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Heat Pumps

Heat Pumps

Heat pumps are going to be one of the best options for heating our homes in a zero carbon future. This is because a heat pump is powered by electricity, which we can supply using renewable energy sources. Through the UK winter, a heat pump can use electricity from wind farms, hydro turbines, and wave & tidal power.

For every unit of electricity, a heat pump can give three or four units of heat. This makes it much more efficient than any direct electric heating, such as an electric boiler or wall heaters. Those direct electric options give only one unit of heat for each unit of electricity consumed.

The running costs of a heat pump will therefore be about one third as much (or less) than direct electric heating. Another important issue is that to run direct electric heating we’d need about three times as much renewable energy generation on the grid. That would make it far more difficult to get to a zero carbon grid.

Is a heat pump currently a low carbon choice?

At present, average carbon emissions from UK grid electricity vary but tend to average out at about the same per unit as mains gas. An efficient heat pump can use one unit of electricity to deliver three (or more) units of heat. This means that overall carbon emissions from a heat pump will already be about one-third as much (or less) than those from a gas boiler. You’ll see a slightly bigger saving against higher carbon fuels like oil or LPG.

Hopefully you’re already on a green tariff, but it’s still important to use less energy in order to keep nationwide electricity demand low. Reducing electricity use is always the most important thing, even if you buy it from a green supplier. So insulating your home as much as possible is important, even if a heat pump could heat the house without insulating it more. Where possible, aim for a home refrofit to a level better than current UK Building Regulations.

What is a heat pump?

You’ll almost certainly have a heat pump in your home already, because small heat pumps power fridges & freezers. The process is complex, but basically the heat pump absorbs heat and then moves it to another place. Most air conditioning units are heat pumps, as are efficient types of tumble-dryer.

Heat pumps for central heating use the same technology to extract energy from the outside air or under the ground. These are low temperatures (below 10°C in winter) but there’s lots of energy when compared to absolute zero (-273°C).

Most of the electricity input is used to run a compressor. In simple terms, this creates a small amount of higher temperature energy from the large amount of energy collected at a lower temperature. The process uses the ‘vapour compression cycle’ to harness the energy available when vapour returns to liquid.

How do I make my heating system suitable for a heat pump?

To work well, a heat pump must be delivering low temperature heat. for many existing homes this will mean using radiators with a large surface area, so that you can supply them with water at about 40 or 45°C. Where wall space is limited, smaller fan assisted radiators will still keep flow temperatures down. The best option is underfloor heating, with a flow temperature of 35°C. However, adding this to an existing house is often not feasible.

It’s important to avoid supplying standard size radiators at 60°C or more, because this will result in poor efficiency and high running costs. It’s therefore not a good sign if an installer says they can just bolt on a heat pump to supply small existing radiators at high flow temperatures. Getting a proper assessment of heat loss is important for then choosing suitable radiators.

You’ll don’t necessarily need super-high levels of insulation for these low flow temperatures to work, but it will make it easier. Old houses that are difficult to insulate well can still make use of a heat pump. It helps if they have a good amount of ‘thermal mass’ – solid stone or brick. This will retain the heat from a low temperature system better than a lightweight home. I mention John Cantor’s book and website below, and his site has a useful page about heating old houses with heat pumps.

What about domestic hot water?

You’ll need a store or cylinder to hold the hot water for your bathroom & kitchen. This should be heated up to 60°C once in a while, to kill off possible legionella. However, this can be done only be once every few days (perhaps just once a week), with the temperature lower the rest of the time to keep the heat pump running more efficiently. Many modern heat pumps will be able to heat a hot water cylinder to 60°C when necessary, to minimise the need to top up with a less efficient electric immersion heater.

In general, when using a cylinder there is more risk of heat loss. For example from long pipe runs, or just from always heating more water than you need. So attention to sizing the store correctly and minimising hot pipe runs is vital for keeping a system efficient. It’s also important to first minimise hot water use by using efficient shower head and spray taps.

