Heat Pumps

A guide to Heat Pumps in 2023

The science of heat pumps is easy to grasp. If you've ever blown over hot food, or your cold hands then you've already experienced the phenomenon of heat transference. The reason we blow on hot food, or over cold hands is the same. For the food, we're trying to extract the heat from the scalding food by replacing it with cooler air from our lungs, the same is true in reverse for warming our hands. The basic principle is simple, heat always transfers from a warm place to a colder one. In the heat pump industry there are many variants, but they all follow or, more accurately, oppose this basic rule.

To underscore this, it may interest you to know that there are already heat pumps in your home or where you work. These may include refrigerators and air conditioning units, but it’s worth remembering that heat pumps can also heat spaces in our homes and businesses as well as cool them.

To trick the natural order of heat transference, heat pumps use electricity to achieve a cold-to-warm transfer. As they don’t physically generate or create heat, they’re not a true renewable technology. Having said that though, because the amount of electricity needed is less than that the energy required to create the same heat, it still counts in many ways. This is one of the reasons heat pumps remain eligible for the government’s Boiler Upgrade Scheme (BUS).

What different kinds of heat pumps are there?

As discussed, all heat pumps extract heat from an external source. Each of the heat pumps on the market today get their heat from one of three potential sources. It’s these sources that give each style of heat pump their names. It’s also worth mentioning that there are also some variants in each category, which are as follows:

• Air source heat pumps - ASHP
• Water source heat pumps - WSHP
• Ground source heat pumps - GSHP

We’ll look at each one in turn and we’ll also discuss the variants in each category, but here’s a list of all the different varieties of heat pumps available.

• Air-to-air heat pumps
• Ductless mini-split heat pumps
• Air-to-water heat pumps
• Water-to-air heat pumps
• Water-to-water heat pumps
• High temperature heat pumps (also known as hybrid heat pumps)
• Horizontal array (or matrix) ground sourced or geothermal heat pumps
• Deep bore or vertical loop ground sourced or geothermal heat pumps

It’s also worth noting that heat pumps are available for both domestic and commercial use, and there are also smaller units that can be used to heat things like swimming pools or hot tubs.

Now, let’s look at them all in detail:

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Air source heat pumps

Any heat pump system that takes its heat from the outside air is an air source heat pump. They all work by extracting heat from outside, amplifying that heat somehow and then distributing said heat indoors. The only real difference between the types of air source heat pumps is what happens after they’ve extracted the heat.

Air-to-air heat pumps

Air-to-air systems do exactly what you’d imagine. These systems collect heat via an external fan before compressing the heat and then circulating it as hot air indoors. You may be interested to know that there’s no functional difference between an air-to-air heat pump and am air conditioner. The good news is though that your air-to-air heat pump system can also be used to cool your property in warmer months too.

The process works like this:

Incoming air is transferred into a refrigerant.
This liquid is then compressed to increase its heat.
Next, the heated refrigerant (now a gas) passes through a heat exchanger.
Here its heat is extracted to generate hot air.
This hot air is then circulated via a series of vents where it’s used internally as heating.
The now cooled refrigerant then returns to liquid form and the start of the system where the cycle begins again.
Now, obviously air-to-air heat pumps don’t generate hot water. This means they are not a true replacement for a full combustioncentral heating boiler. Therefore, this makes them ineligible for the Government’s Boiler Upgrade Scheme (BUS). A shame, because this could have knocked up to £7500off the cost of the installation bill

What does it cost?

The good news is that air-to-air heat pumps are one of the least expensive systems on the market. If you intend to install a simple air-to-air heating system, you can expect to pay between £2500 and £5000 depending on how complex the system is. These systems are not applicable for the Boiler Upgrade Scheme (BUS).

Ductless mini-split heat pumps

These small units are the simplest domestic or commercial heat pump systems around. In essence a mini-split system is an air-to-air heat pump system designed to heat (or chill) a single room. Because they only require two components, they are relatively easy to install and are often retrofitted as addons for properties with existing heating systems.

They function in exactly the same way as bigger air-to-air systems, except without the need for additional ducting. Instead, mini-splits, as they often called, use an external compressor/condenser and an internal conditioning unit to provide heat. It’s worth noting, that these units aren’t known for their looks and often need somewhere to drain extraneous condensation.

What does it cost?

