Heat Pumps: FAQs

What is a heat pump?

A heat pump is a very efficient source of heat for your house using electricity. If the electricity is renewable then you have a very low carbon heating system. It works like a refrigerator, only instead of cooling the inside and dumping waste heat outside, it cools the outside (either the air or the ground or sometimes water) and dumps the heat inside the building. The great thing is you get more heat energy into your house than the energy you use to run the pump: typically 2.5-4 times as much. The extra energy is renewable. For example, when you take heat from the air it has been warmed by the sun.

The super efficiency of heat pumps does however have some limitations and in particular they are less efficient when generating high temperature heat. High temperature systems can run up to about 80C whereas low temperature systems normally go up to 50C or perhaps 55C. If you currently have a boiler and radiators then your boiler is probably feeding the radiators at around 65C. With a lower temperature your radiators will give you less heat. However, many radiators are over-sized, especially if the home insulation has improved since they were installed. Your heat pump installer will calculate the heating requirements of your home and determine what temperature you can run your radiators at and still be warm. If you need to, you can replace some radiators with more efficient/larger ones. Underfloor heating runs at a lower temperature anyway which is ideal for heat pumps.

What different kinds of heat pump are there?

The most common sort of heat pump is a low temperature air source heat pump (LT ASHP). This takes heat from the outside air and heats water for your radiators and hot water cylinder (you will need this installed too if you did not have one already). The outside unit looks like an air conditioning unit with a big fan. A low temperature heat pump can supply hot water at up to 50C

There are three configurations you can have: split system, monobloc, or fully internal. In a split system, the part that heats the water is inside (if you are familiar with how a refrigerator works, the evaporator is outside and the condenser is inside). The indoor unit is about the size of a gas boiler, although it may be integrated with a hot water cylinder, in which case it takes up no more floor area than the cylinder.

In the monobloc all the main parts are outside and the installation is simpler. However, the connecting pipes run at radiator temperature and if it is any distance from the house there will be heat losses. If the monobloc is right next to the house the heat loss will be small.

It is also possible to have a small heat pump with no outdoor unit, just ducts for bringing air in and out. All the working parts of the system are indoors. This configuration is potentially useful for small flats with no ground floor as it can be installed entirely from the inside. Right now there are very few products of this type: Ground Sun makes one.

There are other important variations.

High temperature heat pumps (HT ASHP) can reach higher temperatures (up to 80C is normal) which means that is a more or less direct replacement for your existing boiler, with no change to your radiators. However they are physically larger, cost more, and you lose efficiency when you run at a high temperature this is likely to increase your heating bills by 25% or more.

A ground source heat pump (GSHP) takes heat from the ground, with pipes that run underground, either fairly shallow (e.g. 2m down) but zig zagging over a large area or vertical boreholes. They are more efficient than air source in winter as the ground does not get so cold as the air. However, they are more expensive. The shallow ones need a lot of space and installing them is very disruptive because of digging the trenches. The deep ones are good for cooling in the summer as well as heating in the winter. Water source heat pumps (WSHP) take heat from either a flowing water body (a large lake or river) or an underground water source. These are quite unusual as most people do not have a suitable water body nearby.

You can also get an air-to-air system (A2AHP). This heats your home with warm air like an air conditioning system. Air to air systems can often work for cooling too. You can get ones for individual rooms which fit into the wall. Or, you can have a multi-split unit with one outside unit (the evaporator) supplying air to multiple rooms, each with a small indoor unit (compressor) on a wall or the floor. Air to air systems do not provide hot water so you will need something for that too. Air to air systems are probably your best approach if you do not have wet radiators already for example to replace night storage heaters. They are very efficient.

A hybrid heat pump is a heat pump (usually LT ASHP) with a gas boiler for backup. If everyone uses electricity for heating then demand will be very high on cold winter evenings and the supply system will need a lot of new infrastructure to cope more generation and more distribution too. Flexible heating systems can take some of this strain by using alternative sources, such as gas or stored heat, when necessary. Hybrid heat pumps will be one of those options. You would need to be on a variable tariff, where the price changes according to the supply/demand balance. If supply is tight then the price will be high and this is your signal to switch to the gas boiler.