For more than half of the year you could get enough hot water from solar water heating. Or if you have solar PV panels you could set the heat pump hot water cycle for a time of day when the PV is likely to be generating the power needed.

How do I choose an efficient heat pump?

A ‘coefficient of performance’ (COP) measures the efficiency of a heat pump. A COP of 3 means a heat pump gives 3 units of heat energy for each unit of electricity used. However, the COP excludes top-up heating (from an immersion heater) or the electricity for pumps and fans.

For overall performance, check for a seasonal comparison of total heat output to total electricity use across different weather conditions. This may be given as a seasonal COP (SCOP) or a seasonal performance factor (SPF). These factors will vary for different source and delivery temperatures. Manufacturers will publish tables showing the COP at different temperatures. For example the ratio of electricity to heat may be close to 4 when supplying water at 40°C, but drop to 2.5 if supplying 55°C.

Ground source, air source or water source?

A heat pump is most efficient when you minimise the temperature gap between the heat source and the heat demand. A ground source heat pump (GSHP) should therefore be more efficient than air source. This is because about two metres down the ground stays around 10°C all year, protected from temperature extremes. This heat is almost all solar energy, absorbed by the ground during summer.

With low winter air temperatures, air source heat pumps (ASHP) are not as efficient, but will be much cheaper to install. Their electricity use will increase a bit when outside temperatures get really cold, and if the collector needs defrosting this uses more energy. However, in the UK most of us have a relatively mild winter climate. This means an ASHP should still have a good ratio of electricity to heat in much of the UK.

Some air source heat pumps are air-to-air rather than air-to-water. Warm air heating is uncommon in the UK, and this may only be an option if you’re building or renovating to a very high standard.

Water-source heat pump systems can be very efficient. However, they’re not as common because you’ll need a water source that won’t freeze – such as a spring or borehole.

How much will a heat pump cost?

At the moment installation prices can vary a lot. The best thing to do is to compare quotes from a few installers. The kilowatt rating of the heat pump is a good guide to expected costs. A good home retrofit will allow you to install a smaller (and so cheaper) heat pump.

The price will be related to the size of the heat pump, measured in kilowatts. Recent figures collected for systems installed under the UK government’s incentive scheme (the RHI) suggest that domestic air source heat pumps cost about £1,000 per kilowatt (kW). About half of these air source heat pumps were rated between 6 and 10 kW, putting the total cost between £6,000 and £10,000. A further quarter of installations were rated at 11 to 15 kW, so would have cost a few thousand more.

For ground source heat pumps, the cost for RHI installations is about £1,500 to £1,800 per kilowatt. When you compare ground loop options, trenches are likely to be cheaper than boreholes. These installations were also mainly in the range of 6 to 15kW, which would put total costs in the range of £10,000 to £25,000.

It’s important to be clear that these are average figures and so some quotes could be lower. The relationship of cost to power rating shows that taking measures to reduce your heat demand should save you thousands of pounds on a heat pump installation.

What support is available?

At the moment heat pumps receive support through the domestic Renewable Heat Incentive (RHI). This gives you payments over 7 years, increasing with inflation. You’re paid for the proportion of ‘renewable heat’, which is basically your heat demand minus the electricity use of the heat pump.

The proportion is calculated by multiplying assessed heat demand (from an Energy Performance Certificate) by a factor of (1 – 1/SPF). To illustrate this, with an SPF of 3 we get (1 – 1/3) = 2/3. This means that two-thirds of the heat demand is classed as renewable and eligible for payments.

Currently, ground or water source heat pumps can get 21.29 pence per kWh of renewable heat. Air source heat pumps get 10.92 pence per kWh of renewable heat.

In April 2022 the RHI is to be replaced by an upfront ‘Clean Heat Grant’, to boost the heat pump market. This grant would immediately recoup a large chunk of the installation cost, rather than the longer term payoff from the RHI. We don’t yet know the full details, but figures of £4,000 or £7,000 have been proposed.

