Complete Heat Pump Guide: What Are Heat Pumps & How Do They Work?

Which type of heat pump is right for your home?

There are three main types of heat pump:

• Air source heat pumps (ASHPs)
• Ground source heat pumps (GSHPs)
• Water source heat pumps (WSHPs)

Other types of heat pump include air-to-air heat pumps, hybrid heat pumps and high-temperature heat pumps.

Here are the key pros and cons of each type of heat pump:

Air source heat pumps (ASHPs)

The most common type of air source heat pump (ASHP) in the UK is an air-to-water heat pump. This works by repurposing energy from the external air, concentrating it and outputting it to your heating and hot water distribution system. Air-to-water heat pumps are attractive renewable technology for many self builders and renovators, but there are some key considerations to bear in mind before you specify.

One advantage of an air source heat pump is that the appliance is easy to install in both existing and new houses. This can make ASHPs a relatively straightforward swap for a boiler, although you may need to make insulation and emitter upgrades to ensure good performance. Click here to find out more about installing a heat pump in an existing house.

Freddie and Katie Pack specified a Vaillant air source heat pump via Solaris Energy for their countryside self build, which allows the couple to run a low energy property with minimal long-term costs. Photo: Richard Gadsby

Air source heat pumps are designed to work effectively in temperatures as low as -25°C. However, they are most efficient when the air temperature is higher, so they do have to work harder in peak heating season during the winter.

Nevertheless, ASHPs are significantly more efficient than electrical resistance heating and very popular among self builders for properties both on and off the gas grid. The best air source heat pumps will produce much lower carbon emissions than a mains gas boiler and – in a well-insulated home – can achieve similar running costs.

FIND OUT MORE ABOUT ASHPS

High-temperature heat pumps

Most heat pump systems are designed to warm the home at a low temperature over long periods. The lower and slower the heat, the more efficient it is. But not all homes and distribution systems suit this approach. Some require much higher temperatures, especially when reheating the space or during colder weather. Different refrigerants, or combinations of them, can tolerate greater compression than others, enabling some heat pumps to reach those higher temperatures.

The highest flow temperatures are typically achieved in two ways. One is to use a single refrigerant gas such as R744 (carbon dioxide), which can be compressed to reach 80°C and above. It’s quite energy intensive to get to these temperatures due to the extreme pressures required, so this is better suited to large heat pump systems with a constant high temperature demand.

Available in outputs from 5kW to 12kW, Mitsubishi’s Ecodan monobloc is an R290 air source heat pump with a maximum flow temperature of 75°C, making it suitable for use with existing radiators

find out more about high-temperature heat pumps

Air-to-air heat pumps

An air-to-air heat pump system moves heat from around the outdoor unit, just like an air-to-water appliance, but then transfers into the air supply going into the home. The system delivers heating and cooling via air handling units inside the building, which feature a condenser and fan to distribute the air. With this set-up, you typically won’t require underfloor heating or radiators.

Unlike a traditional air-to-water heat pump (ASHP), most air-to-air systems don’t supply hot water so you will still require a system to heat your hot water supply, such as a hot-water cylinder with an electric immersion heater.

Offering heating capacity up to 5.8kW and cooling to 5.0kW, and A+++ rated for efficiency in both modes, the Daikin Emura air-to-air heat pump features a sleek, contemporary indoor unit

find out more about air-to-air heat pumps

Hybrid heat pumps

ASHPs become less efficient when the air temperature outside is very low (in the coldest parts of winter when you really want the heating on), meaning they need more electricity to deliver the right level of heat for your home.

Hybrid heat pumps include a fossil fuel burner (mains gas, LPG or oil). The heat pump will do most of the work, with the boiler element providing additional heat during colder periods where heating demand is higher. They intelligently select which mode is the most efficient to use based on external temperatures and energy usage. The boiler element can also be used to provide domestic hot water for taps, showers etc, while the heat pump provides space heating.

