Heat Pumps

Heat Pumps

What is a heat pump?

You’ll almost certainly have a heat pump in your home already, as it’s how fridges & freezers work – basically by ‘pumping’ heat out. Most air conditioning units are also heat pumps, as are a small number of new tumble-dryers.

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).

In very simple terms, a compressor creates a small amount of higher temperature energy from the large amount of energy collected at a lower temperature. The actual process is complicated, using the ‘vapour compression cycle’ to harness the energy available when vapour returns to liquid.

Is a heat pump an efficient choice?

A heat pump uses electricity to run the compressor and other components. To be a cost-effective and low carbon option, electricity use needs to be as small as possible. To achieve this you need a well-insulated home, a low-temperature heating system and a correctly-sized heat pump.

A ‘coefficient of performance’ (COP) measures the efficiency of a heat pump. At a COP of 3, a heat pump gives 3 units of heat energy for each unit of electricity used. However, the basic COP excludes any top-up heating (e.g. an immersion heater) or electricity for pumps and fans. Check the source (inlet) and delivery temperatures for any quoted COPs.

For overall performance, make sure you check for a seasonal measure that compares 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). A heat pump with poor seasonal efficiency can have higher running costs and carbon emissions than a modern oil or gas boiler.

How do I specify an efficient heat pump?

Heat pumps work well when delivering low temperature heat. The best option is underfloor heating, with a flow temperature of 35°C. If this is not feasible, choose large radiators that can run at 45-50°C. Avoid supplying water at 60°C or more to standard size radiators, because this will result in poor efficiency and high running costs. Insulating your home beyond current UK Building Regulations levels will ensure that low temperature heating is very effective. And with a lower heat demand you can install a smaller, cheaper heat pump.

A small amount of hot water will be needed at 55°C for bathroom & kitchen. A heat pump that can heat a small amount to this temperature may still be more efficient than topping up with an electric immersion heater. Minimising this hot water use is important, for example with an efficient shower head and spray taps. In the summer, hot water could come from solar water heating or by using solar PV panels to run the heat pump.

Are heat pumps a low carbon option?

At the moment, one unit of grid electricity has roughly double the carbon emissions of one unit of heat from a gas boiler. A heat pump with an SPF of 3 will use about one-third as much fuel as a gas boiler. Therefore, even with double the carbon emissions per unit, overall carbon emissions will be lower. The benefit will be greater against higher carbon fuels such as oil, LPG or direct electric heating.

Hopefully you’re already on a green tariff, but you’d still want an efficient heat pump to keep nationwide electricity demand low. In a zero carbon future, we could run heat pumps using electricity supplied through the grid only from renewable energy sources.

Another factor is that a heat pump contains about 2 kilograms of ‘refrigerant’. This is often a hydrofluorocarbon (HFC), which is about 1500 times more potent as a greenhouse gas than carbon dioxide – so a leak would be very damaging. Some suppliers use the refrigerants propane or isobutane (R290 & R600a), or carbon dioxide (R744) or ammonia (R717). These all have a much lower impact if accidentally leaked.

Ground source or air source?

A heat pump operates most efficiently when you minimise the temperature gap between the heat source and the heat demand.

As ground temperatures are higher in winter than air temperatures, a ground source heat pump (GSHP) should be more efficient. About two metres down the ground stays at about 10°C all year, protected from extremes of heat or cold. This heat is mainly from solar energy absorbed by the ground in summer.

Air source heat pumps (ASHP) may not be as efficient, but are cheaper to install. The efficiency gets lower as outside temperatures drop, and when the collector needs defrosting this uses more energy. In a place with a mild climate they can still perform well. Some ASHPs are air-to-air rather than air-to-water. Bear in mind that warm air heating systems are uncommon in the UK, and may be costly and difficult to install into existing homes.

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

How much will a heat pump cost?

Heat pumps are still a relatively rare option for central heating, and installation prices vary a lot. The best thing to do is to compare quotes from a few installers, and compare to other options (such as a biomass boiler).

A ground source system is likely to cost about £1000 per installed kilowatt (kW). The heat pump itself will be £400 to £600 per kW, with trenches £300 per kW or boreholes £500 per kW. Installation costs for air-source heat pumps can vary from £5,000 to £9,000. Putting in underfloor heating may cost about £2,000.

The running costs (electricity use) depend on heat demand, system efficiency and electricity prices. To supply 12,000 kWh of heat per year, a heat pump with an SPF of 3 will use 4,000 units of electricity at a cost of £600 (at 15p per unit).

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 RHI tariff is paid for the proportion of renewable heat. This proportion is calculated by multiplying assessed heat demand (in the 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.

From 1 April 2019, ground or water source heat pumps can get 20.89 pence per kWh of renewable heat. Air source heat pumps get 10.71 pence per kWh of renewable heat. These payments last for 7 years and increase with inflation.

See Ofgem’s RHI pages to find out about the eligibility requirements and how to apply. There is 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).

Further information

Until recently the CAT store sold the comprehensive book ‘Heat Pumps for the Home’, by John Cantor. Unfortunately this is currently out of print, although we hope it will return. In the meantime it is available as an e-book from Crowood Press, and the author also has a useful website.

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

The BRE have published a heat pump annual efficiency estimator website, to help compare seasonal performance.

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.

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 could run many heat pumps using electricity supplied through the grid from only renewable energy sources, such as offshore wind farms and wave & tidal power that generate through winter.

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.

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