What are heat pumps and how do they work?
Heat pumps are devices that transfer heat energy from outside to inside. The process involves four main steps: evaporation, compression, condensation, and expansion.
Heat pumps take in heat from the air, ground, or water and transfer it to a heat exchanger that contains a liquid refrigerant. This absorbs heat from the outside and evaporates, turning into gas.
This gas then passes into a compressor which increases the pressure and temperature. The gas is then passed through a heat exchanger, where it condenses and releases its heat to warm water for a heating system.
Once the heat has been transferred, the refrigerant gas cools down and turns back into a liquid, ready to be recirculated around the system.
What types of heat pumps are available?
There are two main types of heat pumps used for residential heating in the UK.
Air source heat pumps (ASHPs)
These pumps absorb heat from the outside air and transfer it to a home heating system. They're the most common type of domestic heat pump in the UK and are suitable for most homes.
Ground source heat pumps (GSHPs)
These pumps extract heat from the ground by circulating a water-antifreeze mixture through underground pipes. They're more efficient than air source heat pumps but are more expensive to install, as they require a large outdoor space for the underground pipes or boreholes.
How much do heat pumps cost to install?
An air source heat pump will typically cost between £7,000 and £13,000 to install. A ground source heat pump will typically cost between £24,000 and £49,000 to install.
How efficient are heat pumps?
For every 1kW of electricity that goes into a heat pump, you can expect up to 4KW of heat. But the efficiency of a heat pump depends on several factors:
Air source heat pumps take heat from the outside air, which can fluctuate between -5°C and 25°C for most of the year in the UK. The colder the source temperature, the harder the heat pump must work, affecting its efficiency.
A well-insulated building will retain more heat, meaning that the heat pump works more efficiently.
Ground source heat pumps are generally more efficient as they draw heat from the ground, which maintains a more consistent temperature throughout the year.
How is heat pump efficiency measured?
There are two key metrics that measure heat pump efficiency: the Coefficient of Performance (COP) and the Seasonal Coefficient of Performance (SCOP).
The Coefficient of Performance (COP) is the ratio of the heating or cooling provided by the system to the amount of electrical energy it consumes.
For a heat pump in heating mode, the COP is calculated as: COP = Heating Output / Electrical Input
In cooling mode, it's calculated as: COP = Cooling Output / Electrical Input
A COP greater than 1 indicates that the heat pump is more efficient than a simple electric heater. For example, a COP of 3 means that for every unit of electrical energy consumed, the heat pump provides three units of heating or cooling.
The COP represents the heat pump's efficiency at a specific point in time.
For air source heat pumps, the COP usually ranges from 2.0 to 5.0, meaning that for every unit of electricity consumed, the heat pump produces 2 to 5 units of heat.
Ground source heat pumps can achieve even higher COP values.
The SCOP provides a more comprehensive measure of a heat pump's efficiency over an entire heating season. It accounts for temperature variations throughout the year, offering a more realistic representation of a system's annual energy efficiency.
A good heat pump COP is generally between 3.0 and 5.0
A good SCOP is typically above 3.5, indicating that the system provides efficient heating or cooling over the entire season.
Several factors can influence both COP and SCOP, including the heat pump model, installation quality, and local climate conditions. The SCOP is most accurate when calculated over a full year, as it accounts for both winter and summer performance. During summer, while the heat pump may need to work harder to heat water, it also has access to more energy from the warmer air.
By understanding these efficiency metrics, installers can offer better advice on the potential energy savings and performance of different heat pump systems.
How a heat pump works differently to a gas boiler
A low-temperature heat pump runs most efficiently at 35°C to 45°C; a gas boiler is typically set to run between 60°C to 75°C, although there are moves to reduce this to 50°C to save energy.
Most households with a gas boiler will be used to turning up the heating and feeling an almost immediate effect. But peaks and troughs of energy use are not always the most efficient way to run a heating system.
With most heat pumps, it’s more about maintaining a more constant, level temperature. Yes, they can still give a heating boost when the outside temperature drops, but you have to put more energy in to do that, so their efficiency is reduced. Think of it a bit like driving a petrol or diesel vehicle. Going faster burns more fuel. You get a better rate of fuel economy when driving at moderate speeds.
With heat pumps, it also helps if the building is well insulated, but that’s true of gas boilers too. The less heat energy you lose, the less energy you have to put in.
Why are heat pumps important for the UK?
Heat pumps are a key part of the UK government's plan to reduce carbon emissions in order to reach net zero by 2050. By electrifying domestic heating and replacing fossil fuel boilers with heat pumps, the UK could reduce its greenhouse gas emissions by 15-17%.
The government’s target is for all new installations to use low-carbon heating such as air-source heat pumps by 2035. It has set a target to roll out 600,000 heat pumps per year by 2028.
This is supported by the Boiler Upgrade Scheme which offers a £7,500 discount to homeowners in England and Wales who are looking to upgrade their fossil-fuel boilers to low-carbon heating. This scheme will be in place until March 2028.
How common are heat pumps in the UK?
Despite their potential benefits, the UK is lagging behind neighbouring countries in heat pump adoption.
The UK has just 412 heat pumps per 100,000 people, compared to higher rates in 21 other European countries.
Numbers of heat pumps installed in the UK have been increasing each year since 2017, including a record 59,900 units in 2022.
In order to reach the stated of target of installing 600,000 heat pumps annually by 2028, it’s estimated that we’ll need an extra 50,000 qualified technicians. That represents a huge opportunity for anyone willing to invest in training and understanding how to install and maintain these heating systems.
Installing heat pumps with other renewable technologies
Heat pumps work well alongside other renewable and energy-saving technologies such as solar PV panels and batteries. In the UK, most households with solar panels will generate the most electricity between April and October which could be used to heat hot water and power appliances. But on sunny winter days, they will still generate power that can be used to reduce heating costs too.
By understanding the different types of heat pumps, their benefits, and installation requirements, electricians can play a crucial role in supporting the UK's transition towards more sustainable and secure heating solutions.
Find out more
If you’re a professional electrician or contractor looking to get into heat pump installations, or if you want some advice on choosing the right products, getting specifications and designs for client projects, our dedicated Renewables team are here to help.
Visit cef.mom/renewables for more information or ask in your local branch.