How Air Source Heat Pumps Work

Everything around us contains thermal energy – or heat. Heat naturally flows from a warmer place to a colder place. To provide the heat energy in a home when outdoor temperatures are colder, we need heat to flow in the other direction – from a colder place to a warmer place. But how does it do it?

When the pressure of a gas increases, the temperature also increases. When the pressure decreases, the temperature decreases. This relationship between pressure and temperature is the key to how a heat pump works.

The gas is called a ‘refrigerant’ and a heat pump uses electricity to compress this refrigerant, increasing the pressure and therefore the temperature. 

As the refrigerant’s heat is transferred to your home through the heat exchanger, it cools down a little. The refrigerant is then allowed to expand so that it cools even further. It’s now cold enough to absorb more heat from outside and begin the process again.

The heat delivered to the heat exchanger can then be used to heat your home. This would normally be done using a central heating system – but it could also be done using warm air in either an air-to-air heat pump or an exhaust air heat pump.

With an air source heat pump (ASHP), the cold refrigerant starts its journey in the outside unit (called an evaporator). 

It absorbs heat energy from air blown across a heat exchanger, using fans. Although the air is cool in the winter, there is plenty of energy available because of the large volume of air that passes over the heat exchanger.

How efficient are heat pumps?

Heat pumps are more efficient than other heating systems because the amount of heat they produce is more than the amount of electricity they use. The amount of heat produced for every unit of electricity used is known as the Coefficient of Performance (CoP). So, if a heat pump has a CoP of 3.0, then it will give out three units of heat for every unit of electricity consumed. 

The diagram below shows the basic energy flow of a 14-kilowatt (kW) heat pump to help show how the CoP is calculated. In this example, the heat pump has an electrical power input of 3kW and a heat output of 14kW. The remaining 11kW are obtained from the environment. To calculate the CoP, you divide the heat output by the electrical input, which in this example results in a CoP of 4.7.  

Every heat pump has a published datasheet telling you what its measured CoP is. The CoP is measured at a single point in time, under specific test conditions. However, in real life the heat pump experiences temperature variations throughout the year (ie with ground or air temperatures rising and falling throughout the seasons), so the CoP is not always helpful in understanding what the cost of running the heat pump will be, or its ‘real world’ efficiency over the course of the whole year.

Instead, the Seasonal Coefficient of Performance (SCoP) or Seasonal Performance Factor (SPF) is used to show the efficiency of the heat pump across the whole year. Our  installers will  calculate the SPF based on the system design for your home. This calculation demonstrates how the heat pump should perform given the average temperatures at your location and other details such as the size of your radiators. We will then share this calculation with you before beginning any work. The SPF will give you a better indication of what to expect in terms of running costs and efficiency than the CoP figure.  

While the compressor and pumps need electricity to work, they use less than the quantity of heat they move from outside to inside. The amount of heat energy moved versus the amount of electrical energy used depends on the source temperature and the output temperature, so it varies constantly throughout the year as outside temperatures change.

How this will affect your energy bill will depend on several factors, including:

  • What fuel you are replacing and how much it costs.
  • Your electricity tariff.
  • Which type of heat pump you install and how efficient it is.
  • The design of your central heating system.
  • Your location and its average air or ground temperatures throughout the year.

For people using gas boilers (not LPG or oil boilers), heat pumps are likely to be slightly more expensive to run unless particular attention is paid to ensuring maximum efficiency of the heat pump in the heating system by using best practice radiator / underfloor heating design. However, as utility prices fluctuate over time, we expect that heat pumps will become the cheapest as well as the lowest carbon form of heating available.

For those on LPG or oil, annual variations in prices mean that it’s difficult to give an exact estimate of annual heating costs. For example, heating oil has typically fluctuated between 40-65 pence per litre over the last five-year period, with a general upward trend in price. Unless you can buy oil or LPG at the very cheapest time of the year to cover your entire annual use, heat pumps should save you money on running costs assuming a well-designed system is installed.

The compressor in a heat pump works harder when there is a larger temperature difference between the outside source temperature and the water temperature needed in your radiators or underfloor heating. The less the compressor needs to work, the less electricity the heat pump uses.

While we can’t control the outdoor source temperature, it’s possible to design heating systems that use low temperature water indoors, meaning the heat pump can use less electricity and still heat your home comfortably.

By using radiators with a larger surface area, or underfloor heating, more heat can be delivered into the room without increasing the water temperature. Running the heating system for longer is another way of delivering more heat into the room with lower temperature water.

If you have radiators with a smaller surface area, then the heat pump will have to run at a higher temperature. This means the compressor is working harder to deliver the same amount of heat as it would with larger radiators, or if it had a longer time to run. When the compressor works harder, it uses more electricity, which makes the system more expensive to run.

The aim of a well-designed system is to reduce the heating water temperature as much as possible. The closer the required temperature is to the source temperature (ie the outside air or ground temperature), the more efficient the heat pump will be, and therefore the lower the running costs.