A hybrid heat pump is primarily a retrofit solution for homes where installing a lone heat pump may be difficult or require significant upgrades to ensure optimal operation. Hybrid heat pumps are unlikely to be relevant to new self build homes. The Future Homes Standard will require new homes to use low-carbon heating and won’t be able to make use of fossil fuel heating systems, too.

FIND OUT MORE ABOUT HYBRID HEAT PUMPS

Ground source heat pumps (GSHPs)

Dig down slightly into the earth, and the temperature is about the same as the average in the air. Over the course of a year, this approximates to 12°C in the UK (slightly higher down south and slightly lower in sunny Scotland). A ground source heat pump (GSHP) cashes in on this by extracting warmth via a collector circuit that’s dug into the earth. Of course, the ground and the air are both heated by the sun – so GSHPs are actually making use of solar energy.

Most domestic ground source heat pumps use closed-loop heat collector systems. With this method, a collector fluid (a mix of water and antifreeze) is pushed around a sealed loop, drawing in warmth and returning to the pump at a higher temperature.

A straight pipe collector loop system being laid in trenches as part of an installation by Green Building Renewables

It’s then compressed to raise the temperature even further, before the warmth is transferred into the distribution system (radiators/UFH). The pressure in the loop is then reduced (the fluid is expanded), thus reducing its temperature before it returns to the underground collector.

Where there’s sufficient land available, a horizontally-laid loop is used, with the pipes fitted in trenches approximately 1.5m below ground level (usually in a slinky format to get sufficient surface area).

This is disruptive work, so it’s best suited to new builds where the garden is not yet established. If space is tight, a vertical collector loop can be installed in a borehole, typically running between 50m and 90m deep. Borehole heat pumps are generally more expensive than those using a horizontally-laid collector loop.

FIND OUT MORE ABOUT GSHPS

Water source heat pumps (WSHPs)

The greatest attraction of water source heat pumps (WSHPs) is that the temperature of the heat source is likely to be more stable than either the ground or the air during peak space heating season. That’s because the temperature of the water (which could be a river, stream or lake) won’t drop as much as the temperature of the air or even the ground in winter.

As water is a fluid, even in bodies of still water such as large ponds and lakes, the rate of heat recharge (to the area from which heat is collected) is likely to be more efficient than with a ground source heat pump, which draws its heat from static material with slower heat flows.

Depending on the water source, WSHPs can be either closed or open loop. They can range in size from small domestic at a few kilowatts (kW) right up to large district heaters with outputs of several megawatts (MW).

Closed loop systems work by circulating a heat transfer fluid between the heat pump and the collector. Closed loops are common for small-scale applications, such as in streams and modest rivers. Open loop designs take water out of a lake or aquifer, extract (some) heat and then return the water to its source (or nearby).

The main downside of water source heat pumps is that very few buildings have a convenient, suitable body of water nearby for the collectors.

find out more about WSHPs

Case study Light-filled timber frame self build home in Scotland with an ASHP

Ever since renovating their previous home in West Yorkshire, Brent and Linda Walker had been thinking about undertaking a self build. “When our son was born, we shelved our plans, but when he went to university and we were heading for retirement, we revisited the idea. We realised with the equity in our house, which had gone up in value, and with our various savings, we could make self building a reality.”

Photo: David Barbour

The couple were looking to relocate to Fife, where Linda is originally from. Linda’s brother, who lives in Fife, had been looking for available plots on the couple’s behalf. He mentioned one small plot within a walled garden in the village of Colinsburgh that had been on the market for a long time, but he didn’t think it had much potential.

“The site he found was the one we thought was perfect for our project,” says Brent. “It was south facing, with incredible views, private, not overlooked and within a fantastic community. The only downer was that it wasn’t a serviced plot, but it did have outline planning permission for a two-storey house.”

