How do heat pumps work?

A heat pump is a low-carbon way to heat your home. It's a sustainable replacement for fossil-fuel heating such as gas, oil or LPG, and can significantly reduce your household's carbon footprint.

In future, it's likely that heat pumps will be heating many of our homes, as we aim to phase out the use of fossil fuels to reduce our impact on the climate.  

Although heat pumps use electricity, they are extremely efficient, turning every unit of electricity into three or four units of heat. The UK government has a target to fully decarbonise the electricity grid by 2035.

Most homes can swap out their gas boiler or other home heating system for a heat pump, as it can work with existing radiators and pipework. Find out whether a heat pump is right for your home.

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What is a heat pump? 

In the simplest terms, a heat pump moves heat from one place to another. 

You almost certainly already have a heat pump in your home, though you might not realise it. In your fridge or freezer, a heat pump moves heat from inside the box to outside, reducing the temperature inside the appliance. 

When it comes to home heating systems, a heat pump does the same thing, but it moves the heat from outside the box (your home) to inside, to increase the indoor temperature.

Heat pumps don't actually create heat energy, they simply collect it and move it from the outside air, ground or water, transferring it inside your home, where it can be used in radiators, underfloor heating or warm air heating, and to supply hot water for taps and showers. 

Some heat pumps can also be used in reverse, to provide cooling in warm weather.

Although they are powered by electricity, heat pumps typically deliver three to four units of heat for each unit of electricity needed to run them. This is much more efficient than gas boilers, for example, which produce less than one unit of heat for each unit of gas used.

This makes heat pumps a highly efficient low-carbon technology and, when they're powered by 100% renewable electricity, they can be zero-carbon in operation. The UK government has a target to decarbonise the electricity grid by 2035.

How does a heat pump work?

The term 'heat pump' is commonly used to describe the entire system, though it's actually made up of several components that are installed both inside and outside your home.

They work by absorbing heat from the outside environment – air, ground or water – and transferring it to a refrigerant, which flows through a compressor, condenser, expansion valve and evaporator in a closed cycle. 

The refrigerant is compressed to increase its temperature, then moved to the indoor heat exchanger, where the heat is transferred into the heating system or hot-water storage tank.

Heat pumps work even when the outside temperature is below freezing, as there is always some heat present in any temperature above absolute zero (-273 degrees Celsius). They can operate effectively at -15 degrees, although their efficiency drops in very cold weather. Ground and water source heat pumps are more efficient year-round, since ground and water temperatures are much more constant than the air temperature.

What types of heat pump are available?

Air source heat pumps

There are two different types of air source heat pump: air-to-water and air-to-air. Both types extract heat from the outside air, but they deliver it into your home in different ways.

Air-to-water heat pumps are the most obvious replacement for a gas boiler, as they can circulate heat through a wet central heating system – including radiators and/or underfloor heating – and produce hot water, too. 

Air-to-air heat pumps deliver heat through a fan or warm-air circulation system, but these do not produce hot water, so you will need a separate system for water heating.

Read more about how much they cost to install in our guide to air source heat pumps explained.

Ground source heat pumps

Ground source heat pumps extract heat from the ground using buried collectors, known as ground arrays. Fluid is pumped through the ground array to absorb heat stored in the ground. This is compressed and goes through a heat exchanger, which extracts the heat and transfers it to your home heating system, similar to an air-to-water heat pump.

The ground array may be either a network of coiled pipes – also called 'slinkies' – laid out in shallow trenches, or a vertical pipe inserted into a very deep borehole. 

Both types require a reasonably large garden or other outdoor space for installation. A larger area is needed for slinky pipes in trenches, but boreholes are generally a more expensive option and you will need access for a specialist drilling rig to dig up to 200m down.

Find out more about ground source heat pumps and whether your home could benefit from one. 

Water source heat pumps

Water source heat pumps are similar to ground source heat pumps. The pipe network or pond mat is submerged in water – a river, lake, large pond or the sea – and extracts heat, which is compressed and transferred in exactly the same way.

Water source heat pumps are often very efficient because water temperatures are generally stable throughout the year and are warmer than the average air and ground temperature in winter. 

If you're within 100m of a suitable water source, then this option is worth considering.

Hybrid heat pumps

A hybrid, or dual-fuel, heat pump system combines an air or ground source heat pump with a traditional gas, oil or LPG boiler. The hybrid system switches between the heat pump and the boiler, depending on which is the most efficient at any given time.

In very cold weather, if the heat pump cannot supply heat efficiently, it shuts down and the system switches to the fossil-fuel boiler. This means that a hybrid system is less environmentally friendly, but it could be an option if you can't heat your home adequately with a heat pump alone.

