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Heat pumps

An air source heat pump attached to the side of a house in a garden in England

We answer your common questions about heat pumps.

Last updated September 2023

When it comes to low-carbon heating, it seems everyone is talking about heat pumps. But what is a heat pump? How do they work? And could you heat your home with a heat pump?

Our team of retrofit advisors and energy specialists answer the most common questions asked about heat pumps, an increasingly popular heating system.

Watch our short video where Joe, CSE Retrofit Coordinator, busts some common myths about heat pumps. Or use the links below to find helpful answers.

Common myths about heat pumps explained

What is a heat pump and how do they work?
What are the different types of heat pumps?
Will a heat pump reduce my heating bills?
Will I need to change my radiators to install a heat pump?
Do I need to change my pipes?
Do I have to put in underfloor heating?
Will I need to insulate my house first?
Will I need a hot water cylinder or replace my existing one?
Does a heat pump work with solar panels?
How big does my heat pump need to be?
What are high temperature heat pumps?
I live in a small flat. Can I get a heat pump?
What are hybrid heat pumps?
Do air source heat pumps make a lot of noise?
Do air source heat pumps stop working when it’s really cold outside?
Will the electricity grid be able to cope with all these new heat pumps?
What grants are available to me?
Where to get more information

What is a heat pump?

Heat pumps absorb warmth from the outdoor air, ground or water (even when the air, ground or water feel cold). They use this warmth to heat your home and hot water.

They work in exactly the same way as a fridge or freezer, only in reverse, with the refrigerant and a compressor providing useful heat rather than cooling.  

Heat pumps run on electricity, and for every unit of electricity they consume, a well-installed and reasonably high-spec model will produce 3 units of heat. This represents an efficiency of 300% – much higher than a new condensing gas boiler which may achieve an efficiency of 90%.

In other words, heat pumps are over three times more efficient than gas boilers. And because a unit of electricity currently emits the same as a unit of gas, heat pumps deliver significant carbon savings. They can also deliver savings on heating bills, depending on what heating system they are replacing.

Heat pumps are not a new technology, and millions are already heating homes very effectively across Europe. They are particularly common in the colder Nordic countries and in France and Italy.

Heat exchangers, compressors, refrigerants and the science …

We’re not going to go into all that here! If you want more detail on how they work, the Energy Saving Trust’s In-depth guide to heat pumps is a good place to start.

What are the different types of heat pump?

There are three types of heat pump: air source heat pump, ground source heat pump and water source heat pump. Let’s look at each in turn …

Air source heat pumps

Air source heat pumps are the cheapest and easiest to install and therefore account for the vast majority of installations.

They typically consist of a sealed external unit containing a fan (see photo).

An air-source heat pump outside a bricked wall

Air source heat pumps are very efficient because all they do is transfer heat energy from the outside air to the water in your heating system. In order to do this, they just need a small amount of electricity.

Modern air source heat pumps have undergone a lot of research and development and can achieve efficiencies close to that of ground source heat pumps when installed to a high standard.

Ground source heat pumps

Ground source heat pumps use buried fluid-filled pipes to absorb heat from the ground. These pipes can either be in horizontal trenches around 1m below the surface, or in vertical boreholes which go much deeper, from 60m to 200m. An indoor unit houses the compressor and heat exchanger to transfer the heat to the central heating system and hot water cylinder.

A trench in the earth containing the slinky-type pipework required for a ground source heat pump

Because the temperature a metre below the surface remains around 10°C throughout the year, there is more energy to extract from the ground than from the air during the heating season. This means ground source heat pumps tend to be more efficient than air source heat pumps.

However, the installation can be very expensive and in the case of horizontal trenches require a large area of land to dig up. This means they are typically only used in larger rural properties or for groups of buildings.

Water source heat pumps

Water source heat pumps work in much the same way but require very particular conditions. As a result they are quite rare and need specialist design. A good source for further information is the GSHPA, and the Energy Saving Trust has a useful page, too.

Will a heat pump reduce my heating bills?

When installed to a high standard, a heat pump could lower your energy bills and make your home more comfortable. But there is also a risk that your bills could increase. Whether a heat pump will reduce your bills depends on a range of factors:

What fuel you currently use

Mains gas is typically the cheapest fuel. So replacing this with a heat pump will bring lower savings compared to replacing oil, direct electric or bottled gas.

