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.

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

Glossary

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.

‘Heat pump ready’ is not something that can be answered with a straight yes or no. As a guide, it means you have good roof insulation, wall insulation and at least double glazing. Cavity walls should be insulated and solid walls too (unless you have a small terrace house with hardly any external walls), at least as far as is practical. Also that you are reasonably draught proof and you do not have serious defects such as anything causing damp. If you have mouldy areas already, it could mean thermal bridging which is also a major heat loss.

Less insulation does not mean a heat pump is not possible, but more insulation is always better, and ventilation control improves air quality as well as minimising unnecessary heat loss. Beyond basic improvements such as loft insulation and cavity wall insulation, fabric improvements can take a long time to pay back in energy bill savings - at least they did while energy prices were low. However, they will bring improved comfort and health, they reduce your exposure to energy price rises, and in some parts of the country they will increase the value of your house.

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 Saviing Trust and Green Match blog

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 full details of the boiler upgrade grant have not been released yet (as of February 2022) but we expect that will be worth up to £5,000 and it will 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, this is changing, as the wholesale price of gas has increased massively and this affects the price of gas more than it does electricity. 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.

You also save on maintenance costs. A heat pump needs servicing too but but less often than a boiler, usually every two or three years instead of every year. There is no risk of explosions or poisoning from carbon monoxide with a heat pump.

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.)

You could also reduce your bills by opting for a ‘smart’ tariff (such as Octopus Agile) and a control system that takes advantage such as Homely. You will need a smart meter for this. If you do not have one already, you can ask your electricity supplier for one.

Location

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. 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. 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.

Performance

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.

Noise

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.

Inverter/modulation

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.

Monobloc/split

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 must have antifreeze (glycol) added otherwise 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. Whenever you drain your heating circuit you will need to top up again with both corrosion inhibitor (as you do now) and glycol.

Refrigerant

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 helps you take advantage of time of use tariffs such as Octopus Agile, that vary with wholesale prices. 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. There are different ways to do this. The most common approach is 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.

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.

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 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 best to leave it on all the time, with perhaps just 1-2°C lower overnight. Do not try to increase the target temperature more than say 1°C in an hour. 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?)

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 is extremely unlikely that heating overnight will double your heating demand.

Maintenance

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 two to three years 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 him about the need for anti-freeze (glycol) as well as corrosion inhibitor.

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