High temperature heat pumps

High temperature heat pumps use the same technology as other heat pumps, but are designed to run at a higher temperature – usually 65-80 degrees.  

Standard – or low temperature – heat pumps are at their most efficient when running at 35-45 degrees, which is cooler than most gas central heating systems. To achieve a comfortable level of warmth with a standard heat pump, many existing homes will need larger radiators, upgraded pipework or improved insulation, or a combination of all three. Or you can buy a higher temperature heat pump, but these are more expensive to fit and less efficient than low temperature models, and the external units are larger.

How efficient are heat pumps?

Heat pumps are an energy efficient way to heat your home because they use a small amount of energy to move heat around, rather than burning fuel to create it.

The efficiency of heat pumps is usually expressed as the coefficient of performance, or COP. This describes how many units of heat you get out for every unit of energy you put in. A heat pump with an efficiency rating of COP 3.0 can produce three times more heat energy than the electrical energy it consumes. 

Efficiency varies with the outdoor temperature – the colder the air, ground or water source, the less efficient the heat pump will be. The SCOP, or Seasonal Coefficient of Performance, is the average COP across the whole year. Energy labels for heat pumps show the energy efficiency rating according to their SCOP in heating mode. 

A more efficient heat pump will consume less energy and be more cost-effective to operate.

To get the best from your heat pump, you'll need to understand how to use it most effectively, as it may be different from the way you use your existing heating system. You may need to set your heating to come on for longer than with a traditional gas or oil boiler. Your installer should show you how to control your heat pump system.

Which gas boilers are the most efficient?

Heat pump energy labels

All heat pumps must have an energy label, similar to the label used on gas boilers. It shows how energy efficient the pump is on a scale from dark green (most efficient) to red (least efficient). 

If your heat pump supplies both space heating and hot water, then – like a gas combi boiler – the label will indicate how efficient the heat pump is for each function.

Heat pump energy labels also indicate how noisy they are, displaying the decibel level both inside and outside the home.

Are heat pumps worth it?

There are a few questions to consider to help decide whether or not it's worth installing a heat pump in your home right now.

1. What do you want to achieve? If your main aim is to cut your carbon emissions, then switching from a fossil-fuel heating system (such as gas, oil or LPG) to a heat pump is definitely worth it. The high efficiency of heat pumps, combined with the lower carbon intensity of electricity means you will be reducing the carbon footprint of your home. However, if your main aim is to reduce your energy bills, it is less clear cut. If you're switching from another electric heating system, you can probably make savings. Due to the relatively low price of gas, if you're switching from a gas boiler you'll need the right combination of heat pump and energy tariff to save money, and lower bills are not guaranteed. You can read more about potential air source heat pump costs and savings here.
2. How old is your existing heating system? If you have a relatively new boiler or other heating system, it's probably not worth removing it just yet as it will still be quite efficient. If your boiler is showing its age and needs replacing anyway, it's worth considering a heat pump rather than committing to burning fossil fuels for another 10-15 years.
3. Is your home well insulated? All heating systems work more efficiently in a well-insulated home, but a heat pump could prove to be particularly costly to run if you have little or no insulation. Because they operate at a lower temperature, it can take longer to heat a cold, draughty property and maintain it at a comfortable temperature. It may be better to spend the money on draughtproofing and insulation first before considering a heat pump.
4. Can you get a grant? Depending where you live in the UK, you may be eligible for a grant to help with the cost of installing a heat pump. If you can get that now, it could make a heat pump a good choice - grant schemes come and go, and you may not be able to get the same level of support in a couple of years time. Alternatively, if you are not currently eligible, it could make sense to wait and see what grants are available in future.

Why should you consider installing a heat pump?

• To reduce your carbon footprint: Heat pumps produce no carbon emissions at the point of use, unlike gas, oil and LPG boilers. They do use electricity, so they are not entirely carbon-free, but as the UK's national electricity supply reduces its reliance on fossil fuels, they will become even lower carbon in future.
• To use less energy: Heat pumps are highly efficient, so you'll be using less energy for heating and hot water.
• To lower your energy bills: In a well-insulated home, a heat pump running efficiently could save you money, especially if you are on a heat pump-specific or time-of-use tariff. 

Why might you not install a heat pump yet?

• No space for installation: You'll need some outdoor space, and some indoor space for a water storage tank, pipework, controls and perhaps larger radiators.
• An uninsulated, draughty home: It may prove expensive to heat a poorly insulated home with a heat pump. The heat pump will have to work much harder to replace heat that is constantly leaking out, so can't operate at maximum efficiency. If that applies to you, you should get your home insulated properly before installing a heat pump. It's wise to insulate your home in any case, as it will prevent your home leaking energy, which wastes money, and make your home a more comfortable, healthy and pleasant place to live. 
• Cost and effort of installation: At the moment, heat pumps cost considerably more to install than gas boilers, but help is at hand from the government's Boiler Upgrade Scheme (BUS). There may also be more disruption, especially if you need extra insulation, new radiators and pipework, or other additional work.
• Lower energy bills are not guaranteed: Per unit, electricity is considerably more expensive than gas, so your energy bills may go up if your heat pump does not run as efficiently as planned. 