The efficiency of your existing boiler

If your gas or oil boiler is over 20 years old (and non-condensing) it will be a lot less efficient than a brand new boiler. This means it will use more fuel to provide the same amount of heat and therefore the savings from a heat pump would be greater. Even new boilers often work at below their stated efficiency.

The efficiency of your heat pump installation

This is the critical factor your installer can control with good system design. The efficiency of a heat pump, known as the ‘coefficient of performance’ or COP, is determined by how hard the heat pump has to work to deliver the required amount of heat. This is largely determined by the temperature difference between the source (either the outside air or the ground) and the supply (the water in your central heating circuit). A high COP is achieved by keeping this supply flow temperature as low as possible, which means the emitters (underfloor heating or radiators) need to be bigger to deliver the same amount of heat as a fossil fuel boiler.

The COP will vary throughout the year depending on the source temperature (so air source heat pumps will be much more variable than ground source). The annual average efficiency of your heat pump is known as the ‘seasonal coefficient of performance’ or SCOP, and this is what determines your annual running costs.

Fabric efficiency

If your home is uninsulated and loses heat easily, then having a low flow temperature can be challenging due to the size of the emitters required. The overall heat demand for your home will be higher too, so the heat pump itself will need to be bigger. The more you can reduce your home’s heating demand through insulation and airtightness, the easier it is to achieve a higher SCOP.

Fuel costs

If electricity becomes cheaper relative to gas and oil, then the running cost of a heat pump will come down compared to alternatives.

Time-of-use (ToU) energy tariffs could also lower running costs. Some electricity suppliers are now offering heat-pump specific tariffs. These include cheaper electricity during times of day that the heat pump is likely to be in use.

To reap the benefit of these tariffs, you must ensure that your heat pump is set up to operate during off-peak hours. This can be difficult to setup manually, however there are some smart heating controls that can automatically optimise your heat pump’s operation based on your specific tariff.

Operation and control

Because heat pumps work at a lower flow temperature than fossil fuel boilers they need to be controlled differently. A heat pump will take longer to heat up your home and will work more efficiently if they are left on throughout the heating season. It’s only overnight or when the home is unoccupied for extended periods that they should fall back to temperatures of around 16 degrees.

If you let your home cool down significantly and have to heat it again, it’s likely to be more expensive.

Heat pumps should be installed with ‘weather compensation’. This lowers the flow temperature in your central heating in response to higher external temperatures, therefore maximising efficiency.

This will demand a cultural shift in how we use our heating compared to the on/off cycles we are used to with fossil fuel boilers and electric heaters.

All these factors mean every property will be different. But to get some idea of the savings available, look at the table below (figures from Energy Saving Trust). This shows the potential annual savings of a standard installation in an average three-bedroom semi-detached home, depending on the existing system.

Existing heating systemAnnual savings
Old (G-rated) gas boiler£385
New (A-rated) gas boiler£8
Old electric storage heaters£1,100
New electric storage heaters£830
Old (G-rated) oil boiler£455
New (A-rated) oil boiler£15
Old (G-rated) LPG boiler£460
New (A-rated) LPG boiler£50

Do I need to change my radiators to install a heat pump?

Most wet heating systems (e.g. where hot water is pumped through radiators) have been designed to work with gas or oil boilers which produce water at higher temperatures than heat pumps.

Because heat pumps work most efficiently at lower flow temperatures, they work very well with underfloor heating. That’s because underfloor heating works at a lower temperature and tends to be on for longer periods.

This doesn’t mean heat pumps can’t be used with radiators. But it does mean that they need to be big enough to emit the amount of heat required to heat a room.

Upgrading to bigger radiators could be as simple as replacing single panel radiators with double panel ones. But, in some cases, you may need space for wider or taller ones.

If you add insulation and reduce your heat loss, your existing radiators, or at least some of them, could be adequate. Your heat pump installer will calculate the required output of the radiators in each room.

Radiator against a red wall.

Do I need to change my pipes?

If you have long sections of old or narrow (8mm or ‘microbore’) pipework, you may need to change it. But this is not always necessary. Your heat pump installer will advise you.

Do I have to put in underfloor heating?

Heat pumps and underfloor heating work well together. That’s because underfloor heating works at lower temperatures than radiators and is usually on for longer. It takes more time to heat up and cool down, both of which suit heat pumps.

Heat pumps can work well with radiators as long as they are sized correctly.

Will I need to insulate my house first?