Photo: David Barbour

Research led the couple to Fife Architects, who they engaged before purchasing the site. This allowed them to visualise their self build on the land and decide whether the project was right for them. The design materialised itself as a two-storey, three-bedroom home with a double-height atrium. Sandstone walls, clay roof tiles and stone window surrounds make a nod to the local architectural style.

The new house benefits from power via a solar PV array to the south facing roof, plus an air source heat pump powers the underfloor heating on both levels of the house. Combined with high levels of insulation and airtightness detailing, the project achieved an EPC rating of A for both energy efficiency and carbon emissions.

“As we’re retired and no longer earning an income, we were keen to keep the bills down as far as possible, so we wanted a reasonable level of efficiency,” says Brent. “The house performs so well – we have the world’s least needed woodburning stove!” Linda adds: “The house is very cosy in the way that it’s designed, with all the main living areas facing south.”

SEE THE HOME

How efficient are heat pumps in the UK?

A heat pump’s efficiency is measured according to its Coefficient of Performance (COP). A CoP represents the heat energy you get out of the appliance, divided by the amount of electricity required to run the pump.

So, if you could produce 3 units (usually kilowatt-hours, kWh) of energy out of a heat pump for every 1 unit of electricity input, then the CoP would be three (3/1 = CoP of 3). People selling heat pumps often quote the CoP figure – but the fact it’s achieved in laboratory conditions means it isn’t generally replicable.

Bear in mind real-world heat pump performance depends on a range of factors. These include the time of year (it’s harder to extract useful warmth when it’s very cold outside), how insulated your house is and the proportion of energy used for domestic hot water (DHW).

For space heating, the heat pump will generally be outputting to a temperature of around 35°C-45°C. But the heat pump will have to work harder to deliver water at the 50°C-60°C required for taps, showers etc.

So, if your household has a particularly high hot water demand compared to space heating, this could impact a heat pump’s running costs. That said, a heat pump will still do a better job than an electric immersion heater.

This is what a heat pump ErP label looks like

The best figure to use in order to fully understand the efficiency of an installed heat pump is the Seasonal Performance Factor (SPF). This measures the ratio of heat generated to electric input over the course of the heating system. A heat pump’s SPF is calculated by dividing the total annual heat output in kWh by the total annual electrical input in kWh.

Two recent major government studies (using data from actual installs) have revealed that the median SPF of air source heat pumps is 2.7 to 2.8. So, over the course of a year and depending on your setup, on average you could expect to achieve 2.7-2.8kWh of heat output for every 1kWh of electricity input. That’s far lower than manufacturers’ typical SCoP figures.

What will impact the heat pump’s real world performance?

A heat pump’s real-world efficiency in use depends on a range of key factors, including your property’s location, nature of the building fabric, distribution system, occupancy patterns and proportion of hot water demand versus space heating.

• Location influences the temperature of the heat source. Ground and air temperatures are higher the further south you go in the UK, which can boost efficiency. Local conditions such as exposure also play a part.
• Building fabric determines how quickly heat is lost. Heat pumps are best at delivering heat low and slow, unlike boilers which run hot and hard. Well insulated buildings retain warmth better and suit this steady approach to maintaining indoor temperatures.
• Distribution is critical. The larger the emitters, the lower the distribution temperature can be. Having a smaller gap between the source and delivery temperatures means the heat pump has less work to do. Underfloor heating is ideal but not always feasible to install in existing houses; oversized radiators are an alternative.
• Patterns of occupation vary widely. If you’re going to be in the house most of the time, you can run constant low-level heating, which will support optimal heat pump performance. If you’re out for much of the day and tend to heat your home in bursts, the heat pump will have to work harder for shorter periods – making it less efficient.
• Domestic hot water (for washing etc) represents an increasing proportion of our energy use. A heat pump has to work harder to deliver hot water at around 55°C than to supply space heating at the ideal 35°C.

heat pump efficiency guide

Do you need underfloor heating with a heat pump?