Heat pump FAQs

Is my home suitable for a heat pump?

All types of homes are suitable for heat pumps. Whether a heat pump is the perfect solution for your particular property depends on your specific situation. Gov.uk has an online tool that can give you an idea if your home is suitable.

All heat pumps need space for installation – read more about air source and ground source heat pumps to find out how much space is needed, inside and out. 

As they supply heat at a lower temperature, they work most efficiently in well-insulated homes. 

As we said before, if your home is not energy efficient, you can still install a heat pump, but you may need to add extra insulation or replace your radiators and/or central heating pipework to increase the flow, to make sure you can heat your home adequately.

Will a heat pump save me money on my energy bills?

This depends on your personal circumstances. Heat pumps can cost more to run than gas boilers because they use electricity, which is more expensive per unit than gas. 

However, if designed, installed and operated correctly, an efficient heat pump could save you money on your energy bills, especially if used in conjunction with a specialist heat pump tariff. 

Dr Jan Rosenow,  principal and director of European programmes at the Regulatory Assistance Project (RAP), told us that a household could save as much as 50% and more on running costs with a heat pump compared to a gas boiler but, crucially, 'this requires the heat pump installation to be of high quality, the efficiency of the heat pump to be high and the customer to be on a heat pump tariff'. 

If you currently use oil, LPG or electric heating – which are generally more expensive to run than gas boilers – you are likely to save more money with a heat pump.

Read more about installation costs and potential savings in our air source and ground source heat pump guides.

Do I need planning permission for a heat pump?

In general, no. 

You might need to seek planning permission from your Local Planning Authority (LPA) if:

• the volume of your air source heat pump unit (including housing) would exceed 0.6 cubic metres
• there is an existing air source heat pump on the building or within the gardens or grounds
• your proposed location is within one metre of your property boundary
• your proposed location is on a pitched roof or less than one metre from the edge of a flat roof
• your proposed location is on a wall that fronts a highway, and any part of that wall is above the level of the ground-floor storey
• your property is a Listed Building
• your property is located within a World Heritage Site or conservation area.

If you're still unsure, Nesta has more information to help you decide if a heat pump is right for you.

Can I get a grant to install a heat pump?

The government is currently inviting people to apply for £7,500 off the cost of a heat pump via its Boiler Upgrade Scheme (BUS). 

Find out how the Boiler Upgrade Scheme works, whether you might be eligible and how to apply. 

Will a heat pump supply me with hot water?

If you are replacing a combi boiler with a heat pump, you will probably need to install a hot-water tank or cylinder when you switch to a heat pump. 

If there's no room in your home, the cylinder can be installed in a garage or other outbuilding, or there are horizontal models designed to be installed in the loft space – be aware, though, that placing it in an unheated space may decrease its efficiency. 

Or you could install a heat battery, which is generally smaller than a hot water cylinder. 

If you use relatively little hot water, another option is to install a heat pump for your heating system only, and add an instantaneous water heater for your hot water requirements.

Do I have enough electricity supply to power a heat pump?

An 80A fuse connection is sufficient for ‘any typical household looking to install a combination of low carbon technologies’. That’s according to the Electricity Networks Association (ENA) which represents the companies which operate the electricity wires across the UK.

New homes are built with an 80A or 100A fuse. Older properties tend to have less. For example, we’ve come across a case of a homeowner who shares a 60A supply between three flats in one building.

To find out your home’s supply, check the main fuse for your house (usually found near your electricity meter). If it isn’t clear (some older fuses are encased in black boxes) ask your Distribution Network Operator (DNO). You can find yours using the ENA’s online tool.

If your home’s electricity supply is too low, it can be problematic if you want to install a heat pump. At worst, if you try to use too much power you could blow the main fuse in your house.

The installer of your heat pump should assess the maximum load of your property. If your heat pump's demand would exceed it then they should check with your DNO if your fuse and service cable can take the extra load. If they can’t, your DNO will need to upgrade your supply.

DNOs will usually upgrade you to 80A or 100A (single phase) for free, according to the government's Department for Energy Security and Net Zero (DESNZ), and it can be done quickly. 

If you have a three-phase supply then you’ll need to pay to upgrade the cable, fuses and meter. The rules on this changed in April 2023; previously you’d have been charged for other parts of the upgrade too.