The more insulated and efficient your home’s fabric (walls, roof, floor, windows), the less heat it needs to stay warm. This means it’s easier to deliver this heat at a low flow temperature, which means a more efficient heat pump and lower bills.

If your house is leaky and uninsulated then having a low flow temperature can be challenging due to the sheer size of the emitters required. The overall heat demand will be higher too so the heat pump itself may need to be bigger. The more you can reduce the home’s heating demand through insulation and airtightness, the easier it is to achieve a higher SCOP.

The shape and detachment of your home has a big impact on its fabric efficiency even before considering insulation.

A detached house will lose much more heat than a mid-terrace of the same size. This means it can be harder to run heat pumps efficiently in larger houses with lots of uninsulated external walls. But it is possible – you will just need large radiators or underfloor heating.

Fabric first

A ’fabric first’ approach which prioritises improving thermal performance with insulation and airtightness before looking at technologies like heat pumps is generally advised. These tend to be permanent changes to the building which reduce fuel bills and improve comfort, without running costs or maintenance. It also allows a smaller heat pump and radiators to be installed later.

However, this doesn’t mean everything must be insulated before installing a heat pump. Insulating floors and solid walls can be very expensive and disruptive, and sometimes it isn’t possible. You may also need a new heating system when you don’t have the funds to do everything at once.

In this instance, it could make sense to do basic fabric measures, install a heat pump and then, for example, add external wall insulation at a later date.

This shouldn’t mean your heat pump is oversized, as modern heat pumps are inverter-driven which means they can modulate down and operate at a lower capacity without losing efficiency. 

Broadly speaking, as a minimum, you should aim for the loft to be insulatedcavity wall insulation (where possible), reduce heat loss though windows (with double or secondary glazing, shutters and curtains) and a safe level of draught-proofing (with necessary ventilation provision).

Will I need to have a hot water cylinder or replace my existing one?

If you have a heat pump, you will need a hot water cylinder as they do not produce hot water on demand like a combi boiler. If you currently have a gas combi, you’ll need to find a suitable location for a cylinder.

This is usually less of a problem than people think. There are lots of possible places to put them in most homes. Your heat pump installer will advise you.

Even if you have a cylinder already, you may need to replace it for one that is ‘heat-pump ready’. This basically means having a larger heating coil which heats the water at lower temperature. If you keep your old cylinder, you’ll save the cost of a new one (around £1,500), but you may incur higher running costs because a newer cylinder will have a larger coil and be better insulated.

Does a heat pump work with solar panels?

Yes, a heat pump can work with the electricity supplied by a home’s solar photovoltaic (PV) panels. Unfortunately, the supply and demand of the two technologies are not ideally matched. In winter, when you want to heat your home with your heat pump, your solar PV won’t be generating as much electricity as in the summer.

That said, solar panels will provide electricity for the heat pump to produce hot water from spring to autumn, and some heating on cold, sunny days. The contribution will depend on the size of the PV system and the heat pump.

There is a device called a solar PV-diverter which takes any surplus electricity generated by the PV panels not being consumed in the home and uses it to heat the water in your cylinder. This maximises the usage of your PV and reduces the amount that a heat pump is used for hot water when the PV isn’t generating, increasing its overall efficiency. However, careful consideration needs to be given to how the two systems are installed and controlled.

Solar panels on the rof of a family home.

How big does my heat pump need to be?

Gas and oil boilers work at a higher temperature than heat pumps and generally speaking, these installers have not spent a lot of time calculating your exact heating needs. A heat pump and emitters need to be sized much more accurately or the system won’t work efficiently.

If it is too big, you’ll pay more than you need to, and it may not work as efficiently as it could. If it is too small it won’t provide enough heat to keep you warm.

This means you need a competent person to do a room-by-room heat loss calculation survey. This will give you an idea of running costs before you commit and costs around £350. These surveys are based on assumptions around the thermal performance of building materials; newer surveys are available that measure the actual heat loss of a building which could lead to more accurate heat pump sizing.

What are high temperature heat pumps?

High temperature heat pumps use a different refrigerant to produce heat at higher temperatures. This means you may not need to change your radiators or improve your insulation. They are seen as more of a direct replacement for fossil fuel boilers.

However, higher temperatures require more energy, meaning bills may be up to 25% higher than a regular heat pump. They are also more expensive to buy.

