Air source vs ground source heat pumps: A complete guide

Whether you’re concerned about being more environmentally friendly or you’re looking to reduce your energy bills, investing in a heat pump might be a worthy investment.

But which heat pump should you choose? Air source or ground source?

In this complete guide, we'll be comparing these two heat pumps to determine which one would be right for you. So, without further ado, let's take a look:

What is an air source heat pump?

Air source heat pumps (ASHPs) are a relatively new form of home heating system. Generating hot water by using natural heat energy in the outside air rather than fossil or non-renewable fuels, ASHPs are being championed for their eco credentials and lower carbon emissions. Some models of air source heat pumps can operate as both heating and cooling systems, making them well-suited for people living in areas with colder winters and hotter summers.

How does an air source heat pump work?

Heat energy from the outside air gets drawn into a network of tubes. These tubes are filled with refrigerant gas, which gets compressed and increases the temperature from cold to hot liquid.

This hot liquid then goes through a heat exchanger that warms up the water for the radiator and taps.

What is a ground source heat pump?

A ground source heat pump is a device that draws heat from the ground and transfers it into your home. Due to radiation from the sun, heat gets stored beneath the surface of the Earth with nothing to cool it down.

Through a network of pipes which are filled with water (or a combination of water and antifreeze), this renewable energy heating system takes this solar energy and circulates it into your home.

Ground source heat pumps can also be installed in two ways: vertically and horizontally.

• Vertical ground source heat pumps take up less space, however, the installation process needs specialist equipment and you may not receive as much heat.
• A horizontal system, on the other hand, requires a significant space and the installation process requires digging trenches.

Water source heat pumps are another option, but are less common.

How does a ground source heat pump work?

The ground collectors are placed underground, either vertically or horizontally, and refrigerant inside circulates around to absorb heat energy that is naturally stored in the ground. Inside the heat pump, the warm refrigerant is compressed and passes through a heat exchanger, heating the rest of the home appliances.

Much like the air source heat pumps, the fluid is cooled down and the cycle repeats itself.

What’s the difference between an air source heat pump and a ground source heat pump?

Now, it doesn't take a rocket scientist to work out one obvious difference between air source and ground source heat pumps; ASHPs generate heat from the air, whereas GSHPs generate it from the ground!

But, besides this, are there any other differences between these two types of heat pumps? Let's take a look. 

Air source vs ground source: How much do they cost?

The price of air source heat pumps

On average, an air source heat pump costs in the region of £10,000 to buy and install. In some instances, this figure could be much higher, with larger or older properties typically needing bigger and more expensive pumps to meet their heating requirements. Why? Because older houses tend to be less well insulated than modern new-builds meaning they will lose more heat through walls, attics, windows and doors. Larger homes will inevitably have larger hot water requirements than smaller homes, simply because there will be more rooms with more radiators!

This price can certainly be off-putting, which is why the government has implemented the Boiler Upgrade Scheme to provide £5,000 grants for homeowners purchasing their heat pumps.

If we look at the annual running costs, an air source heat pump amounts to £1,360 which is slightly more than a gas boiler's running cost (£1,236 per year). That said, air source heat pumps shouldn't require as much maintenance as a boiler or ground source pump, so the hefty price tag may be worth it in the long run.

The price of ground source heat pumps

Ground source heat pumps have far more complex and labour-intensive installation processes than air source alternatives meaning that, perhaps unsurprisingly, they cost considerably more, too.

If your ground loop is already buried, you can expect to pay around £24,000, but if you need to dig a borehole, it'll cost you around £49,000.

Now, ground source heat pumps are still eligible for the government’s Boiler Upgrade Scheme grants, but you’ll only be able to claim a maximum of £6000 towards the cost of purchasing and installing one. When we compare this to the total amount you can expect to pay, it’s immediately clear that the current financial incentives don’t really scratch the surface. Ultimately, this makes ground source heat pumps an unaffordable heating system for the vast majority of homeowners, especially at a time when we’re all feeling the pinch more than ever.

Air source vs. ground source heat pumps: Space requirements

How much space does an air source heat pump need?

