Getting an air source heat pump

Converting to a heat pump is usually the best way to get off gas heating, saving tonnes GHG emissions per year. Picture: W.carter, CC0, via Wikimedia Commons

What does getting a heat pump involve?

This guide is about the process of replacing your gas or oil boiler with an air source heat pump (ASHP) in a domestic central heating system. This is the most common type of heat pump. An air source heat pump takes energy from the air outside and uses it to warm the central heating circuit and your hot water cylinder. It works the same way that a refrigerator pumps heat out of the food store and into your kitchen. This heat is renewable as it is replaced by warmth from the sun.

If you are not sure about whether a heat pump would work for you, you could book a visit to a home with a heat pump near you or read our guide Are you heat pump ready? or watch some of the Open Eco Homes videos such as "Living with a heat pump" or "Low carbon heating options".

Other kinds of heat pump include:

  • Ground source heat pump: takes heat from the ground instead of the air. They are more expensive to install (another £8,000 or more) and not everyone has a suitable piece of ground.
  • Air to air heat pump: takes heat from the air and uses it to warm air in the room, like air conditioning. Air to air heat pumps can often provide cooling too. You would have to replace all your radiators with air handling units and you need another solution for hot water.

There are advantages and disadvantages of all types of heat pump. See our FAQ page

Getting a heat pump for the first time is more complicated than getting a boiler replacement, because there are decisions to be made, extra plumbing and electrical cables to go in and often additional equipment. So there are a number of phases involved. Normally:-

  1. A salesperson/surveyor comes to do a preliminary survey and offer. This should involve a discussion on what sort of heat pump you want, where you want to put it, and your expectations in terms of efficiency and running costs. They may also offer a quote at this stage or ask for a deposit which pays for the next stage.
  2. They will then send someone to do a full house survey. The purpose is to calculate how much heat you need in each room, and also check that the radiators are capable of delivering this heat at a heat pump friendly temperature. You will agree with them which radiators (if any) need upgrading and what the new ones will be. (You can argue this is wasted time and money if every company does this from scratch. However the quality of the survey is key to the quality of the installation and no installation company is prepared to risk using a competitor’s survey.)
  3. The installation itself - normally one to three days work, but there is likely to be preparatory work involved too.
  4. Living with your heat pump afterwards including maintenance


COP - Coefficient Of Performance is a measure of the efficiency of the heat pump. It varies with the temperature outside (for an air source heat pump), what flow temperature it has to provide and also how hard it has to work - i.e. how much heat it has to generate compared to its maximum capacity.

Flow and return temperature - these are the temperatures in the fluid leaving and returning to your boiler or heat pump. Inbetween, the fluid has been round the radiator circuit or the through the heat exchanger in your hot water cylinder.

Modulation - this is the ability of a heat pump or boiler to run at less than maximum capacity - keeping up a gentle jog rather than a series of sprint/rest cycles.

Monobloc/split system - a monobloc system is a heat pump in one unit, which is located outdoors. A split system has an external unit and an internal unit. There are pros and cons to each.

SCOP is the annual average COP.

Before you start

Other information sources

We strongly recommend you talk to other people with heat pumps about their experience - we all learn from experience, preferably other people’s. Also do look around for professional information - Heat Geek has a series of videos, the first one is: What To Ask Your Heat Pump Surveyor and we also strongly recommend this one: How To MAXIMISE Your Heating Efficiency In 3 Simple Steps. Also here is another interesting video describing an installation- .

Finally there is more detailed information on our Heat Pump FAQ page.

Are you heat pump ready?

Heat pumps can heat most houses but you get much better performance (i.e. lower bills and less noise) if the house is well insulated and draught proofed first. If you do not do this first but you do it later, you will end up buying a heat pump that is larger than you need (both physically and in cost). Or you may buy a heat pump that is too small to start with and use backup heat in cold weather. However, to qualify for subsidies you normally need to install a heat pump that is sized for the whole house heating.

See also Are you Heat Pump Ready.

Do not believe the myths

If you consult your normal boiler service people they may try to persuade you to carry on with a boiler - and ‘hydrogen will be here soon’. This is not true: hydrogen (by which we mean 100% hydrogen, not just a 20% blend by volume that is actually only 6% by heating value) is decades away because the whole gas grid will need to be converted and this is going to be a huge undertaking, even if it does actually happen.

