Skip to main content
Energy at home

In depth guide to heat pumps

A heat pump captures heat from outside and moves it into your home. It uses electricity to do this, however the quantity of heat delivered into your home is greater than the quantity of electricity used to power the system.

As a heat pump captures heat that is already present in the environment, the system itself emits no carbon dioxide emissions.

How does a heat pump work?

Everything around us contains thermal energy – or heat. Heat naturally flows from a warmer place to a colder place. To use 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 fluid increases, the temperature of the fluid increases. When the pressure decreases, the temperature decreases. This relationship between pressure and temperature is the key to how a heat pump works.

The fluid is typically referred to as a refrigerant. The heat pump uses electricity to compress this fluid, increasing the pressure and therefore the temperature.

As the fluid’s heat is transferred to your home, it cools down a little. The fluid 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 can then be used in your home’s central heating system (in an air-to-water or ground-to-water heat pump) or passed into hot air blowers (in an air-to-air heat pump).

How efficient are heat pumps?

Heat pump efficiency is determined by the amount of heat energy provided for every unit of electrical energy the heat pump uses. This is known as the Coefficient of Performance (CoP). The CoP is calculated by taking the heat energy provided and dividing it by the amount of electricity used to deliver that heat.

The CoP is measured under specific test conditions. However, because in real life the heat pump experiences temperature variations throughout the year (i.e., with ground or air temperatures rising and falling), the CoP is not always helpful in understanding what the actual cost of running the heat pump will be, or its ‘real world’ efficiency.

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. Heat pump installers must calculate the SPF based on their system design for your home. The installer should tell you about the SPF before beginning any work. While it cannot be completely accurate, the SPF should give you a better indication of what to expect in terms of efficiency than the CoP.

Will installing a heat pump help save money on my heating bills?

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.

Average heat pump efficiencies in the UK tend to vary between an SPF of 3 to 4, with ASHPs tending towards the lower end and GSHPs towards the higher end of the scale.

Designing and operating your heat pump system

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.

Do I need permission to install a heat pump?

Before installing, 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, and it is best to check with your local planning department before proceeding, especially if you are 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. Ask your installer to do this for you, as they will have all the information required and are best placed to complete the forms.

Getting the most out of your heat pump

Electricity tariff

An electricity tariff is how your energy supplier charges you for the electricity you use. It’s typically made up of a price per unit (kWh) of electricity you consume, and a daily standing charge. Because heat pumps use electricity, finding a low-cost electricity tariff can help keep your running costs low. We have more information about choosing an electricity supplier and tariff.

What kind of heat pump is right for me?

Air-to-water heat pumps are the most common type of domestic heat pump in the UK and are suitable for many types of home.

If you have a garden or large outdoor space, you may be interested in finding out more about ground-to-water heat pumps.

If you don’t have radiators or underfloor heating, and can’t or don’t want to install these, you might be interested in finding out more about air-to-air heat pumps. Air-to-air systems are more commonly associated with smaller properties such as flats and park homes.

You can find more information on the differences between ASHPs and GSHPs in our blog.

Latest blogs

Go to blog

Air source heat pumps vs ground source heat pumps

12 October 2021

We took a look at the key differences between the two most common types of heat pumps - air source and ground…

Applying energy efficiency principles in your new kitchen

5 October 2021

Are you thinking about a new kitchen? If so, it might also be the time to consider replacing some of your old…

Powered by sunshine: why electric vehicles and solar panels are a perfect match

9 September 2021

We spoke to head of services delivery Matt Fraser to find out why solar panels and electric vehicles are the future of…

Last updated: 14 October 2021