It may also surprise you to learn that despite their simplicity, these units can be pricy to install. How much will depend on factors like the size of room and ease of access. Therefore, mini-splits are more common in commercial properties where aesthetics matter less and function matters more. As with other air-to-air systems, mini-splits don’t create hot water and are ineligible for the Boiler Upgrade Scheme (BUS). Number wise, expect to shell out between £1500 and £4k for the pleasure.

Air-to-water heat pumps

Like air-to-air heat pumps, air-to-water heat pumps extract heat from the outside air. The only real difference between these systems is the addition of a hot water system on the inside. While air-to-air heat pumps blow hot air around a network of vents and ducts, air-to-water heat pumps instead transfer their heat into hot water. This means they operate much more like traditional gas or oiled fuelled central heating. Like traditional central heating systems, these systems use a network of pipes and radiators to heat a property.

Although, air-to-water heat pumps function like gas and oil-fuelled systems there are difference between them. The biggest difference lies in how the systems work. Unlike traditional central heating systems, air-to-water don’t physically generate heat. There is no combustion or emissions and air-to-water heat pumps do not use fuel at all. Instead, air-to-water heat pumps use electricity to take advantage of and amplify the naturally occurring process of heat transference.

Another main difference lies in a phenomenon called the Coefficient of Power (COP). In combustion-based heating, the energy is provided by the chemical reaction of burning fuel. This process is better suited to generating higher temperatures quickly, while heat pump technology is better at maintaining a lower, yet more stable constant temperature.

To achieve this, air-to-water heat pumps work best with a reliable source of external heat. Without getting too technical, getting the most from such a system requires a situation where gap between the external and internal temperatures is as low as possible. To work efficiently, heat pump technology also requires some stability in the temperature of the internal space. Put simply, the property can’t have many leaks and therefore should be well insulated.

There are some heat pumps that deal better with poorly insulated houses and higher temperature gaps, and we’ll cover those in a moment. The important fact to remember is that although all heat pumps, except air-to-air versions, function like central heating, they won’t turn your house into a sauna.

What does it cost?

When it comes to installing an air-to-water system a lot can depend on how much remodelling is required. Sometimes it’s possible to use a lot of the existing infrastructure like the pipes and radiators. In a perfect world though, these should be redrilled. Heat pumps nearly always require 15mm pipework, so if you're on microbore the likelihood is you will need to sheel out around £4000 for a new pipework system.

Also, you may need to add more or larger radiators and you’ll definitely want to ensure your home is properly insulted. If you don’t need a lot of extra work, and air-to-water heat pumps system could set you back £8k to £12k. If you do need to add some upgrades though, you can add another couple of grand to the price. The good news is that an air-to-water heat pump system is eligible for the Government’s Boiler Upgrade Scheme (BUS). This incentive can knock £7500off thebill so it’s worth checking.

Water source heat pumps

Like air, water contains hidden heat. As the sun’s solar power blasts us across space, much of the heat is retained in bodies of water. With a water sourced heat pump it’s possible to extract this heat and use it to heat our homes and businesses. Just like air source heat pumps, water source heat pumps do not create heat. In fact, the only real difference here is where the heat comes from.

Water, in general, retains more heat that the open air because it’s less prone to other forces like air current and winds etc. No, there’s a simple reason why not every property is suited to a water source heat pump system and one you may have guessed already. The fact is, water source heat pumps as the name suggests, require a nearby source of water. The source needs to be reliable and stable too.

You can’t install a water source heat pump if the water ever dries up for example. So, ideally to work efficiently, a water source heat pump needs a body of water like a pond, reservoir, canal, or river. Of course, not everyone has a stable water supply to hand which makes these systems location dependant.

There are two common water source heat pump systems in use in the UK. Apart from being near a suitable body of water, installing a water-based heat pump system can involve planning permission and a fair amount of red tape.

• Water-to-air heat pumps
• Water-to-water heat pumps

Water-to-air heat pump

Although not as common as water-to-water systems, water-to-air heat pumps do exist. You may be interested to know that when Canary Wharf first opened its doors to potential tenants, the exhibition arena was heated and cooled by a heat pump. The air blowing system was designed to draw heat or cooler air from the dock water outside before feeding it back into the building as hot air through a network of ducts and vents.

Water-to-water heat pump

These systems function in exactly the same way as an air-to-water system. The only difference between them is that the heat is collected via a series of submerged pipes rather than extracted from the air. As ground water retains heat better than the air, the Coefficient of Power (COP) is far greater than an equivalent air sourced version.

What does it cost?

