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How Does a Heat Pump Work? UK Guide 2025
What is a heat pump?
Simply put, a heat pump is a device that transfers thermal energy from one point to another. A common example of this can be found in our refrigerators where the evaporation and cooling of a liquid is used to lower the temperature of the storage compartment.
Nowadays, this technology can also be used to extract the potential thermal energy from the air outside, or even from the ground, feeding it directly into your home where it is compressed, and the heat transferred using a series of coils. It sounds like magic but the science behind it is relatively simple.
In a nutshell, an air source heat pump, for example, works by extracting heat from the air outside using a fan and refrigerant, amplifying it via a compressor, and then feeding it into your house using your central heating system. This process works on a cycle that is powered by a relatively low amount of electricity in relation to the heat generated.
Here is the step-by-step process of how an air source heat pump works:
- Draws in air: The heat pump draws in air from outside, no matter what the temperature is outside.
- Heats the refrigerant: The air is passed across a heat exchange surface, which heats a refrigerant liquid and turns it into a gas.
- Compresses the gas: The gas is then moved through a compressor, which increases the pressure and temperature.
- Transfers heat: The heated gas is passed across the internal heat exchange surface, where it transfers heat into your home's central heating system or hot water.
- Cools the gas: As the heat is transferred, the gas cools and turns back into liquid form.
- Repeats: The liquid is pumped back outside to start the cycle all over again.
Heat pumps can be up to three times more efficient than gas boilers. They can also be cheaper to run than some conventional heating technologies. Even in cold weather, heat pumps can still work effectively. They can take heat energy from the air or ground, even when it's very cold outside.
Although they use a small amount of electricity to run, heat pumps are considered highly efficient and clean because they don’t depend on burning fuel to create heat. They perform well in moderate climates such as the UK and can provide heating during the winter, even at temperatures of -20 degrees Celsius.
It’s also possible to combine heat pumps with other heating systems, like gas in hybrid configurations.
There are three main kinds of pump depending on the type of medium, or source they extract heat from, namely air, ground, and water, and they are now beginning to be incorporated into modern builds on a regular basis.
Find out about the different types of heat pumps here.
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How do Heat Pumps Work?
Heat pumps can be an efficient way to heat your home. Although there are different types of heat pumps, they all basically work in the same way. They transfer heat from the outside environment into your home or office via a four-step process, evaporation, compression, condensation, and expansion.
Heat is taken from the air, ground, or sometimes water by the heat pump and transferred to a heat exchanger that contains a liquid refrigerant. The heat absorbed by the refrigerant evaporates and is turned into a low-pressure, low-temperature gas. This gas is transferred to an electrically powered compressor that compresses the refrigerant. The compression causes the pressure of the gas to increase, which raises the gas temperature. Once the hot gas reaches the heat exchanger, it’s circulated and transfers its heat to a cold-water circuit. As the water absorbs the heat from the gas it heats up.
When the water reaches the desired temperature, normally around 55 degrees, it is sent to your home’s radiators and underfloor heating to warm your house. Because the heat is transferred to the water circuit, the refrigerant cools down enough to turn it back into a liquid. When the cooled refrigerant moves through an expansion valve the pressure is lowered which allows it to absorb more heat energy. It’s then pumped back into the heat exchanger to repeat the cycle.
How does a heat pump work in the UK?
On the whole heat pumps can work extremely efficiently in the UK. They can offer 300% - 400% efficiency through most of the year, although, in the case of air source heat pumps, this figure will start to drop off when the outside temperature drops below 0°C. Most air source heat pumps will work at around 200% efficiency down to -10°C and often boast attractive efficiencies down to -15°C to -25°C.
Heat Pump Technology
On the outside of an air source heat pump, you will find a system of coils with refrigerant in it over which the outside air is drawn using an impeller or fan. The liquid in the refrigerant coils absorbs all the heat and because it warms up, begins to evaporate. This gas then goes through a compressor that increases the temperature dramatically. The coil then moves inside the building towards some inner coils where the heat is released. The refrigerant meanwhile flows back outside where it picks up more heat and the process starts all over again.