Here are some good references that bust other myths, such as the heat pumps only work in new homes, or that they are excessively noisy. Energy Saving Trust and Green Match blog

This short video showcases an install in a Victorian Terrace house with no solid wall insulation - getting excellent performance and bill savings compared to gas.

What is a reasonable price for a heat pump?

The cost of installing a heat pump for the first time is much higher than replacements will be afterwards, because there are all sorts of extra things that need doing - as well as the heat pump itself you will probably need a new hot water cylinder, possibly a buffer tank and lots of new plumbing. The overall cost is typically £7,000 to £15,000 depending on the size and complexity of the system. See How much does it cost to install a heat pump for a more detailed discussion.

The boiler upgrade grant (as of October 2023) give you £7,500 for an air source heat pump and it is be paid up front, unlike the renewable heat incentive which paid out over the following years.

What will the heat pump do to my energy bill?

Until recently the price of electricity was about five times as much as gas so that it was almost certain your bills would increase. However, wholesale gas prices have been very volatile lately and the relative cost of a heat pump is dependent on these prices. You may pay a bit more or a bit less, it depends on your situation. Your installer will give you an estimate of the running costs with the heat pump based on the particular system you buy. They should ask you what price you pay for your electricity to do this.

If you disconnect from gas completely then you no longer have to pay a standing charge which will save you about £70-£100/year. However, there may be a charge for the disconnection; this varies depending on who your supplier is. There will be a charge if you want to remove all the gas pipes and meter box as well, but you do not need to do that.

If you have solar PV panels and/or a battery system this will help reduce your bills too. You may find that your export goes down as you are using more of your own power. The main requirement for the heat pump in summer is to heat your hot water so you may find it useful to adjust the timing on that.

There is a useful tool for estimating energy bills here. (You may need to update the energy prices as these are changing rapidly and variable from one supplier to another.)

Heat pump tariffs and controllers

With a smart meter you can take advantage of time-of-use tariffs designed for heat pumps such as Octopus Cosy. This has three hours of cheap electricity twice a day: early morning and in the early afternoon. Then there is a period of expensive electricity during 4-7pm which is normally peak time. With a home battery you can use the cheap times to charge the battery and run the heat pump at lower cost. In most cases one charge should get you through the peak period, though it depends on the size of your battery and your house. If you do not have a battery, you may find it useful to heat the house warmer during those periods so that you need less energy at the more expensive times.

There are also special tariffs available for homes with EVs, usually with cheaper electricity overnight. If you have a home battery as well then once you have topped up your EV you can use the rest of the time to charge your home battery for the heat pump.

Optimising your heat pump schedule for different tariffs like this is tricky but you can get a smart controller such as Homely to do it for you. Most heat pumps come with basic weather compensation which means that they turn the radiator temperature up or down depending on the outside temperature. The goal is to keep the heat pump running at the lowest temperature that will keep your home sufficiently warm. If you vary the thermostat setting, for example lower overnight, then you need extra heat to warm the home up in the morning - some heat pump controllers will handle this too by increasing the radiator temperature. However, this is hard to optimise as there are tradeoffs in heating for longer but at a lower temperature., Homely learns how fast your home warms up and cools down and combines this information with weather forecasts to anticipate your heat demand and set the radiator temperature for the best efficiency. Homely can also take advantage of tariffs such as Cosy and even Octopus Agile, which varies the price of energy each half-hour according to wholesale prices. You will need to give the controller some leeway in your temperature requirement: up to 1, 2 or 3°C above the setpoint.


It helps to consider in advance where to put the heat pump and other equipment. Your installer will advise you on what is possible. You will need:

  • A heat pump - either an all-in-one outdoor system (monobloc) or a split system which has a smaller outside unit and an inside unit as well
  • A hot water cylinder.
    • If you do not have one already, you need to think about what size you need and where it should go. If space is a serious problem you can get a compact thermal store instead such as a SunAmp system. This is about a third the size but more expensive.
  • Possibly a buffer tank
    • Depending on what sort of system you get, you may need a buffer tank as well. Your installer will advise you. Buffer tanks are typically 20-50 litres. They might go underneath your hot water cylinder
  • Expansion vessel to maintain constant pressure in the heating circuit. This is relatively small, typically 5-15 litres.