Another way to configure a hybrid system is with an under-sized heat pump. Then when the heat pump is not enough you can use the boiler. This system would be cheaper to install but will use a lot more gas so the overall carbon emissions will be higher.

Exhaust air heat-pumps are quite unusual. They take in air from inside your house rather than from outside. You typically use it with air extracted from kitchens and bathrooms. If the house is very draughty or poorly insulated then this will not be enough and you will need additional heating. However if your home is air tight and well insulated an exhaust air heat pump can be very efficient. There is more about exhaust air heat pumps on the Design Buildings Wiki.

What about hot water?

Heat pumps cannot supply hot water on demand like a combi boiler so you will need a hot water cylinder. Even if you already have a cylinder, if you get a low temperature heating system you will need a new one with a larger heat exchanger.

You should set your hot water thermostat as low as you can. You can probably get away with water at 50C most of the time (just mix in less cold when you run a bath); the heat pump will need to supply a little hotter than this to get the cylinder up to temperature. You may find you need a bigger cylinder than you had before, because with the lower temperature you will add less cold to get the temperature you need.

It is often said that you should heat your hot water cylinder up to at least 60C once a week to avoid the danger of legionnaires disease. Building regulations for domestic premises (as of December 2020) do not actually require this, though they do say that the hot water supply system must deliver water free of bacteria. If you choose to run a weekly legionella cycle, a high temperature heat pump can do it directly. If you have a low temperature heat pump you will need a backup immersion heater. If you have a hybrid heat pump you can use the gas boiler.

If you are using warm air heating (air to air) then you will need a separate heating system for your hot water. This could be a cylinder with an immersion heater or an instant water heater. The cylinder can be heated with off-peak power allowing you to take advantage of lower tariffs. With the instant water heater you only heat as much as you need.

You can combine a heat pump system with a solar hot water panel as well: see our solar thermal FAQ for more information on this.

Will I need to make changes to my radiators?

The main concern with the rest of your heating system is your radiators. They may not be efficient enough at low temperatures to heat your room adequately. Your installer will do calculations on each room to determine what the radiators will have to deliver in cold weather. (The definition of cold weather varies somewhat depending on where you live.) Then if you have some radiators that are not big enough they can be upgraded. This may mean they will be thicker, with more internal fins, rather than being taller or wider, or you can add a radiator fan to increase the heat output. There is a good description of radiator types here (The Green Age) and a review of radiator fans here (Mighty Gadget). Also, there is a table of different types of radiator and their heat output per unit area in this report by Carbon Trust on heat pump retrofit in London (table 6). For example if you currently have single panel radiators running at 70C you can upgrade to double panel double convector radiators to run at 45C.

Your installer will use something like the MCS Heat Emitter guide in his calculations. This has a table indicating what oversize factor you need, and hence how big the radiator. For example, suppose the room has a heating demand of 0.9 kW and the nominal radiator power for the existing radiator is 2.0. That means you have an oversize factor of 2/0.9 = 2.2 The nominal radiator power corresponds to a flow temperature of 70C. Reading off the chart, with an oversize factor of 2.2 you can run your radiator at 55C. The chart also suggests you can get an efficiency of 2.4 with an air source heat pump or 3.1 with a ground source one. However, this also depends a great deal on the actual heat pump model and you may get better than this.

Chart from the MCS heat emitter guide. Note that although this refers to SPF it is for space heating only and is for the heat pump only (corresponding to SPF(H2), see What does COP mean and SPF)."

If you want to run at a lower flow temperature you will need to improve your insulation or draught proofing to reduce your heating demand, or upgrade your radiators. To run at 50C you need an oversize factor of 2.4 so in the above example you would need a radiator power 2.2 kW. To run at 45C you would more power still, as shown in the table below. The tricky bit is determining your heating demand - your installer will do all this for you.