Price wise, unless there’s a need to drill bore holes, the cost of a water source heat pump sits between that of an air-to-water and a horizontal geothermal system. In terms of cash that’s somewhere between £10k and £15k. there is some good news though. Water-to-water heat pumps are eligible for the Government’s Boiler Upgrade Scheme (BUS). This incentive can knock up to £7500off thebill so it’s worth checking.

Hybrid or high temperature heat pumps

Before we get to ground source heat pumps let’s take a look at one last air source variant. We mentioned briefly an air source system that functioned more like traditional gas-fired central heating. These are devices designed to offer a similar level of heating as traditional central heating without the need to opt for expensive insulation.

Hybrid systems, as the name suggests, are a combination of a heat pump and another heat source. The secondary heating is often supplied by a gas fired boiler which can help the heat pump system reach higher temperatures quicker, hence its other name. These systems combine the best if both systems to provide a heating solution for those with hard to insulate properties, or who Perhaps live in colder climates. In the latter case the secondary heat source becomes more of a backup than the main heating system.

What does it cost?

While you won’t see much of a difference when it comes to the running costs of a hybrid system, although you will likely see a drop in emissions. As for installation, these systems can cost a little more than a dedicated air-to-water heat pumps as they are more complex to operate. Numbers wise, you can bank on spending between £7k and £13k depending on the options you want.

There’s more bad news too, because hybrid systems count as fossil fuel central heating, hybrid heat pumps systems are ineligible for is eligible for the Government’s Boiler Upgrade Scheme (BUS).

Ground Source Heat Pumps

Heat doesn’t only exist in the outside air. Valuable heat can be drawn from the ground to keep our houses warm too. As the sun heats the planet, solar energy is trapped in the ground and can be extracted by ground source heat pumps. These systems, also known as geothermal heat pumps use one of two different methods to extract dormant heat from the ground. These variations give their name to each system which are as follows:

• Horizontal array (matrix) ground source heat pumps
• Deep bore (vertical loop) ground source heat pumps

Although the systems appear quite different, and the price difference between the two is vast, they work in the same way.

Horizontal array (matrix) ground source heat pumps

Better suited to properties which have ample outside space, horizontal heat pumps use a twisty labyrinth of pipes to harvest the heat from the ground. If you’ve ever had a look behind your fridge freezer, you’ll have a good idea what this looks like. Horizontal ground source heat pumps use a similar crazy lattice of pipework like those to the rear of your fridge except on a bigger scale. The array or matrix as it’s often called, is laid about a metre deep, usually under a terrace, lawn, or garden.

The array is filled with a similar refrigerant found in the air source pumps we mentioned earlier. Apart from that, the system works the same. The only difference is that the heat is extracted from the ambient temperature of the ground rather than the air. The reason for opting for a ground sourced system over an air source system comes down to that magic phrase Coefficient of Power (COP).

As the ground temperature is less prone to change, it provides a more stable and a higher temperature source of heat. Like water, the ambient temperature of the earth is more resistant to external forces. So, if a horizontal array provides a great source for a ground-to-water heat pump why is there another variant? It’s a good question and to answer it let’s look at that other option.

Deep bore or vertical loop ground source heat pumps

Unlike a horizontal array, deep bore heat pumps don’t require a lot of outside space. This is the key to why they exist at all. Of course, there are other factors to consider, but if space or access is an issue, a deep bore or a vertical loop heat pump could be the answer. Ideal for buildings where it’s impossible to lay swathes of shallow pipes, deep bore heat pumps use drilling equipment to burrow down into the earth.

Sometimes extending a hundred metres or more, these holes are then used to house a single or loop of pipe into the ground. The effect is same, the refrigerant filled pipes extract heat from the surrounding earth and then the heat pump system works like all the others.

What does it cost?

The main difference between the ground source heat pump systems, often comes down necessity or cost. If you have the space, then the horizontal option is usually the first choice. Price wise, a horizontal coilheat pumps system has a starting price around £20k but can cost up to £35k or even more.There is some brighter news though, both ground source heat pump variants are eligible for the Government’s Boiler Upgrade Scheme (BUS). This incentive can knock up to £7500off that bill so it’s important to check if you qualify.

Is a heat pump suitable for my property?

Heat pumps are great for most properties, but there are many factors to consider before you break out your debit card and not every home is ideal. Let’s look at each in turn.