For air source heat pumps, you'll need space outside for the pump unit to pull in air and space inside for the water storage tank. Any walls or objects should be a minimum of 30cm away from the back and sides of the heat pump and there needs to be 1.5m of space in front, with nothing to obstruct the space.

What about a ground source heat pump, how much space does one take up?

As expected, ground source heat pumps require more space than air source pumps. Much more. This is because, rather than being a standalone unit that can sit adjacent to your home, GSHPs have many metres of complex pipework, all of which must be installed underground.

If you do decide that a ground source heat pump is the heating system for you, though, you’ll have two options when it comes to actually installing it outside your home; vertically or horizontally. Let’s take a look at these in a bit more detail.

• Vertically

Vertical heat pumps take up less space than horizontal heat pumps and are much more efficient. One of the main issues you'll face with vertical heat pumps is access to the drilling rig since the pump will be confined below the ground.

One borehole that is 25cm across and 100cm deep is enough for most UK homes. Larger properties, however, may need several boreholes with a 5-6m gap between them.

• Horizontally

Horizontal heat pumps cover more ground and can be trickier to install and maintain. The trenches for ground loops are 100m wide and 1-2m deep. This much should hold 200m of pipe - 100m away from the property and 100m towards it. The size of your property will impact the number of trenches you'll have, especially if you want to heat your home adequately.

Ground source pumps come in all shapes and sizes. Some are small enough to fit in an airing cupboard, whereas others are as big as a washing machine. In general, heat pump space should be around 1x3m in size.

Air source vs. ground source heat pumps: Maintenance and lifespan

Maintaining an air source heat pump

An air source heat pump warranty is usually two to three years, depending on the manufacturer.

To maintain your air source heat pump and improve its longevity, clean your unit regularly with a soft brush and sweep away any dirt or debris that may be blocking the system. You can also replace your air filters, though this job will require an installer.

In order to meet and maintain your heat pumps warranty, you must have annual servicing. However, much of the general maintenance can be carried out by yourself, including keeping  your system clean. 

Maintaining a ground source heat pump

Like air source heat pumps, ground source heat pumps are also relatively low maintenance.

Again, regular maintenance is necessary to keep a healthy heating system running smoothly, and annual servicing is a must. Looking after your heat pump will help increase its lifespan, so it’s worth providing it with the TLC it needs!

Air source vs. ground source: Output and performance

How much heat can air source heat pumps generate?

In comparison to a boiler, heat pumps produce less heat.

An air source heat pump loses efficiency below 0°C since they rely on outside air. Once the temperature falls, so does the heat output. During the winter months, you may end up having to prioritise what’s more important: a warm room or hot water.

Because of this reason, air source heat pumps can be unreliable and could actually cause a lot more problems for you and your family. 

How much heat can ground source heat pumps generate?

Since these heat pumps are buried, they can produce a stable amount of heat that isn't impacted by changing climates.

A ground source heat pump can generate around 3 to 4 kilowatts of heat for every 1 kW of electricity that gets absorbed. Since we’re using heat from the ground, the energy efficiency is much higher, as nothing below six metres can dramatically alter the temperature.

That being said, the size of your ground loops will determine how much heat your pump will be able to extract and once your ground pump is installed, it’s very difficult to go back and make changes.

Once a substantial amount of heat has been taken from the ground, the temperature will fall from 10℃ to 0℃ or colder. As a result, the heat pump will need to work really hard to circulate the heat and produce the same amount for all heat outputs in the home.

A four bedroom house, for example, may have differing temperatures of water and two rooms might not be as warm as the other.

Should you replace your boiler with a heat pump?

Most UK homes were built for boilers, so replacing it with an entirely new system can be challenging.

Heat pumps can be great as an environmentally friendly alternative but you should consider whether you can heat your household on one alone. Another option could be to use both systems, where your boiler is your backup, safe source of heat, but you try using heat pumps where and when you can.

Heat your home with BOXT

While heat pumps do have their plus points where the environment is concerned, they just aren’t right for everyone. Don’t worry, though, this doesn’t mean that there aren’t eco-friendly options available to you.

At BOXT, we offer a wide range of options so you can find the best boiler for your home by answering a few questions. Get a boiler quote now.