Heating demand (kW) Flow temperature (C) Oversize factor (from chart) Radiator power (kW) = demand x oversize
0.9 55 1.9 1.7
0.9 50 2.4 2.2
0.9 45 3.1 2.8

You are quite likely to already have oversized radiators, especially if you have installed efficiency measures in your home. If the radiators were sized for the heating demand before you insulated then your heating demand will have gone down and your oversize factor has gone up.

As well as the radiators, there are other factors in your heating system that may need to be upgraded. For example, for running at a lower flow temperature you need to pump water around the heating circuit more quickly. If the pipes in your heating circuit are very narrow the resistance could be too great for your pump. 10mm or more you should be OK for individual radiators but the main pipe runs need to be larger. If you have microbore pipes throughout this could be an issue.

Will I need to make changes to my underfloor heating?

If you have underfloor heating, that should be fine as underfloor heating usually runs at a low temperature. If you were running it before from a gas boiler, this provides water that is too hot for the underfloor heating so you will have a blending valve to reduce the temperature. This will need to be adjusted or even removed to make the system work at the temperature you get from the heat pump. (We have heard of one case where this was not done and the result was the floor did not get warm enough. It took many visits from different engineers to diagnose the problem).

What is a buffer tank? Do I need one?

Heat pumps give very poor efficiency if they have to switch on and off frequently this is called cycling. The same is also true with your boiler, but it is even more important for heat pumps. There are some heat pumps that can modulate their power (typically down to about 30%) but some have fixed output so when heating demand is low they will be on for short periods at a time. To avoid cycling, you need a minimum amount of thermal capacity in the heating circuit. There will be some in the radiators and pipework but if all your radiators have thermostatic radiator valves then they dont count as thermal capacity as they are not in the circuit all the time.

BRE (Building Research Establishment) says that buffer tanks are rarely installed in this country (report here) but they can make a big difference to performance. The disadvantages of a buffer tank are the extra cost and space to put it. There will be heat loss from the buffer tank. However if it is in a heated area of the house, that heat is still useful.

Whether you need one depends on both the heat pump you have selected and your particular situation - so ask your installer whether you should have one and if so what size.

What is a thermal store? Do I need one?

A thermal store is usually a large tank of water used to store heat. You can also have a heat battery which is similar but uses a special material that can store heat in a smaller space than water. There are two main uses for a thermal store: 1) To combine heat from different sources for example from solar hot water panels, or from solar electric panels if you do not need the power, and/or from a biomass stove as well as a heat pump. 2) To store heat when it is cheap, to use later when it is expensive useful if you have a time of use tariff that it expensive at peak times. An electric battery will do the same job and take up less space but it will cost more.

A thermal store supplies heat, not hot water so it has heat exchangers for the heat coming out as well as the heat going in. See also How is a thermal store different from a hot water cylinder on Nicola's blog.

If you have a thermal store you probably do not need a buffer tank as well, but it depends on how it has been configured.

Screen grab from an Octopus Agile account. Chart shows electricity demand for a dwelling with a heat pump avoiding the expensive peak period

Will it reduce carbon emissions, save money?

Running on electricity means it can be very low carbon. The emissions from electricity are reducing rapidly. Even at the current level you will certainly save carbon emissions compared to natural gas or oil (to be better than gas you would need an SPF of 1.15 whereas you should get at least 2.5) and this is going to get even better over time as we get more low carbon electricity.

Saving money is less certain. The maintenance costs are similar to existing boilers so the difference in running costs depend on energy prices and efficiency. If gas is a third the price of electricity, you need efficiency of at least 270% (SCOP = 2.7) to reduce bills (assuming your existing boiler is 90% efficient). This is certainly possible with a low temperature system. If you switch completely away from gas you can avoid the standing charge on that as well as the fee for actually using it. Standing charges vary but could easily be another 70 per year in savings.