How much will a heat pump cost to run?

It is possible with good design and the right choice of electricity tariff to get the running costs of a heat pump to be about the same or lower than a gas boiler. As mentioned at the start, this would make it far cheaper to run than direct electric heating, and should also make running costs cheaper than oil or LPG.

In CAT’s Zero Carbon Britain report we propose refurbishing all homes to reduce heat demand. An example in the report is of reducing the annual heat demand for an average house from about 10,000kWh down to 4,000kWh.

With the right choice of electricity tariff, a well-designed heat pump with a seasonal COP of about 3.5 would use about £150 per year to meet that reduced heat demand. A gas boiler would have pretty much the same running costs. For the original higher demand of 10,000kWh, the costs would be closer to £400 for each option. Please note that these are the prices before the current spike in wholesale energy prices in late 2021, which will lead to higher prices for householders. However, with gas and electricity prices both rising, the comparsion between them may well end up the same.

I give a link below to a case study published by Trystan Lea of OpenEnergyMonitor. He used an ‘agile’ variable rate tariff from Octopus to keep running costs very low. Octopus themselves recently published an article about choosing the right tariff for running a heat pump.

There is also a tariff designed for heat pumps from Good Energy. Other energy companies may well start to offer similar tariffs for heat pumps, so check around to see what is on offer.

Further information

In Summer 2022 CAT is running the one day course Renewables for Households: heat pumps.

The CAT store sells the comprehensive book Heat Pumps for the Home by John Cantor, who also has a very useful website.

On Trystan Lea’s website he gives a detailed example of designing and running a heat pump efficiently in an older house. By opening the menu at the top you’ll see many pages with lots of data about how he evaluated heat loss, sized radiators, and so on.

Trystan Lea is one of the people at OpenEnergyMonitor, who supply monitoring equipment for heat pumps. This can help you understand how a heat pump is running, and improve efficiency and running costs.

For more on the Renewable Heat Incentive (RHI), see Ofgem’s RHI pages for the eligibility requirements and how to apply. There’s also an official Renewable Heat Incentive Calculator to help with working out the payback.

You can find RHI-accredited installers through the Microgeneration Certification Scheme (MCS). Another listing of heat pump professionals can be found at the Ground Source Heat Pump Association.

Ethical Consumer have assessed several brands that manufacture heat pumps, looking at their environmental and ethical record.

This page was written by CAT’s Information Officer Joel Rawson. You can contact me with further questions (choose ‘free information service’ on the form).

See also the related questions below for more details.

Related Questions

How much land is needed for a ground-source heat pump?

Trenches should be at least two metres deep to harness a consistent year-round heat source. They will need 50-80 metres of pipe per kilowatt (kW), or 10 metres of ‘slinky’ coiled pipe per kW, with at least 5 metre distance between trenches with coils. So a typical 8kW heat pump requires around 400m2 of ground area for slinky coils. Note, however, that this will depend on a number of factors, including ground conditions.

Boreholes need 20-50 metres of pipe per kW, and will usually be 100-150 metres deep. You may need 2-4 pipes per borehole, or more than one borehole. The Pipe diameter should be 20 to 40mm for best performance: large enough to reduce pumping power but small enough to increase flow velocity and cause ‘turbulent flow’ (giving better heat transfer).

Bear in mind that installers trying to reduce costs might skimp on the length or bore of pipe, or the depth of the trenches.

How does air temperature affect the efficiency of an air source heat pump?

As the air temperature outside drops, the gap increases between that and the temperature needed in the building. An air source heat pump (ASHP) will then use more electricity.

In a damp and cold climate, frost will build up overnight on the external part of an air-source heat pump. An energy-intensive defrosting cycle then has to be used, so the efficiency will decrease and running costs increase.