A standard air-to-water or ground-to-water heat pump needs to be able to store hot water for when you need it. The size of hot water cylinder required will depend on the volume of hot water that you need, but often it can be fitted inside a cupboard measuring 80x80cm. A hot water cylinder allows the heat pump to gradually heat the water, with the cylinder storing the hot water for when you need it.

While most heat pumps can provide water at 55°C, hot water in the cylinder will need to periodically reach 60°C or higher to kill harmful bacteria. While some heat pumps can deliver water to this temperature, most systems are designed to use an immersion heater (an electric heater within your hot water cylinder) to top up the temperature to the required level.

An air-to-air heat pump does not generate hot water, so you will need to consider an alternative way of heating your water, such as using an immersion heater.

If you don’t have space for a hot water cylinder, you still have options. Some hybrid systems are designed so that the heat pump provides space heating, while the boiler provides hot water instead.

You could also consider installing a heat battery, which takes up less space than a hot water cylinder. Instantaneous hot water heaters are also available and can be installed under your kitchen sink to provide a smaller volume of hot water.

Conventional fossil fuel boilers were typically designed to deliver water to your radiators at 75°C – though modern condensing boilers should ideally run at lower temperatures to run as efficiently as possible. In comparison, a heat pump would ideally circulate hot water to the radiators or underfloor heating at temperatures between 35°C and 45°C, depending on the outside temperature. This is when the heat pump will work most efficiently with the lowest running costs.

Fitting underfloor heating after the house has already been built can be challenging. Instead, you may opt to install a heat pump and continue using existing radiators at a higher temperature or install radiators with greater surface area to provide enough heat using lower water temperatures.

Radiators come in many different shapes and sizes. Often single panel radiators can be swapped for double or triple panel radiators to increase surface area, without needing to increase the amount of wall area taken up. Moving from single panel radiators to triple panels can more than triple the heat output without taking up more wall space. Radiator upgrades can be a cost-effective way of improving the efficiency of your heat pump and reducing running costs.

Underfloor heating should be designed to work with water no warmer than 45°C in most cases, so it’s safe to assume that if you have underfloor heating it should work well with a heat pump.

Some wet central heating systems installed in the 1970s have a particular type of pipe called ‘microbore’, which has a small internal diameter. This can be problematic when upgrading to a heat pump. The very small pipe diameter means the heat pump cannot transfer water quickly enough to the radiators. If you think you have microbore pipe, speak to your installer when they come to survey your house.

Your heat loss survey will calculate the energy required to heat your home by considering its size and the level of insulation and draught-proofing it has. Improving the insulation of your home makes your home more comfortable and reduces your heating costs.

If your home is well insulated, each room needs less heat to stay warm, so the radiators can provide this heat using lower temperature water, allowing the heat pump to run more efficiently.

This saves energy and money in two ways: by reducing your heating requirement and increasing the efficiency of your heat pump’s output.

Another benefit is that reducing your overall heating need could mean you require a smaller sized heat pump, with lower running costs and lower purchase cost than a larger heat pump.

Draught-proofing can also reduce your overall heating need by reducing heat loss through uncontrolled ventilation.

In some homes, increasing insulation may not be practical or cost-effective. Insulation installers can visit your home and advise where rooms could be improved with appropriate insulation, and if you live in an older home built before 1919, or in a listed building or conservation area, we recommend speaking to insulation specialists with experience of working with these types of properties.

If you can’t install insulation, or increase your radiator size, there are alternatives available. 

Before installing a heat pump, it’s important to check if you need to apply to your local planning authority for permission. Most heat pump installations are considered ‘permitted developments’, meaning no permission is required. However, there are exceptions, such as installing a cascade system and it’s best to check with your local planning department before proceeding, especially if you live in a listed building or conservation area.

Find out more about getting permission.

You should also inform your local district network operator (DNO) that you are planning to install a heat pump. The DNO is the company responsible for bringing electricity from the network to your home. You can ask your installer to assist you with this, as they will have all the information required to complete the forms.

To find out more about the process for notifying a DNO of your heat pump installation, you can refer to the ‘Connecting electric vehicles (EVs) and heat pumps’ section in this guidance from the Energy Network Association.

Further guidance on registering a heat pump (as well as similar devices including solar panels, electric batteries, and electric vehicle chargepoints) in England, Scotland and Wales can be found on the UK Government website.

Getting the most out of your heat pump


To ensure you’re getting the highest performance from your heat pump, it’s important to use your controls effectively. Depending on the controls you have, and the level of heating and hot water you need, there are different ways to get the most from your heat pump. Setting the system controls can sometimes be confusing so the Team at Caplor are on hand to answer any queries you may have about operating your system.


As with any heating system, a heat pump needs to be well maintained to operate as designed – though luckily most heat pumps tend to be easy to maintain with minimal input required from the end user.

With regular scheduled maintenance, you can expect a heat pump to operate for 15 years or more.

Typical checks include a visual inspection of the water pump, external pipes, fittings and electronics. Ground source heat pumps may also occasionally need to be re-pressurised or have the quality of the antifreeze checked, which can be done by a professional every 2-3 years.

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