In a perfect world, heat pumps work best in the following setting

• well-insulated property
• not too close to other houses
• replacing existing gas, electric, LPG or oil-fuelled central system

Here’s why…

Insulation

Heat pump systems are not as dynamic as fossil-fuelled or electric central heating. The heat is more consistent and runs a little cooler than traditional central heating. Therefore, to ensure a steady temperature and avoid putting pressure on the system it’s essential to insulate your home well.

Ideally, you’ll want cavity wall and loft/roof insulation, as well as double glazing. If you have solid brick walls or it's not possible to to cavity insulate for some reason, then it's definitely worth looking at some options like internal or external wall insulation. It may even be worth hiring a thermal camera to look for any leaks you can repair. Remember, heat pumps aren’t designed to crank up the heat to furnace levels. They work better when they’re keeping things nice and comfortable, they're designed to create an ambient temperate instead of a blast of heat.If you’re constantly losing heat to drafts, you’ll soon find your heat pumps bills start to rocket.

It's worth mentioning that in order to qualify for the government's Boiler Upgrade Scheme, you will have to have a recent EPC with no advisories for cavity or loft insulation.

Proximity to other properties

There are legal requirements governing how close to your neighbours your heat pump unit can be. Installing one also depends on planning regulations and permissions as some systems need significant external infrastructure.
As of October 2023 the rule is that in England and Scotland an ASHP must be installed 1mor more from the neighbours parimeter. In Wales this is increased to 2m.

Why replace an LPG or oil-fuelled boiler?

This comes down to pressure. Like LPG and oil-fuelled boilers heat pumpsystems work better at lower temperature. This is because heat pumps prefer the pipes more often used by oil and LGP boilers, generally speaking 15/22mm. It’s not impossible to use existingradiators, but you may find changing some of the infrastructure amps the efficiency.

Of course, it’s possible to fit any property with a heat pump…
The suggestions above are just that. It’s possible to install heat pumps in any home. That said, some systems are a bit needier than others. Air-sourced heat pumps won’t give you much trouble. What they lack in efficiency compared to ground-sourced systems, they more than make up for in easy installation. Assuming your home has insulation, suitable pipes and radiators, you’re good to go. Ground-sourced heat pumps though, well, that’s another matter.

What do you need to install ground-sourced heat pumps?

The first thing anyone looking to add a ground-sourced heating system needs is deep pockets. Typical installations need either a borehole or a horizontal coil and neither comes cheap.

Both designs have their merits and while there may be a price difference it’s likely to be nominal. The real difference between these two systems is the space they occupy and the efficiency they produce.

As a rule, vertical heat pumps suit environments where there’s less available space and it’s possible to drill a deep bore hole. Vertical systems are often more efficient too as they rely on extracting heat from stabler lower ground. Although even a horizontal heat pump will return far greater efficiency than you’d get from an ASHP or traditional central heating.

Horizontal heat pumps, by contrast, are better where there’s easy access to a wide yard or garden. These systems are also easier to maintain as they are easier to access than their deep-bore cousins.

Both systems extract heat from the ground, which is then compressed, exchanged, and distributed via a system of pipes and radiators.

In short, if you live in an urban setting or don’t have a lot of space outside, a vertical geothermal heat pump system could be the answer. If on the other hand you have a big garden or drilling a deep borehole is impractical then you should opt for a horizontal geothermal heat pump system.

Heat pump installation process, reliability, and lifespan

In this section, we’ll look at the installation process for air source and ground source heat pumps, also known as geothermal heat pumps. Then we’ll discuss what to expect when it comes to longevity, warranties, and maintenance.
We’ve discussed this before, but the first stage in any heat pump installation is to check your insulation. Because of the way heat pump systems work, good insulation is essential to ensure their efficiency.

Ensuring you have enough insulation before you install a heat pump isn’t optional, it’s a must.

Air source heat pumps

Air source heat pump systems are the easiest to install. Once you’re happy you have sufficient insulation the next phase is the planning stage. Ignoring the internal elements, for now, to install an air source heat pump you’ll need suitable outside space. Now, there are planning restrictions on how close these units can be to neighbouring properties. It’s a good idea then to make sure you clear any permissions before you start.

Where you place your external unit also depends on its size, and how much noise it makes. Obviously, smaller quieter units are easier to live with nearer the house. Either way though, you’ll need suitable flat wall space where it's possible to drill through the necessary walls. Choosing the right place to install your heat pump is critical to the functionality of the system.

• You’ll want to keep it away from direct sunlight.
• It will need clearance on both sides of the fan to ensure enough airflow.
• You don’t want to put it anywhere it could pick up contaminants like oil or other fumes or gases.
• You’ll need to protect it from the elements, in particular heavy snowfall.
• It’s a good idea to mount it on mounts to reduce the amount of vibration it produces.
• Remember your external heat pump also needs an electrical power supply.