But, in some homes, it’s not possible to improve your insulation or fit large enough radiators to keep flow temperatures down. In these cases, a high temperature heat pump can still offer significant carbon savings over fossil fuel heating. But you’re less likely to save on your bills, especially compared to mains gas, unless electricity prices fall further relative to fossil fuels.

I live in a small flat. Can I get a heat pump?

Air-to-air heat pumps are most suitable for small flats. They cost around £4,000 to £6,000 to install, depending on size, which is much cheaper than an air-to-water heat pump. Note that air-to-air heat pumps are not included in the Boiler Upgrade Scheme.

What are hybrid heat pumps?

Hybrid heat pumps combine two technologies, a heat pump and a boiler. They are designed to address the issues associated with installing a heat pump in large and poorly insulated homes. The system itself decides which heating technology to use. For example, most of the time it would use the cheaper heat pump, but on a very cold day when the heat pump would struggle, it chooses the boiler.

This might be a good option in a large heritage property that’s hard to insulate. But you are effectively buying two heating systems so it’s likely to cost a great deal more. Hybrid heat pumps are not included in the Boiler Upgrade Scheme.

Do air source heat pumps make a lot of noise?

This is a common concern. The noise level of an air source heat pump will largely depend on how hard it needs to work, as well as its make, size and manner of its installation. Strict noise limits are a key factor in their production and installation, and manufacturers are always working on reducing noise levels.

The rated noise level is measured at one meter from the heat pump and reduces significantly as you move away from it. They typically reach 40-60 decibels, which is the equivalent of a quiet office.

It is also worth noting that heat pumps are mainly active during winter when people are not spending much time outside. Their operation for hot water during the summer can be time controlled.

Do air source heat pumps stop working when it’s really cold outside?

No, they don’t. Air source heat pumps are designed to work in temperatures as low as minus 25°C. And they are common in many countries that are significantly colder than the UK such as Norway and Finland.

Of course, they have to work much harder when it’s cold, but the same is true of all heating systems. They should still be twice as efficient as a gas boiler or direct electric heating when the temperature falls below zero.

Air source heat pumps may begin to freeze in very cold weather, but they have a defrost mode which prevents this from spreading throughout the system. And they are normally placed a few inches above the ground and above all but the heaviest falls of snow.

When installing, it’s important to ensure there is a run-off drain. This is so that the condensate doesn’t pool and freeze in very cold weather, creating a hazard.  

Will the electricity grid be able to cope with all these new heat pumps?

As more homes switch to heat pumps, and electric vehicles (EVs) replace petrol, there will be a huge increase in electricity demand.

Will the electricity grid be able to cope? In the short term, yes. Problems with grid connection are not widespread. In a few local areas heat pumps and EV chargers are not being connected to the grid due to capacity reasons, but these are the exception.

Meeting future demand, however, will be a serious challenge. Investment is critical. The government estimates the grid’s spare capacity – currently around 50% – will be used up by rising demand over the next ten years or so unless significant upgrades are made.

The utilities that manage the electricity network, known as distribution network operators, are confident that the grid will be ready. They say that preparation for this has been central to their business plans for a long time.

Demand flexibility is key

While the networks are preparing, there needs to be a sustained strategic approach from the government and Ofgem to ensure that the necessary planning and investment takes place.

One of the keys to this is ‘demand flexibility’. This means spreading the demand for power throughout the day, for example, so that not everyone is charging their car at the same time. The government estimates this could reduce costs by up to £10bn a year by reducing the need for electricity generation and for upgrades to network infrastructure.

What heat pump grants are available to me?

The Boiler Upgrade Scheme is a government grant designed to encourage property owners to install low carbon heating systems.

It offers £7,500 off the cost and installation of an air source heat pump and ground source heat pump.

The government is encouraging heat pumps because large-scale take up will make a significant contribution to the UK’s net zero commitment. Around three-quarters of household emissions associated with our energy use come from heating and hot water. If we move away from gas and oil and switch to electricity generated from renewable technologies like solar and wind, we’ll make huge reductions in our carbon emissions.

We will also be reducing the amount of gas we have to import from abroad. This will save money and reduce dependency on other countries.

More information

The Heat Pump Association can provide further technical information about heat pumps.

The Energy Saving Trust have lots of information on heat pumps aimed at householders.

If you’re serious about installing a heat pump, the Microgeneration Certification Scheme have a list of approved installers.

The Heat Pump Federation has helpful guidance for procuring heat pumps, which you can download from their website.

And finally, Heat Geek have a network of approved installers who have completed advanced in-depth training.

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