If you currently use electricity directly for your heating then you will certainly save by using a heat pump. Even if you are using night storage heaters, you should still make savings, though not so much because you will need to run the heat pump during the day (unless you have a large thermal store). You might be better off switching back to standard rate electricity as Economy 7 is expensive during the day.

It is also worth looking at time-of-use tariffs that offer better value for households with heat pumps. Octopus Agile (see screengrab, right) is very good value at the moment provided you avoid peak times. Depending on the efficiency of your house it might be that you can turn off the heat for three hours (usually 4-7pm) without it cooling too much, or you could use a thermal store or an electric battery to cover the peak period. Also Good Energy has a tariff for people with heat pumps that is lower in the autumn and winter. There are likely to be more of these special tariffs in the near future.

You will be able to add a battery or thermal store to your system later, as long as you have space for it. You dont need to get everything at once. The installation costs for heat pumps are considerably more than for a replacement boiler but you can get help with them from the Renewable Heat Incentive. A low temperature heat pump costs less to install than a high temperature one, as well as less to run. However you may have to upgrade your radiators as well.

Can I get a grant

See our finance page for information on grants for renewable heating.

What does COP mean, and SPF?

These are all measures of efficiency. COP stands for Coefficient of Performance and it indicates how much heat you get out of your pump for every unit of electricity you put in under particular conditions (e.g. 7C outside, supplying heat at 45C). If the COP is 3.0 that means you get 3 units of heat for 1 unit of electricity so the efficiency is 300%.

SCOP is a seasonal average COP. It is basically the average COP considering a whole year with a typical range of weather and heating demand. This is for space heating only, and it is for the heat pump, not the heating system as a whole.

SPF is the Seasonal Performance Factor and this does include hot water provision too, so is lower than SCOP. Strictly, it includes heat supplied to your hot water cylinder from the heat pump but not use of the immersion heater. In order to qualify for the RHI your system must have an SPF of at least 2.5. SPF can be measured at different boundaries. It is usually quoted as SPF(H2). This includes just the heat pump with associated pumps and fans and this is the one that is used in the RHI criterion. SPF(H4) includes other components that are needed to drive the heating circuits and is therefore more accurate for calculating your heating bills. As you include more of the system the SPF goes down a little. For example, a study on actual heat pumps by UCL (For DECC in 2017 but based on data collected up to 2014), found air source heat pumps average SPF(H2) was 2.65 (and hence would qualify for the RHI) while the average for SPF(H4) was 2.44.

What efficiency can I expect?

Heat pump manufacturers report either SCOP for the year or COP under different conditions. The radiator oversizing chart above shows the sort of SCOP you can expect depending on your radiator flow temperature, although this is conservative and you may get better than this depending on your particular heat pump. You might like to watch Trystan Lea (video link) telling us about his installation in a terraced home with solid walls - achieving a mean COP of 3.9.

You can expect better efficiencies from a ground source heat pump than an air source heating pump. For example, with a flow temperature of 45C you can expect an SPF of 3.0 with an ASHP or 3.7 with a GSHP. However a high temperature ASHP will give you less efficiency at least when you run it at a high temperature. For example, you might get 4.2 at 35C (good for underfloor heating), or 3.6 at 50C but only 2.9 at 65C (see chart). 2.9 is still not bad but remember that this is the SCOP, not the SPF so the SPF will be a little lower.

This chart shows technical data reported for an actual heat pump.

For a more accurate SPF calculation based on an actual heat pump product, you can try this estimator from BRE (Building Research Establishment). This takes into account the flow temperature and your buildings heating demand compared to the heat pump heating output how over-sized is your heat pump. It ignores buffer vessels.

