Air source heat pumps vs ground source heat pumps

Heat pumps are an effective and energy efficient way to produce hot water to heat your home. They work by absorbing heat from the environment and transferring it to a fluid, which is compressed to increase its temperature. This heat is then transferred from the compressed fluid into the central heating system, to use for both heating and hot water.

The main difference between the two types of heat pumps is simply where they get heat from: air source heat pumps (ASHPs) absorb heat from the air whereas ground source heat pumps (GSHPs) absorb heat from the ground.

The two videos below explain how each type of heat pump works:

While the basis of the two technologies is the same, the cost of having them installed differs. Some cost factors are common to both air source and ground source heat pumps, including:

• The size of your house.
• Whether it’s a newbuild or existing house (newbuild helps keep costs down).
• How much preparation work needs to be done for the conversion, such as whether you need a new electricity supply from your fuse box to the location of the heat pump.
• Whether the installer recommends upgrading radiators to help improve efficiency.

The cost of an air source heat pump is approximately £7,000 – £13,000.

A ground source heat pump on the other hand can cost between £14,000-£19,000. The additional cost for the ground source heat pump installation comes from the external work in the garden, where either ‘slinkies’ or ‘boreholes’ need to be installed.

Despite the greater upfront cost of installing a ground source heat pump, this type of pump is more efficient when it comes to heating your home, which results in higher fuel savings and lower energy bills. Renewable Heat Incentive (RHI) payments are also higher for ground source heat pumps than for air source heat pumps, meaning you will save more in the long term when opting for a GSHP.

You can find a more detailed breakdown of costs and savings on the pages below:

• Ground source heat pumps
• Air source heat pumps

The efficiency of a heat pump is governed by the ‘source’ temperature, in this case either the air or the ground. The colder the source, the harder the heat pump must work. This means that when we compare ground source or air source heat pumps for efficiency, we need to consider where the house is located and what time of year it is.

Air source heat pumps are working with air temperatures that fluctuate between -5°C to 25°C for most of the year. There are of course some days where it can get much colder, but for most locations in the UK, this temperature bracket covers at least 95% of days in the year.

Ground source heat pumps extract heat from the soil, where the temperature doesn’t reach as high, but also doesn’t drop below freezing. In most circumstances, the soil temperature will stay above 5°C throughout the year, as long as the ground loops, which extract heat from the soil, have been designed correctly.

This means there are some times during the year when the ASHP will be more efficient than the GSHP, but when it gets really cold, the ground source heat pump will be more efficient than the air source heat pump.

In addition to being cheaper to install, another advantage of an ASHP is that it doesn’t need to use energy to pump fluid around the pipework outside, as happens with a GSHP. So, on days when the air is the same temperature as the ground, an ASHP can still be slightly more efficient.

On balance, however, GSHPs tend to be more efficient over the year, but the extra cost savings will depend on where you are in the country and the environmental conditions. It would be easier to justify the added cost of a GSHP the further North you live.

Air source heat pumps generally continue to work at temperatures of around -15°C, while some can work at much lower temperatures. However, if you see these sorts of temperatures regularly, you might find that either a GSHP or hybrid heat pump is a better option.

Air source heat pumps are easier and quicker to install than ground source heat pumps as they do not require any land to be dug up for installation. They look like air conditioning units and are mounted outside the property, typically next to an external wall. Some ASHPs also have an inside unit about the size of a traditional boiler. Types without an inside unit are called ‘monoblocs’, whereas those with an inside unit are referred to as ‘split’ systems.

The size of the ASHP will vary depending on your home’s heat demand – the larger the home, the larger the heat pump unit you’ll need. Your installer will let you know what size is suitable for your property.

While they don’t take up too much outdoor space, the unit will be visible from the outside of your property, and the fans generate noise when the heat pump is running. Your installer will therefore need to consider whether any neighbours might be affected by this noise. Heat pumps are generally very quiet except in the very coldest weather, and it’s easy to hold a normal conversation within a couple of metres of the outside unit.

Air source heat pumps usually fall under ‘permitted developments’ and therefore don’t require planning permission. However, if you live in a national park or listed building it would be wise to check with your planning authority first.

When thinking about installing a ground source heat pump, one of your first considerations should be if you have enough outdoor space to fit a ground loop. The ground loop can be installed in two ways, vertically or horizontally, but each will take up a certain amount of space in your garden and you’ll need to check the ground is suitable for digging.

If you want to put the pipes in vertically (using boreholes) because you don’t have much area to work with, this will increase the cost of installation. A small to medium house might only require one borehole, but bigger properties may need two or even three. Opting to lay the pipework horizontally is a cheaper method of installing a ground source heat pump, but you’ll need a lot more space, so it’s only suitable if you’ve got a large garden. As a rule of thumb, you need about 2.5-3 times more land than the floor area of your house, so a 150m² house would need roughly 400-450m² of land. This would need to be unobstructed by trees (to avoid roots), with road access for the digger.

You will also need some indoor space to fit the heat pump unit, which is about the size of a large fridge. It may or may not include an integrated hot water cylinder, depending on the model.

As there is a lot of planning and labour required to install a ground source heat pump, the entire process can take several weeks. Once installed, however, the pipework for your GSHP will not be visible and will not change the look of your property or functionality of your garden.

Despite the similarities in the way that they work, these two types of heat pump have different setups and requirements. Both offer energy saving benefits that could help you cut energy costs and heat your home efficiently and effectively.

If you’re limited by budget or space restrictions, it might be easier to choose between an air source and ground source heat pump, however others will need to take more time to carefully consider the needs of their household.

Any potential installer should be able to discuss all these issues with you in more detail to help you find the right solution for your needs.