Heat pumps work best when they’re used to run low-temperature distribution setups, such as underfloor heating (UFH). UFH converts your entire floor into a heat emitter; the increased surface area allows for a much lower flow temperature to create a cosy environment – so less energy is required from the pump to raise the warmth of the water.

Underfloor heating is a great option for new self build homes, and can now be relatively easily installed in many existing buildings thanks to the proliferation of low-profile retrofit UFH systems.

However, it won’t suit every home. If you want to go for a heat pump but can’t install underfloor heating, your heating engineer may recommend upgrading to oversized radiators. Where single panel radiators remain, these can be replaced by twin panel designs – if not, the distribution system will have to be extended, taking up more wall space.

Colin and Elaine Frier self built a brand-new SIPs home in Tarbert, Argyll & Bute. There is no mains gas supply in Argyll, so the heating system consists of an underfloor setup powered by an air source heat pump. There is also a mechanical heat and ventilation recovery (MHVR) system that makes sure the climate in the house is kept fresh. Photo: David Barbour

How much does it cost to install a heat pump?

Heat pump installation costs can vary significantly depending on the specific project and the work required to ensure the system is correctly sized for the home, installed and capable of meeting the household’s heating requirements.

Heat pump costs can start from as little as £7,000, ranging up to £40,000+ depending on the type and size of heat pump you need.

• ASHPs cost an average of £13,681 (installed)
• GSHPs/WSHPs cost an average of £31,200 (installed)

_{Data supplied by Microgeneration Certification Scheme (MCS), reflective of Jan-Sep 2026 installation data.}

Aiming to increase the number of switches to heat pumps (instead of gas boilers), the government introduced the Boiler Upgrade Scheme (BUS). The BUS provides grants of up to £7,500 to encourage homeowners to replace their existing heating systems with more efficient, low-carbon alternatives. This includes air, ground and water source heat pumps and in some cases, biomass boilers.

You can access the following under the Boiler Upgrade Scheme:

• £7,500 towards an air source heat pump
• £7,500 towards a ground source heat pump (this includes water source heat pumps)
• £5,000 towards a biomass boiler.

As of the 21st July 2026, off-gas-grid homes can access an enhanced grant of £9,000 to help them make the switch to low-carbon heating.

Applying the standard £7,500 BUS grant to the average installation costs above would bring the average ASHP installation down to around £6,181, while a GSHP or WSHP would cost around £23,700 after the grant. These are indicative figures: the actual cost will depend on the property, the system specified and any additional work required as part of the retrofit.

To qualify for the Boiler Upgrade Scheme grants, your heat pump installer must also be accredited under the MCS.

NIBE’s F2040-6 air source heat pump was selected as part of a low-carbon overhaul of this Victorian terraced house, which was also upgraded with insulation and other energy efficiency measures. The setup – also featuring a 200L hot water cylinder and additional heat buffer vessel – powers underfloor heating downstairs and radiators upstairs. “The house is always at a stable temperature, and my bills are lower than with the gas heating that existed before,” says owner Richard Lowes

How much does a heat pump cost to run?

The answer to this depends on those key factors such as the overall performance of property you’re installing the heat pump into, what proportion of demand is for space heating vs hot water, the type of heat pump you’re using and – ultimately – the SPF your heat pump achieves.

If your engineer calculates that an ASHP is going to deliver 310% efficiency in your home, for instance, you can then use that as your baseline when comparing running costs versus other heating systems.

We can use the prevailing energy price cap as a starting point for calculating whether a heat pump will be cheaper to run than a gas boiler.

One unit of electricity currently costs 26.32 pence per kilowatt-hour (26.32p/kWh) under the energy price cap (October to 31 December 2026), compared with 7.97p/kWh for gas. This means electricity costs around 3.3 times as much as gas per unit of energy.

The overall efficiency of the heating system needs to be considered when comparing the running costs. Assuming a gas boiler operates at 90% efficiency, a heat pump would need to achieve an overall Seasonal Performance Factor (SPF) of around 3.0 to match the running cost of mains gas.