When comparing quoted COPs, check what source and delivery temperatures they’re based on, and if hot water is included. Ask installers for figures that reflect winter air temperatures. Here are some example figures you may see for the COP of a heat pump at different outdoor and delivery temperatures:

Temperature (Inlet) Temperature (Delivery) Heat Pump COP (7kW)
-7°C 35°C 2.3
2°C 35°C 3.0
7°C 35°C 3.4
7°C 45°C 2.8

If you live in a place where very cold winters are common, then the extra investment in a ground source system may be worthwhile, because you’ll always have a source temperature of about 7°C.

A German study (Frauenhofer Institute) found that ASHPs in new buildings achieved an average COP of 3.0, while those added to existing buildings had an average COP of 2.6 (very few of these had underfloor heating).

An early Energy Saving Trust field trial of several heat pumps across the UK found a wide variance in performance. Only a few reached an acceptable COP of 3 or more – however, many were early installations. In a second phase of this trial, various remedial measures were taken to improve the systems. After this, the average seasonal performance factor (SPF) for ASHPs was 2.45, compared to 2.8 for ground source systems. All but one of the GSHPs being monitored met the benchmark standard of 2.5, compared to 9 of the 15 ASHPs. The best performing heat pumps in the field trial supplied underfloor heating.

Can I power a heat pump with solar PV panels?

It sounds great in principle to heat your house using a heat pump, and get the electricity needed using solar photovoltaic (PV) panels.

However, the UK climate makes this impractical. Very little solar energy is available at the time of the year when your heat demand is greatest. A fairly large 4kW solar PV roof (around 30m2) will produce around 15kWh of electricity per day in May or June, but only 3 or 4 kWh on a typical day in December or January. A heat pump may need about twice as much electricity as this, plus you’ll have several other electricity demands to meet.

A solar PV array can still be a good investment in itself, generating low carbon electricity to use in the home or to export and contribute to decarbonising the grid.

If you live in a rural area, you might have wind or hydro power available to you, which give more energy in winter. However, most homes don’t have a suitable site for these energy sources.

In a zero carbon future we will be able to run heat pumps using electricity supplied through the grid from renewable energy sources that generate power in winter. These are mostly large-scale – such as offshore wind farms and wave & tidal power.

Are air source heat pumps noisy?

There will be some noise – ask your installer and also check technical literature on manufacturers’ websites for figures. The external part of an air source heat pump (ASHP) is basically the same as an air conditioning unit, but they do vary a lot – so don’t judge all ASHPs by the noisiest air conditioner.

Bear in mind that performance has improved in recent years as use of the technology has grown. Ten years ago, about 65 decibels (db) may have been quoted for the noise level at 1 metre from a collector unit. New units may now be able to run at less than 50db – but do check the noise rating at full power. By comparison, normal conversation may be at a noise level of 50db, a busy office about 60db, and a busy street about 70db.

Decibels are measured on a logarithmic scale, which means that an increase of 10dB will correspond roughly to a doubling of loudness. Measurements of environmental noise are usually made in ‘Acoustically Weighted Decibels’, or dB(A), which includes a correction for the sensitivity of the human ear.

Should I sign up for a green tariff?

Signing up for a ‘green tariff’ from a company focused only on renewable energy is a great way to support the renewable energy industry. Changing your supplier is now very easy, and in most cases won’t make any difference to your supply.

One issue is that the small companies that specialise in renewable energy may not be part of the ‘Warm Home Discount’ scheme (although if they get enough customers they will be brought into it). This scheme gives a rebate to people at risk of fuel poverty, such as those receiving Pension Credit Guarantee Credit and some others. If this applies to you then you’ll need to stay with a larger provider to get this rebate.

Which green tariff?

We recommend choosing a company that only supports renewable energy. This means your money will not indirectly go to operate or build fossil-fuel power stations.