However, as with boilers, heat pumps do not always perform as well as they are supposed to. You can get poor performance for all sorts of reasons to do with both the heat pump installation and also configuration. Here are some of the common ones:

  • An oversized heat pump or too small buffer tank, causing it to cycle on and off frequently.
  • Heat pump too small, or ground loop too small for a ground source heating pump, so that backup heat is often needed.
  • Asking the heat pump for higher temperature heat than was designed
    • You need to set the hot water thermostat as low as you can, with a legionella cycle once a week to sterilise the water.
    • Weather compensation controls should reduce the radiator temperature demand most of the time.
    • Avoid big step changes in thermostat settings as this requires a sudden burst of heat that can only be provided by high temperatures. Try not to increase by more than 1.5C per hour.

In the early days of the RHI there was a survey done of how well heat pumps performed. The data was collected between 2011 and 2014 and at that time there were a lot of poor installations. The average efficiency was only 2.6 for ASHP and 2.9 for GSHP (for SPF(H2)). Just two thirds of ASHP and three quarters of GSHP met the required minimum efficiency for the RHI.

Is my house heat pump ready?

In principle a heat pump can be as large as necessary, as long as you have somewhere to put it. The limiting factor is more likely to be your heating demand and radiators/underfloor heating. You need the demand to be low enough that you can supply it with low temperature heat preferably 45C or less. Otherwise you will need a high temperature heat pump and that will also be less efficient.

Assuming you do not wish to paper your walls with radiators, you need your home to be well insulated to start with. If you have good insulation in walls and loft, double glazed windows throughout and reasonable draught proofing you should be OK, though you may need to upgrade some radiators. One clue is how rapidly your home loses heat overnight. If the temperature plummets and you need more than an hour of heat to warm it up in the morning, then you would do better to install some more energy efficiency measures before the heat pump. The Carbon Trust report on heat pump retrofits in London (section 4.3) suggests that if you have an EPC rating of C or better without the benefit of PV panels you can probably run with low temperature heat. If you have PV panels then this will improve your rating but does not improve your heating efficiency so the rating is not a good indication of your heat pump readiness.

Another approach is to install underfloor heating. This is generally good as you get a very large surface area.

You also need somewhere to put the heat pump. For an air source heat pump you need somewhere to install the external unit well ventilated and exposed to the elements, such as an outside wall or flat roof with around 4m2 space around it. The unit itself can be quite small e.g. 1m x 0.5m x 1m high but it depends on how much heat you need. Also it needs to be reasonably close to the house - not more than say 15m.

Will I need a backup heating system?

If the heating system has been configured to MCS requirements it should be designed to provide you with enough heat 99% of the time. That means there may be a few days a year when it is a bit chilly and a backup system with a wood stove would be nice. Your backup system should not be based on electricity, since you will be using it when heating demand is high and the electricity supply system is already under strain. In any case, it is a good idea to have some kind of heating that is not reliant on electricity in case of power cuts.

Unless you have a heat pump that modulates power at part load it is more efficient overall to slightly undersize the heat pump and use backup heat in cold weather. However, this will not be eligible for the RHI.

What does an air source heat pump look/sound like?

This is the outside unit of a NIBE 8kW split system at the back of a terraced house. The internal unit is in the house above the hot water cylinder.

The external unit looks like an air conditioning unit. The units vary in size, depending on how much heat you need. Inside there is a fan. There will be some noise, so you won't want to put it immediately outside your bedroom window. However fans can be very quiet if they are well designed. Larger units with slower fans will generally be quieter. If your neighbours are concerned it is worth pointing out that the heating is very unlikely to be active when they are relaxing in the garden in the summer.

What does a ground source heat pump look/sound like?

When it is in the ground, the heat pump is invisible. Here are some pictures from drilling boreholes, and you can find some more useful pictures and diagrams of ground loops on the EST website .

There will be a little noise from the pump.

What if the ground is frozen or there is a frost?

All heat pumps will be frost resistant and the circulating fluids contain anti-freeze. Even air source heat pumps should be able to operate down to air temperatures of -15°C, although they will not be very efficient in those conditions. If frost builds up on the external heat exchanger up it will automatically enter a defrost cycle.

Will my garden get more frost? (Where does the heat come from?)