The useful heat produced is pumped into air ducts and can be transported to various locations around the building or can be used to heat water that feeds into radiators. Ground source heat pumps work a little differently, getting their heat source from the ground outside or a body of water.
These systems use either closed loop for the ground or open loop piping for water. A closed-loop ground-source heat pump uses a continuous supply of refrigerant to perform the same sort of action as an air-source heat pump whilst an open-loop ground-source heat pump uses the water from a well or lake to do the same job.
For larger-scale installations, you might like to opt for an absorption heat pump that works in the same way as an air source one but uses ammonia instead of a refrigerant. These are more often found in industrial installations, but recent developments have seen them being utilised for big commercial and domestic properties.
The other major component of any heat pump is the ducting that transfers the heat from the main source to the rooms in the house. The ducting is often the problem when it comes to retrofitting a house as it can be invasive and expensive to install. For this reason, some domestic properties go for mini-split heat pumps that are less problematic to install.
One of the newer initiatives in heat pump technology is the development of thermodynamic panels that transfer both the heat from air and solar radiation to warm up your water and rooms. These can generally be fitted on the side of a house but can also be found on the roof, looking a lot like solar panels.
Types of Heat Pumps Available and How They Work
If you’ve read our other pages, you’ll know all heat pump heating systems work the same way. They extract heat from an external source, increase that heat, and then move it around. The only real difference between them is where they source the initial heat and how they share it. Heat pumps are not a new technology and therefore their application is relatively widespread. As such, you can find lots of little variants, but the following represent the most common systems.
- Air to air
- Air to water
- Water source
- Ground source (also known as geothermal)
Air-to-Air Source Heat Pumps Explained:
An air-to-air source system is the easiest to explain because we’ve all seen these devices in action. Indeed, you’re likely to have used one in your life because they’re also known as air conditioners. It’s a lesser-known fact that air conditioners can also provide heat. In the simplest of terms, these machines extract heat from the outside air by using a special fluid. Then, during the next stage in the circuit, the device makes the fluid hotter still by compressing it. The increased heat is then transferred to internal air and circulated around the home as heating. At this point, the refrigerant returns to liquid form. Then it makes its way back to the start and begins the process anew. As you may have guessed air conditioners follow the same process but in reverse. They extract heat from internal air before swapping it for cooler air from outside.
The system works because of the laws of thermodynamics. So, we know that even very cold air still contains heat. With the right device then, it’s possible to extract heat from air with temperatures as low as –20c. Because of the simplicity of these devices, Air-to-Air heat pumps are very common. In fact, they are so common, that some systems include additional individual air-to-air units within them. These can heat or chill a specific room or area in a house like a cold room or a larder.
Air-to-air units often work alongside other styles of heat pumps. Some houses have small individual air-to-air heat pump systems in different rooms rather than using a single system. It’s not uncommon to find air-to-air heat pumps using vast tracts of ducts to distribute heat exactly where it’s needed. Other smaller systems though are self-contained with almost no pipes at all and simply exchange heat through a single unit.
Air-to-Water Source Heat Pumps Explained:
Like air-to-air source heat pump systems, air-to-water source heat pumps extract heat from the air outside. Here though, is where things change. Unlike air-to-air heat pumps, air-to-water heat pumps distribute the extracted energy in the form of heated water. This means, of course, that the system needs a series of pipes and radiators to effectively and efficiently spread the energy around. In most cases, there’s no need to replace existing pipes and radiators. However, heat pumps operate at a lower pressure than traditional central heating, and updating the pipes can help. Thanks to the requisite infrastructure, air-to-water heat pump systems often operate as a single, multi-room circuit. You won’t find many, if any, systems designed to heat a single room.
Water-Source Heat Pumps Explained:
Water-sourced systems, as the name implies, work by extracting heat from a water source rather than from the outside air. Like air, external bodies of water always have residual heat. In fact, water holds more heat than external air. These systems can be site-specific as they need to be near a suitable body of water. The water source could be a canal, a local lake or reservoir. To function efficiently, water-sourced heat pump systems must be close to the water source. Otherwise, they can lose heat through expansive lengths of pipe. As with the other heat pumps, we’ve discussed, the heat transfer happens in the same way.