All electricity providers are required by the government to include some electricity from renewable sources. If they just offer a green tariff as one of a range of tariffs, then they may be simply charging a premium for electricity they’re legally required to produce! This is why we recommend companies that invest your electricity bill payments only in more renewable electricity.

If enough people sign up for renewable energy tariffs with these suppliers, then demand for renewable electricity will rise above the minimum government requirement. Therefore, as well as signing up yourself, encourage others to do the same.

The Ethical Consumer website gives a ranking based on the ethical and environmental record of electricity & gas suppliers. You have to be an Ethical Consumer subscriber to see the whole report, which gives more details.

I’m on a green tariff – so can I use as much electricity as I like?

It’s important to bear in mind that signing up to such a tariff does not mean you can leave all your lights on because it’s all zero carbon!  If you use more electricity through your green tariff it means that less renewable electricity is left for those that are not on green tariffs. This means that more fossil fuel will be burned to meet their share of energy use.

Also, every means of generating electricity has some environmental impact, including the energy and materials that go into manufacture and installation. Energy saving measures are vital, because it’s them much easier to meet our electricity needs with energy sources such as wind farms, and wave & tidal power. Our Zero Carbon Britain project has a lot more details about how we can meet all our energy needs using only renewable energy.

Is underfloor heating good, and how is it installed?

Wet underfloor heating is definitely something to consider if you are replacing your heating system, undertaking renovation work, or building from scratch. It gives a very even temperature over the floor area, and works well with renewable energy sources and well-insulated buildings. Underfloor heating is slower to respond to changes or to heat up from cold, so is best suited to well-insulated buildings and works well if you have good levels of thermal mass.

The system should be sized to run with a flow temperature of 35°C, compared to radiators which may run at 60°C or more. To allow for a low flow temperature, the pipes should be spaced at 100mm or less. It’s therefore a good match for a ground or air source heat pump as these are more efficient when supplying low flow temperatures. Underfloor heating also works well with a modern condensing boiler, because this will then run in condensing more more often.

The radiant heat given off by the floor results in high comfort levels. In our experience this means that, in practice, you can run it at lower temperatures and save more energy. It should be possible to have the house at a temperature 2 or 3 degrees lower than with conventional radiator use. It’s great for rooms with high ceilings because the heat goes up the centre of the room. You’ll also have more free wall space without radiators.

There are several ways of installing underfloor heating – see below for examples based on what we’ve done at CAT. It’s possible to do it yourself but you’ll need good building and plumbing knowledge & skills. A professional installer can do a pressure test to check for any leaks and ensure it is stable before covering it over. 15mm bore pipe is better than 10mm, as it’s easier to then pump the water around. A manifold connects the pipes to the heating system and heating controls just as in a normal heating system.

Solid Floor

Start with a limecrete or concrete subfloor above a damp-proof membrane. Above this, add a layer of solid insulation (could be cork), which should also run up the edge of the floor to stop heat escaping into the walls. Then lay the underfloor heating pipes (cross linked polyethylene or barrier pipe), running back and forth along the length of the floor. Various fixing systems are available to hold pipes in place. Over this lay a screed of limecrete or concrete, covering the pipes by at least 50mm. Finally, add a floor finish such as tiles, slates or stone.

Suspended Floor

Insulate between (and perhaps also below) the joists to prevent heat loss, and then lay a subfloor over the joists. The underfloor heating pipes lie on top of this, between battens. To ensure that the heat is evenly distributed, you can either fill the gaps with a weak sand/lime mix, or fit aluminium plates to the pipes to dissipate the heat evenly through the floor. The floorboards then sit on top of this, held by the battens.

The timber must be really dry – if it isn’t, it may shrink and crack with the heat. The moisture content of timber for a wooden floor with underfloor heating should be about 8% for retrofitting and 10% for a new build. Alternatively, lay it loose for the first year, so that adjustments can be made for any movement. The surface temperature shouldn’t go above about 30° or the timber may distort, and you should leave a gap around the edge of the timber floor to allow for expansion.

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