An air source heat pump should have little impact on your garden as the air you make cold quickly mixes with other air. Also, on average your home leaks heat faster than the heat pump pulls it in. If it did not, your home would get hotter and hotter, which it definitely does not do. The electricity you use to drive the heat pump adds energy to the system as well, so there will be no overall cooling on the outside due to an air source heat pump.

If you have a ground source heat pump, then the ground surface above the ground loop will be cooler than it would have been and there is a possibility of more frost. This is more likely to be an issue with a shallow heat pump system. The ground is rewarmed mostly by the sun but it takes a long time both to cool down and to warm up again. If it is correctly sized the effect should be small.

If you have a ground source heat pump for an air conditioning system which does cooling as well as heating, then you are using the ground as a huge thermal store. Cooling your home in summer warms the ground up and stores heat for the winter. However this sort of system would not qualify for the RHI.

What maintenance does it need?

Heat pump systems need very little maintenance usually, but you should have an annual check to ensure all is OK. Air source heat pumps need a regular check that the filters are clear. You should be able to do this yourself. They do not need an annual safety check, as your gas boiler does. However there are a lot of moving parts that could go wrong so a regular maintenance check is sensible.

How long will it last?

Ground source heat pumps should have a design life of around 25 years. Air source heat pumps may not last quite so long as they have more moving components. You can expect a five year warranty.

How much does it cost?

A ground source heat pump will cost around 14,000 - 19,000 to buy and install. This includes a hot water cylinder you might not need this if you already have one and you are installing a high temperature heat pump. An air source heat pump is likely to cost 9000 to 13,000 (according to the EST) with high temperature systems at the top end again this includes the cylinder. If you need to make changes to your radiator system that will be extra.

Air to air heat pumps are cheaper and you can install them for a part of the house only for as little as 2,000-2,500. You can make substantial reductions in gas use if you turn down the gas heating thermostat and use an air to air heat pump for top-up heat in the area you use the most.

There is financial support available for most air source heat pumps. At the moment the Renewable Heat Incentive (RHI) provides payments over seven years, if you have a qualifying installation. However, you still have to find the costs up front. This will be replaced by a voucher scheme called the Clean Heat Grant which will provide payment at the time of the install.

Does it need planning permission?

Ground source heat pumps do not need planning permission unless your home is a listed building or in a conservation area. Air source heat pumps are a permitted development under most circumstances- see the planning portal for details . Also heat pumps will have to comply with building regulations but your supplier will probably be qualified to self certify their work.

What should I consider when talking to a supplier?

You must use a supplier certified with the MCS certification scheme in order to qualify for the RHI. You should also compare quotes and ask for reference sites (as with any supplier), and check out the details of the warranty - what is and is not covered. You should have a minimum one year warranty on the installation.

Your installer will estimate the efficiency you can expect and what your heating bills will be like. This needs to be based on the heating system needs for your particular house, not just the average given in a product brochure. However, this is still based on a standard heating schedule and thermostat setting, rather than your actual heating pattern.

If your supplier has not recommended a buffer tank, ask them if the heat pump modulates power output under part load conditions. If it does not, ask them if a buffer tank is appropriate, or if you need to keep TRV valves open to maintain enough thermal capacity in the radiator loop.

Your heat pump supplier may not be able to dig trenches or drill bore holes themselves in which case they should at least project manage that part of the work, unless you really want to of course.

Where can I get more information?

We thoroughly recommend this video:

The approach that Trystan has taken is excellent and provides a really good example of how the use of an ASHP in retrofitting an older home with conventional radiators is achievable and improves the comfort of the home.

The cost of the ASHP @ 2.4K including VAT and overall materials including new rads, hot water tank was under 5k for a 77m2 home . In 2020, Trystan's home reduced C02 emissions by 70% versus a gas boiler and the unit costs of energy/kWh was comparable to gas.

The introduction of smart meters, time of use tariffs and further decarbonisation of the grid will continue to support the decision to consider the retro fitting of ASHPs in many many homes within the UK.


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