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.

This heat pump has been sited in a front garden but hidden from the road by a hedge. The vegetation has been cut back to allow air flow around the unit. The gardener will need to keep an eye out in case it grows back too much.

At least part of the heat pump is outside - that is where it gets the heat from. For an air source heat pump (ASHP) this looks like an air conditioning fan and it needs to be at least 30cm from a building, with lots of room for air flow on at least two sides. Ideally it should be in a warm spot such as against a south-facing wall but this is not critical. However, the closer to the house the better. It needs something hard to stand on and with reasonable drainage (for when it defrosts itself, like your fridge or freezer). It will generate quite a cold blast on a cold day so it is not compatible with delicate plants in front of it. If placed within vegetation, eg a hedge, the shrubs must be pruned back minimum of 30cm away from the back and side of the heat pump

Heat pumps are normally classed as ‘permitted development’ so you do not need to get planning permission. However, this presumes they are not visible from the street and they are not close enough to neighbours to be a noise nuisance. They do make some noise because they have pumps inside and they have a big fan.

If the heat pump is some distance from the house you may need preparatory works to dig a trench for the electricity cables and pipes for the heating fluid.

The heat pump also needs to connect to the plumbing for your heating circuit. This is probably easiest from the place where your boiler is now. There will need to be a new pipe run from the outside unit to those connections.

Your heat pump will also need its own electric circuit which needs to go from the unit to your main consumer unit. However this will just be normal electric wire and it can usually run along the skirting board.

Choosing an installer - first survey.

As with all major works you should invite a number of installers to quote. You will find some suggestions here. How can you tell who to go with? Here are some guidelines.

Choose installers who are MCS registered and who have a track record as heating engineers. Heat Geek certified installers are usually good - and so is the Heat Geek consumer advice.

Never agree to accept a quote on the day - say you need to think about it. If they employ high pressure sales tactics, forget it. If they try to sell you a high temperature heat pump or a hybrid system (a heat pump in combination with a boiler for backup) in the first instance, reject them. These may be offered in the last resort, but high temperature heat pumps will give you lower efficiency and a hybrid system can never be zero carbon. A high temperature system can be sold as a direct replacement for a boiler, supplying radiators at 80°C but this will give you terrible bills.

Similarly, if they try to sell you a very large (e.g. more than 10 kW) system and you only have a small house, then they are going to overcharge you. As a general rule a mid sized house (90 m2) would need 6-8 kW.

Before they quote they will need to check some things to see what needs doing:

  • They will ask for your bills or a recent EPC in order to estimate the size of the heat pump required.
  • They will discuss with you where the heat pump will be installed.
    • You can have a monobloc unit, which is all in one, or a split system which has an outside unit and an inside unit. There are advantages either way - they should be able to discuss this with you.
  • They should check the radiator pipework to see if it is large enough for the job.
    • Some homes have microbore pipes less than 15mm which would almost certainly need to be upgraded. If they don’t check, they must have vastly inflated the price to cover it anyway, or they don’t intend to upgrade them which is worse.
  • They should check your electricity supply, to check the capacity and that there is a spare circuit on your consumer unit.
    • The heat pump uses a lot of power, like an oven, so it needs its own circuit. This will mean a new consumer unit if there are no spare slots. Their electrician will do this. If you do not already have a hot water cylinder with an immersion heater you will probably need another circuit for that as well.

Some heat pumps have a high starting current and if you have other high consuming high power (such as an EV charger) then you may need to upgrade your main fuse. If it needs to be upgraded your electricity distribution company (here it is UKPN) should do this for free. However, this needs to be done before they can install the heat pump and it can take months to arrange.

  • They will discuss your hot water needs.
    • Even if you have a hot water cylinder already they may recommend a new one. It is very important that the cylinder has a good heat exchanger. The existing cylinder may be old and scaled up or the heat exchanger may be too small.

Selecting a heat pump

Your installer will probably offer you a choice of two or three different heat pumps. If you prefer, you can of course select a heat pump range yourself and select installers that provide it. Either way, here are some features to look out for.