Internally, you may be able to use much of the existing pipework and even radiators, but they must be up to the job. It’s worth considering upgrading this infrastructure if your supplier recommends doing so.
Inside your home, you’ll also need space for the pumps, control system, and heat exchanger. This shouldn’t take up much more room than your existing central heating system.

How long will it take?

Overall, the entire process should take between two to five days depending on the size of your house and the complexity of the system.

Ground source heat pumps

Ground source heat pumps are a good deal more complex than their air source cousins. They come in two different varieties, and both involve significant groundwork and disruption. At their heart, ground source heat pumps work the same way as air source heat pumps. Internally then, the systems involve the same amount of effort, distribution, and equipment. It’s outside where things change.

Deep borehole heat pumps
This type of heat pump requires that two to three vertical shafts be dug to depths between 70 and 200 metres. As you can imagine there’s a lot to consider when drilling holes that deep. Deep bore ground source heat pumps are often only used when a property lacks outside space. It takes a lot of garden to lay a horizontal matrix and if you don’t have it, you don’t have it. This means they are more common in urban areas like city centres. However, drilling 70 to 200 metre boreholes is no picnic. For a four bedroom, detached property you will generally require three bore holes at a cost of around £5000 per hole.

Horizontal matrix ground source heat pumps
Like their deep-bore cousins, horizontal matrix heat pumps extract heat from the outside ground. The ground is consistently warmer than the outside air, so they are more efficient. Instead of relying on deep pipes though, these heat pumps use a series of curled pipes dug to a uniform depth of approx. 1.2 metres.

Depending on the space you have available this pipe work can vary in style and size, but it’s possible to make use of some tight spots. If you watch your supplier install your horizontal pipework, you’ll notice it looks a lot like the pipes on the back of your fridge, which in a way it is. As it’s easier to dig down a few feet than burrowing 70-plus metres, horizontal matrixes are far easier to install. Because they’re also easier to dig up, subsequent maintenance is easier too.

This affects the price, and the swing is significant. If you imagine a ground source horizontal heat pump can set you back £20k, then a deep borehole system can cost up to twice as much.

How long will it take?

Neither system is easy to install. Two to five days might do for an air source heat pump, but plan for two to four weeks for a ground source system. Borehole ground pumps on average take a little longer. Either way, you’re looking at a month or so before the system is up and running.

Shelf life, maintenance, and warranties.

Although, not exactly new technology, heat pumps are in the middle of a boom. This means that efficiencies, prices, and lifespans are improving all the time. Therefore, you need to keep one eye on the future when considering your installation. That said, a typical heat pump installation will likely last between 10 and 15 years, but you’d be unlucky if they didn’t reach the full 15 years. Some systems can last 50 years, it all depends on how well installed and how well maintained it was.

Despite their attractive shelf lives you may get a nasty shock when it comes to guarantees and warranties. Of course, they vary, but an average parts-only warranty is five years. Some bigger names will stretch theirs’ to 10. On average though, if you can get over five years for your warranty, you’re doing OK.

Things get even worse when it comes to labour. Most suppliers will only guarantee their handiwork for between 12 and 24 months. Quite why there’s such a discrepancy between actual lifespan and warranties is a bit of a mystery. If we had to guess, we’d say it’s down to the cost of fixing things when they go wrong. Can you imagine trying to find a leak in a 70-metre borehole, or having to dig up forty feet of horizontal pipework when the hot water goes off?

In general, air-source systems are easier to maintain since the worst that can happen is the external unit fails. A failure with a ground sourced heat pump system however can lead to major disruption. This is why maintenance is so important and why it’s a good idea to get the best warranty you can find.

In short
Both air source and ground source heat pumps are efficient and effective replacements for traditional central heating. However, it’s a good idea to read up, research, and learn everything you can about them before taking the plunge.

What about the future?

As the industry and technology grows, common sense tells us that new innovations will come to market. No doubt, new prototypes and new designs are already under testing. So, while for now this represents a good cross section of the market, it’s always worth doing your research just in case there’s something new just up the road.

If you’d like more information about heat pumps, of if you’re considering having a heat pump installed, please take the time to read through our guides. The fact you’ve found us already means you’re one step ahead of the public at large, but contained within our pages you’ll find accurate and up to date information about all aspects of the heat pump world.