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 direct electric heating, which gives one unit of heat for each unit of electricity consumed. The running costs of a heat pump will therefore be far less: about one third as much. In addition, to run direct electric heating we’d need about three times as much renewable energy generation. That would make it 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 from about one to one-and-a-half times as much as from 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 are already less than half of 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 have an efficient heat pump, in order to keep nationwide electricity demand low. Using less electricity is always the most important thing, even if you buy it from a green supplier. To achieve this you need a well-insulated home, low flow temperatures around your heating system, and a correctly-sized heat pump.

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 should be delivering low temperature heat. The best option is underfloor heating, with a flow temperature of 35°C. However, adding this to an existing house is often not feasible.

An alternative is to have radiators with a large surface area and supply them with water at 45 to 50°C. Avoid supplying standard size radiators at 60°C or more because this will result in poor efficiency and high running costs.

You’ll need to have decent levels of insulation for these low flow temperatures to work. A heat pump should therefore be part of a home refrofit to a level better than current UK Building Regulations. Getting a proper assessment of heat loss is important for then choosing suitable radiators. For example:

  • In a room with a heat loss of about 1700 watts, a radiator with a peak output of about 2000 watts needs a flow temperature of over 60 degrees C to keep the room warm.
  • In a room with a smaller heat loss of about 1000 watts, a radiator with a larger peak output of about 3500 watts will still keep the room warm on a flow temperature of about 45 degrees C.

The second option is much more suited to a heat pump, and will keep electricity consumption low.

You’ll still need some hot water at about 55°C for your bathroom & kitchen. A heat pump system can be designed to heat a hot water cylinder to this temperature. This approach should be more efficient than topping up with an electric immersion heater. It’s important to first minimise hot water use by having efficient shower head and spray taps. In sunnier months you could get hot water from solar water heating or by using solar PV panels to run the heat pump.

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. A COP may be 3.5 if supplying water at 35°C, but drop to 2.5 when suppying 55°C.

The BRE have produced a heat pump annual efficiency estimator website to help compare seasonal performance. You can choose specific models of heat pump and compare their efficiency in UK temperatures.

Ground source or air source?

A heat pump will have maximum efficiency 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 should be much cheaper to install. Electricity use will increase when outside temperatures drop, and if the collector needs defrosting this uses more energy. However, if you live somewhere with a mild climate an ASHP can perform well. For example in many urban areas.

Some ASHPs 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 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.

Running costs will depend on heat demand, system efficiency and electricity prices. In CAT’s Zero Carbon Britain report we propose refurbishing all homes to reduce heat demand. An average house could have heat demand reduced from about 10,000kWh to 4,000kWh. A heat pump might then use about 1500kWh of electricity to meet the reduced heat demand. 1500kWh of electricity could cost about £200 per year on an average tariff. This would be about the same cost as suppying 4,000kWh from a gas boiler.

What support is available?

Heat pumps receive support through the Renewable Heat Incentive (RHI). The amount you get back is based on the heat demand calculated in your home’s Energy Performance Certificate (EPC).

The domestic RHI gives you payments over 7 years, increasing with inflation. The payments are for the proportion of ‘renewable heat’, which is basically your heat demand minus any electricity use.

The proportion is calculated by multiplying assessed heat demand (from your EPC) by (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 so 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.

Further information

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.

In 2021 we are running a new one day course: Renewables for Households: heat pumps. See also the related questions below for more details.

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

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).

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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