Fun fact - when Canary Wharf first opened its doors in the ‘80s, the exhibition centre for the development had a water-sourced heating system. This heat pump extracted heat from the water in the docks that surrounded the complex, compressed it and circulated it around the arena.
Ground-Source Heat Pumps Explained:
Often called geothermal heat pumps, ground-sourced heat pumps work like water-sourced heat pumps. The difference is that they extract heat from the ground outside rather than water. Like the source that water-sourced heat pumps draw their heat from, the ground temperature is consistent. This means ground-source heat pumps can offer the same or better efficiency as water-sourced circuits. In addition, they don’t need a nearby expanse of water. The downside to ground-source systems is the price. Because they extract heat from underground, they need a lot of preparation. This can involve drilling deep boreholes or laying extensive pipework outside the property. Maintenance can also be expensive for the same reasons.
Things to Consider when Installing a Heat Pump:
The age of your property:
If you are living in an old house, you may not have adequate wall and ceiling insulation which can lead to substantial heat loss and make it difficult for a heat pump to maintain a comfortable temperature. Your heat pump’s efficiency can be further impacted by significant drafts from leaky windows and doors. Your radiators are more likely to not be compatible with the lower water temperatures produced by a heat pump. Older homes may have limited space for outdoor unit placement or for running necessary pipework, potentially complicating installation. If you live in a historic or listed building the appearance of an external heat pump unit might not be aesthetically pleasing. If extensive renovations are needed the cost of the installation may be prohibitive.
Insulation and double glazing:
All homeowners must check how well their home is insulated paying particular attention to wall and roof insulation. In some cases, triple glazing may be recommended to prevent heat loss.
Radiator size:
It’s important to check whether your radiators are going to be big enough for your needs. Your installer will advise you on whether you need triple glazing for your system.
The size of your property:
The size of your home will determine the size of the heat pump system you require. For example, a 100 sq. m house might need a 4kW heat pump, while a 200 sq. m house might need an 8kW or 16kW pump.
Enough space to house the unit:
Adequate space is required for the external unit.
If your home is not adequately insulated or you live in an older property where insulation or radiator upgrades wouldn't be feasible, then a high-temperature heat pump could be a better alternative.
The Pros and Cons of Heat Pumps
Whilst they are not technically renewable technologies because they depend on a little electricity from the mains, heat pumps are a highly efficient way of keeping your house or office building warm. For the UK, where the climate is fairly moderate, heat pumps can work all year round even at the lower temperatures found in our winters. The problem for older buildings is that heat is produced at a lower level than in other systems which means that properties that are prone to draughts and are not properly insulated will struggle to keep the right ambient temperature.
Generally, if you have the right property, a heat pump will save you money over oil or gas-based heat sources. For instance, an air source heat pump could save you around £440+ a year, though there are a lot of variables involved, not least the type of system you choose and the kind of property you have.
The UK government is on a big drive to push heat pump technologies right now. Indeed, they plan to phase out traditional boilers by 2050. So, now is not only a good time to ask if a heat pump is right for you, but it may also be the last chance to go another route.
To help you make up your mind we’ve put together a simple pros and cons list.
Advantages of Heat Pumps:
Running cost
The amount you can save on running costs will depend on what type of heating system you are replacing. You can make substantial savings if you are switching to a heat pump from an oil, LPG, electric boiler system or an incredibly old gas boiler system. There isn’t, however, an enormous difference between the running costs of a heat pump compared to an efficient gas boiler as heat pumps run on electricity which is more expensive than gas currently. If you can remove your gas standing order or better yet, power your heat pump from your own solar cells the savings will be substantial.
Efficiency
Once operational, heat pumps will outperform fossil fuel boilers in every conceivable metric.
Emissions
Another no-brainer is that because heat pumps only extract existing heat the other emissions generated come from the small amount of electricity required to run the fans and pumps.
Low maintenance
Although more complex to install, and we’ll get to that, the actual mechanics of a heat pump system are very simple and require far less tinkering than traditional central heating.
Heating and cooling
That’s right, most heat pumps can heat or cool your home. This is something your old boiler just can’t compete with.