The key parameter is the SCOP which is the Seasonal Coefficient of Performance - the average efficiency you can expect over a year. The higher the SCOP the better. If SCOP is 3 that means you get three units of heat for every unit of electricity: 300% efficient.

The SCOP will vary depending on the flow temperature the heat pump has to supply to your radiators. Ideally you should configure for 45°C or cooler for space heating and then you should get SCOP of 3.5 or higher. 55°C is also OK but your SCOP will be worse, maybe 3.0. If you need to go higher than 55°C you will need a high temperature heat pump. However the heating temperature you need depends on your heat demand and what sort of radiators you have so it is not possible to finalise this until after the full survey.

The SCOP in the brochure is a guide to what you should get but it also depends on the weather and will vary year to year (see Why my heat pump performed worse this winter). It also depends to an extent on the heat loss of your home. If you have a very efficient home you may actually get lower efficiency because the heat pump has nothing to do unless the weather is very cold.


All heat pumps make some noise and there are rules restricting noise that might annoy your neighbours - but what about you? Most people are fine with the sort of noise a heat pump emits. But if you're sensitive to noise, you may want to look more closely at this.

Each heat pump has a maximum noise level specified in A-weighted decibels (db (A)). The ‘A’ weighting adjusts for our hearing, as we are more sensitive to some frequencies than others. However people vary too, the immediate surroundings may either muffle or echo the sound, and what you might find acceptable with background traffic noise might not be acceptable in a quiet rural area. Here are just a few pointers:

  • The Noise Abatement Society created a "Quiet Mark" for best-in-class products heat pump recommendations can be found here. Note however, that only products submitted for evaluation by their manufacturer will feature here.
  • You will probably want to look not just for a variable speed pump, but also for a variable speed fan. With these, the noise figure at full-pelt will only be emitted in mid-winter, when you and your neighbours are not sitting in the gardens, and only briefly when topping up the hot water.
  • Some models feature a "quiet mode", which allows you to schedule times of day where maximum output shall be reduced. This is only a useful extra on a machine that's otherwise quiet already; if it's a noisy design, with this feature thrown in as a concession, you may find you need to run it in quiet mode all the time - and then find you need to turn it off in cold conditions.
  • When going for an "audition", be aware that your noise perception is very dependent on the surroundings. You won't learn much from visiting a site next to a busy road. A garden surrounded by hedges will sound very different from one surrounded by brick walls.


When boilers were first invented they were terribly inefficient at part load. Then they invented modulating boilers which can give good performance down to a third or a quarter of their full load. Heat pumps are the same. This is important because a boiler or a heat pump has to be sized for the coldest day which means they are running at part load almost all the time. If they cannot modulate enough they have to cycle on and off and doing this often is terrible for efficiency and shortens the lifetime of the heat pump too. Cycling can be minimised with a buffer tank (see below) but this takes up more space. Heat pumps which can modulate are often called inverter heat pumps.


Whether you need a monobloc or a split system depends mostly on where you have to put it. However apart from layout there are a few other factors you may be interested in.

  • If you have a monobloc then heat wasted by the heat pump is lost to the outside so there is a slight loss of efficiency. Also there is heat lost in the pipes that run between the unit and the house so these need to be as short as possible and very well lagged.
  • If you have a split system then (usually) the pipes between the unit and the house carry refrigerant. This means you need a refrigerant engineer to install them. It also means that refrigerant leaks are more likely which means the choice of refrigerant is important (see below).
  • With a monobloc, the pipe between the unit and the house carries heating circuit fluid. This usually means antifreeze (glycol) in the heating circuit it could be disastrous if, for example, you go on holiday and turn the heat pump off and there is a big freeze while you are away. However, most people leave the heating on at a low level when they go away in order to protect the house and water pipes. When you have glycol, then whenever you drain your heating circuit you will need to top up again with glycol as well as corrosion inhibitor (as you do now). There is a cheaper alternative which is to add an antifreeze valve. This will automatically protect the heat pump by draining fluid from the heating circuit if there is a risk of it freezing inside the heat pump. See here for a good explanation of the pros and cons.


