills, especially if replacing conventional electric heating.
  • Income through the UK government’s Renewable Heat Incentive (RHI). (air-to-water)
  • The return on investment against oil and LPG heating systems can take as little as five years for residential dwellings.
  • Lower carbon emissions.
  • No fuel deliveries needed.
  • Can heat the property as well as the 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 the property efficiently. You will also notice that radiators will not 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 really also upgrade your insulation to get the most out of this system. Fitting larger radiators or underfloor heating would also be an advantage.

Deciding whether an air source heat pump is a suitable choice

To tell if an air source heat pump is right for a particular property, there are a few key points to consider:

  • Location:- There needs to be a place outside the property 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.
  • The type of heating system used:- ASHP do particularly well with underfloor heating systems or warm air heating because they operate at low temperatures. Properties without an existing central heating system will require one to be installed for an air source heat pump to work.
  • Insulation:- Because ASHP perform best when producing heat at a lower temperature than traditional boilers, it’s important that the property is well insulated and draught-proofed to minimise heat-loss.
  • Which fuel is currently used:- The system is more likely to pay for itself if it is replacing an expensive system like electric heating. Currently, you would be ill-advised to switch from mains gas as there would not be a tremendous saving on the heating bill.

Cost & Savings

The main cost of a heat pump is the upfront payment for the product and its installation. The pump will use a small amount of electricity, but the running costs are minimal especially if solar panels are also connected. Installing a heat pump is not especially disruptive work, but it would make sense to carry out this work at the same time as other property renovations.

The price of a full installation can vary considerably depending on the area of the country and the installer you choose. An ASHP costs more than a new gas or oil-fired central heating system and the typical cost ranges from about £6,000 for a flat requiring 5Kw output to about £16,000 for a detached house requiring possibly 9Kw. The main factors are:-

    • The output required
    • Size of the property
    • The current level of insulation
    • The current heat distribution system
    • The size of the pump
    • The brand of the pump 
    • The pump’s performance and efficiency 
    • The room temperature required
    • Simple or advanced controls (e.g. a weather-compensated thermostat).

Depending on the current state of the property, you may also need to allow for the possible costs of upgrading radiators and the attendant pipework, installing underfloor heating and extra insulation. 

The best way to get an accurate quotation is by way of a professional survey, which is normally a free service absorbed by the installation charges.  

The running costs of an installed system will vary depending on the size and insulation levels of the property, what type of distribution system is installed, the room temperature required, as well as the CoP of the system.

The payback time (how long it takes to recoup the cost of the system in energy savings) depends on:

    • The efficiency of the system (including how well the heat is dispersed and the insulation level in the property)
    • The type of fuel and heating system being replaced
    • Income from the Renewable Heat Incentive (RHI) 
    • The ongoing price paid for electricity
    • How the generated heat is used within the property.

The likely savings made each year in an average sized, four-bedroom detached home at current figures range from about £400 to £1,300 a year depending on what type of system is being replaced. However, be aware that if replacing a modern high efficiency heating system, an ASHP could work out to be more expensive.


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 but with regular scheduled maintenance, you can expect an ASHP to operate for 20 years or more.

You should check annually that the air inlet grill and evaporator are free from leaves or other debris as well as removing any plants that start to grow near the heat pump. You should also check the central heating pressure gauge in your house from time to time.

The installer should provide you with written details of any other maintenance checks you should undertake to ensure everything is working properly, but a professional should service the heat pump every two to three years.

Antifreeze can be used to prevent the heat pump from freezing in cold winter weather and 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. If your domestic installation unusually has external refrigeration pipes these will need to be serviced annually by a refrigeration engineer. 

Planning permission

Before starting any work, check with the planning authorities about the rules concerning the siting and external appearance of the ASHP.

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