Safety
Because there’s no combustion, there’s little to no risk of fire from toxic fumes from a heat pump. This can also have a positive effect on your insurance costs.
Grants and subsidies
As the government has identified heat pumps as a viable way to reduce national emissions it’s keen to support their installation. You’ll find lots of grants, loans, and other subsidies to help with the cost of your installation wherever you live. The Boiler Upgrade Scheme (BUS) is the main UK government scheme to provide grants to help property owners in England and Wales replace fossil fuel heating systems with more efficient, low-carbon heating systems.
The BUS offers grants of up to £7,500 for air-source heat pumps (ASHPs) and ground-source heat pumps (GSHPs).
You can check whether you are eligible for this grant and find out more about it by contacting us at the following link: https://www.renewableenergyhub.co.uk/search-installers
Disadvantages of Heat Pumps:
Upfront cost
As discussed, the price of a heat pump system can be a double-edged sword. They're cheaper to run than many traditional heating systems but expensive to fit. However, as mentioned there’s lots of help available if you know where to look.
Complex to install
Some heat pumps are simple to fit but ground-based systems in particular are the opposite. If you’re considering a ground-sourced heat pump, be prepared for some pretty hefty disruption.
Ambient noise
While the technology is improving it’s worth remembering that heat pumps do require pumps and fans to work. These mechanisms do, of course, make a noise. Like we said sound levels are improving, and you may never hear them, but it’s something to consider if you have sensitive hearing.
Sustainability
We’ve put a question mark here because although the technology is itself considered to be renewable, there are questions about how they are manufactured.
Some like it hot!
If you’re a fan of turning up the dials and basting in the glow of a roaring fire, you may want to skip a heat pump installation. Heat pump systems are better suited to providing consistent comfortable conditions rather than turning things up to 11.
Are Heat Pumps Worth it?
Heat pumps are increasingly worth it in the UK due to their energy efficiency which often provides a financial incentive, the Boiler Upgrade Scheme, and environmental benefits. Heat pumps are far more efficient than gas boilers, generating three to five units of heat for every unit of electricity they use.
Unlike traditional heating systems, heat pumps run on electricity rather than fossil fuels. This means that as electricity is currently three times more expensive than gas in the UK, running a heat pump often costs about the same as running an efficient gas boiler system. However, significant savings can be made if you switch from oil, LPG, electric boiler systems or an old gas boiler system.
Additional savings can be achieved by combining a heat pump with a solar panel (PV) system, although it's not the perfect marriage. You need heating in the winter when the days are shorter and darker and your solar panels aren't generating as much. This said, solar PV will still generate electricity even in the depths of winter, so a well sized solar PV system will help cut the running costs of a heat pump.
Some models of heat pumps have the added benefit of providing both heating and cooling, making them particularly attractive for the varied climate in the UK.
The upfront installation cost can be higher than for traditional systems even with the £7,500 Boiler Upgrade Scheme. That said, heat pumps can certainly work well in the UK and in many instances can help to reduce your energy bills, whilst lowering your carbon footprint.
Although you won’t see a vast difference in the costs of a heat pump versus a traditional gas boiler system if you can remove the need for your gas standing order and even better than this, run your heat pump from solar power you have generated yourself, then substantial savings can be made. Removing the gas standing order alone will save you at least £230 a year.
We've been experiencing a large increase in requests for information about heat pumps. Many people are looking to install a heat pump to future-proof their property. As the UK moves away from fossil fuels, there's an increasing likelihood of carbon taxes and pressure on the gas infrastructure, leading to higher running costs for fossil fuel boilers. Currently, homeowners are not mandated to replace their gas boilers and it doesn't seem like they will be any time soon. This said, the government will need to continue low-carbon initiatives in order to achieve our net-zero targets by 2050.
Heat pump efficiency in colder climates has improved, making them more suited to our winters, comparable in running costs to mains gas boilers, and a great option to oil, LPG, and electric boiler systems.
Find out if a heat pump is right for your property and check out our How much do heat pumps cost article for further information.
How Does a Heat Pump Work? UK Guide 2025
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