The working fluid in a heat pump is called a refrigerant. (I think this is because the first heat pumps were for refrigerators.) Traditional refrigerants are terrible for climate change and ozone depletion and this is a problem in case they leak. They should not leak but they sometimes do - this is the most common problem with air conditioning systems. Your regular maintenance should check for this.

However, refrigerants are rapidly improving in this respect. R32 is common now and it has a global warming potential (GWP) of ‘only’ 677 - an order of magnitude better than older ones. There are some heat pumps using propane (R290), or other gases with GWP < 10. Be sure to ask what the GWP is for your heat pump’s refrigerant.

If your GWP is < 1,000 (e.g. R32 or better) then even if there is terrible leakage, your carbon emissions will still be far lower than using a gas boiler.

Controls and monitoring

You should aim to use the heat pump integrated controls if at all possible as this is the best approach to get good performance. Heat pumps can work with an external thermostat too, but controls designed for boilers may not give good results.

Having said this, there are some third party thermostats that are particularly designed for heat pumps such as the Homely thermostat. This is a ‘smart’ system that learns how fast your home heats up and cools down and checks weather forecasts to anticipate your heat demand. It can also work with time of use tariffs such as Octopus Cosy and Octopus Agile. The Homely will make sure you take advantage of the cheap times. This also helps to reduce your carbon emissions from use of electricity as cheap prices often means more renewable power. You will need a smart meter if you want to use any TOU tariffs.

The control system should adjust the radiator flow temperature depending on the heat demand - so that it can run more efficiently with a low temperature whenever possible. All heat pumps should at least have the capability for weather compensation, where the system selects a lower flow temperature when it is mild outside. The outside unit will have a temperature sensor for this. Heat Geek's video How To MAXIMISE Your Heating Efficiency In 3 Simple Steps explains more about weather compensation and how to use it. Weather compensation can be effective but it needs tweaking to get the best performance.

Some heat pumps can do load compensation as well, which means that if the room temperature is below the target the flow temperature is increased and vice versa. The goal is to run the heat pump continuously at the lowest temperature possible, giving highest efficiency and the best controllers achieve this automatically. However, load compensation works best if you have a constant thermostat setting. If the thermostat setting is increased by several degrees (e.g. in the morning from a lower setting overnight) a load compensation controller may respond with a sharp increase in radiator temperature. It will warm you quickly but not so efficiently. Smart ones such as Homely will manage this sensibly.

It is very important that you can check your heat pump performance. You don’t need to do this all the time, but if your electricity bill is higher than you expect you need to be able to find out definitely if this is because of the heat pump. As a minimum, you need to have two ‘meter readings’ from your heat pump: how much heat it has delivered and how much electricity it used to do this. This tells you the overall efficiency. Some heat pump controls give you this information but others do not (and one wonders why they don’t want you to know). You can also have separate meters fitted for this, but heat meters are quite expensive.

It is also very useful to be able to check the flow temperature being generated by the heat pump and the temperature of the returning water - again, so you can check on how it is functioning if there is a problem. If you don’t have this you can, if necessary, tuck some cheap temperature sensors under the lagging on the pipes.

Heat demand survey

This survey is like an EPC survey, only more detailed. It needs to include the radiators as well as the heat loss and the surveyor has to do calculations for each room independently. The survey normally takes a few hours. If you have floor plans for your house they will save a lot of time.

The survey should report the overall space heat demand of your home. It is worth checking this against a recent EPC: look for the bit that reports space heating demand in kWh/year. If there is a big difference from the survey (e.g. more than 10%) you should question it. Your installer will probably err on the side of overestimating your demand rather than underestimating. A larger heat pump will cost more and may be physically larger than you need, which makes it harder to hide from view (if that is of interest to you). It may also end up costing more to run.

Also, we hear that heating demand is often over-estimated for homes that have good draught-proofing because the assumptions in the survey - and in your EPC calculation - err on the generous side. This can make a big difference, especially in a home that is also well insulated. So if your home is very cosy and the survey suggests higher demand than your actual bills, consider getting an air tightness test done. We have known this reduce the size of the heat pump required by a third.