It’s worth bearing in mind, too, that if your plot is off the mains gas network, a well-installed heat pump is always likely to deliver bill savings compared to options such as oil or LPG boilers. Off-gas-grid homes can now access an enhanced grant of £9,000 under the Boiler Upgrade Scheme to help them make the switch.

Why are heat pumps better for the environment?

It’s true that heat pumps are much lower in CO2 emissions compared to gas boilers. A kilowatt-hour (kWh) of electricity currently produces an average of 0.131kg of CO2 emissions, whereas gas produces about 0.182kg (according to the UK government’s 2026 greenhouse-gas conversion factors). So, electricity is already significantly less carbon-intensive than gas per unit of energy.

Heat pumps produce more units of heat for the units of electricity that they consume. A straightforward electrical resistance heater, such as a storage heater, immersion heater or electric radiator, only gives you 1kWh of heat out for each 1kWh of electricity input. An air source heat pump, on the other hand, turns 1 unit of electricity into 2-3 units of heat.

The ratio between the heat delivered and electricity consumed over a heating season is known as the Seasonal Performance Factor (SPF). An ASHP achieving an SPF of 2.8, for example, would deliver 2.8kWh of heat for every 1kWh of electricity consumed over the period.

Using the 2026 emissions factors, an ASHP achieving an SPF of 2.8 would produce around 0.047kg CO2 emissions for each kWh of useful heat delivered. A gas boiler operating at 90% efficiency would produce around 0.203kg CO2 emissions per kWh of useful heat. On this basis, the heat pump would produce around 77% less carbon emissions than the gas boiler.

Over the next decade or so, the UK government plans to further decarbonise the National Grid by increasing the proportion of mains electricity that is generated from nuclear and renewable sources, rather than fossil fuels such as coal and gas, so heat pumps should become an ever more attractive option to decarbonise heating.

If your main aim is to minimise carbon emissions from space heating, note that sustainably sourced biomass (including logs, wood chips or pellets) is almost zero carbon in use, as long as woodland is replanted and the fuel does not have to be transported great distances.

how to build an eco house

What’s it like to live with a heat pump?

Heat pumps are a largely hands-off technology, much like boilers. Once installed and configured, you should rarely have to touch the controls. Other renewables require significantly more intervention, which isn’t for everyone. With biomass, for instance, you’ll need to take regular delivery of pellets and possibly do some manual loading of the fuel.

If designed and installed correctly by an MCS (Microgeneration Certification Scheme) accredited engineer, homeowners can expect their heat pump to deliver effective performance for approximately 15-20 years. This compares well to boilers and alternative renewable heat source options.

So, heat pumps are a good choice for those who would prefer a low-input, low-maintenance system. Modern products feature high-quality smart controls to optimise performance. The latest generation of heat pumps are also very quiet in operation. If noise is a particular concern on your project, look for Quiet Mark certification.

Dene and Debbie Happell’s ambitious contemporary timber frame design in the Trossachs National Park pleased local planners thanks to its sympathetic materials and sensitive location among the trees. When it came to choosing the heating system, the couple were keen to fit a renewable system. “We found ground source heat pumps too expensive and there’s more work because you have to dig a bore hole and fill it in afterwards,” Dene explains. “So, we went for an air source heat pump.” The unit sits at the back of the house in a small sheltered hut. Photo: Douglas Gibb

What maintenance do heat pumps require to ensure a long lifespan?

A heat pump that has been specified and installed correctly can last for over 20 years. Bear in mind that some individual components may require repair or replacement during this time as they are subject to wear due to the heat pump’s outdoor location.

Regular maintenance can help keep the system operating efficiently and identify potential problems before they lead to a major failure. You should get your heat pump serviced by an MSC-certified engineer at least once a year, and preferably before winter when the heat pump will be working hardest.