Based on the room by room heat demand the surveyor will determine if any of your radiators need upgrading in order to supply enough heat on the coldest days. If you have underfloor heating this should be fine. Upgrading the radiators usually means thicker ones rather than taking up more wall area. Or you can fit fans on them to make them work more effectively.

There is a tradeoff here - if you don’t want to upgrade your radiators you can run at a higher temperature, with less efficiency (see performance above). A ‘heat pump ready’ home can be heated comfortably with a flow temperature of no more than 55°C, sometimes 45°C or less. If you need to go above 55°C then you need a high temperature heat pump but this will mean bigger energy bills.

The installation

Installation normally takes a day or two. It involves at least an electrician as well as plumbers and possible refrigerant engineers. Here is a checklist you can use to make sure they finish the job properly.

  • If preparatory works are done by a different contractor than your heat pump installers, make sure that they fully understand what is required. Otherwise the installation will be delayed while the problems are sorted out.
  • If your heating circuit is old and possibly sludgy it could use a power flush.
  • If you have to have a lot of radiators changed, make sure they are balanced afterwards (otherwise you will find some never get hot).
  • Make sure that all exposed pipework is very well lagged so that your heat is going into the right places. This is particularly important for pipes coming in from outside or going through unheated spaces such as under suspended floors.
  • Make sure they commission the system properly - they should run it up (even if it is summer) and make sure all the radiators warm up nicely and that the system reacts to the thermostat correctly. Also they should check that the hot water cylinder heats up fully

Living with your heat pump

The radiators will be cooler

The radiators will not be as warm as before. This is absolutely normal and is not a problem as long as they can keep the house warm.

The new heating system will not warm up as quickly as before. It is generally recommended to leave it on all the time, with perhaps just 1-3°C lower overnight. If you want a quicker response you will need to run the radiators hotter which loses efficiency - it is not easy to predict but you could easily end up using more electricity than you would have done with constant temperature! (See also Will heating your house constantly use more energy?) You may find that you can reduce your daytime thermostat setting 1-2°C as the stable temperature reduces air currents and the constant heat means the walls will be warmer. This will offset at least some of the increase.

If your house is so leaky that this is going to vastly increase your bills - insulate it first. However, this limitation does not mean that the HP is not your best solution. Bear in mind that even if you get as little as 200% efficiency, this is still twice what you would get from any other electric heating system. It would be extraordinary if you doubled your heating demand. See also Are you heat pump ready.

Hot water temperature

Your heat pump can probably heat your hot water to at least 50°C and that is hot enough for all purposes around the house from washing up to running a bath. Hotter than this can cause serious burns. To heat your hot water cylinder to a higher temperature you may need to use an immersion heater for topping up, or the heat pump itself may have an internal boost heater - either way the extra heat supply will only be 100% efficient. Some people set an even lower temperature such as 45°C. This will save energy as the heat pump is more efficient at the lower temperature.

You might want to heat the hot water cylinder to a higher temperature depending on circumstances.

  • If the cylinder is too small you may run out of hot water - e.g. if you want to run a 100 litre bath and your hot water cylinder is only 100 litres, well that is cutting it rather fine. If you heat the water to a higher temperature you can mix in more cold water to get the temperature you need.
  • If your cylinder is very large, so that water remains in it a long time, then there is a risk of bacteria growing in it such as legionella . For example if you have a 200 litre tank but you use only 60 litres/day, then the water will be in the cylinder for several days. You can either set the temperature to at least 50°C to prevent bacteria growth or heat it sometimes to 60°C to kill everything off (a sterilisation cycle). There is an excellent video about this from Heat Geek.

Your heat pump controls may allow you to configure a regular sterilisation cycle. If not, and you have to use an immersion heater directly, try to ensure that the immersion heater is only run after the heat pump has done what it can. This will get the best overall efficiency.


An air source heat pump needs some regular maintenance, some of which you can do yourself - keeping the fans clear of vegetation and making sure that the drainage is clear so that it can de-ice.

It should have a professional service every year to check the system pressures, glycol levels, clean filters and so on. You may need parts replaced from time to time.

If you have a monobloc system then whenever your plumber drains your heating circuit, you should remind them about the need for anti-freeze (glycol) as well as corrosion inhibitor.

Share this page