Which should you choose?
Air source and ground source heat pumps both use naturally occurring heat to provide an efficient, renewable way of heating your home and hot water. As their names suggest, air source systems draw heat from the air, and ground source from the earth.
But despite some clear similarities in the way that they work, these two types of heat pump have very different setups and requirements. As we discuss below, the one that’s more suited to you will depend on how much you’re willing to spend, and how much space you have available outside your home.
The efficiency of heat pumps is substantially higher than other forms of heating system. Heat pumps are capable of operating at 350 or even 400% efficiency (meaning that 1 unit of electricity creates 4 units of heat). This is because heat isn’t generated like in a gas boiler or electric heater; it’s simply carried into your home and concentrated.
Overall, ground source heat pumps are more efficient than air source heat pumps. This is because heat is transferred through the ground via the movement of water, and as a liquid, water can hold more heat than air can. In more technical terms, the heat capacity of water is around 4 times greater than air.
Air source systems are also subject to fluctuating temperatures. During the winter when the temperature outside drops, air source heat pumps must work harder to maintain your usual warm temperature inside. Their rate of efficiency drops, so they use more electricity.
To contrast, the temperature underground is almost constantly 10oC all year round. This is why ground source systems can keep your house feeling cosy even in the dead of winter without sacrificing efficiency.
Air source wins on this one. The external unit of an air source heat pump sits outside (usually against an external wall), and is similar to an air conditioner unit in shape and size. This is connected to an internal unit containing the circulation pumps and hot water, which is typically smaller than the average boiler.
Ground source systems absorb heat from the earth, so a network of pipes must be laid underground. There needs to be digger/JCB access for the installation of these pipes. As a rough guide, this pipe network usually needs to be twice the floor area of your house – including multi-storeys – to meet your home’s demand for heating and hot water (subject to insulation).
If you don’t have a large amount of space or you’re unwilling to dig up your garden, you can opt for a vertical system of pipes, where boreholes are drilled 60 to 100 meters deep. The downside to vertical pipes is that drilling boreholes is more expensive than laying vertical pipes just a few meters below ground.
These pipes feed into an internal unit, which sits inside your home. This internal unit should fit comfortably in a small utility room.
Air source systems are substantially cheaper to install than ground source heat pumps. For ground source systems, landscaping and diggers are involved in the installation, and there are additional components required that aren’t needed in air source systems. However, both types of heat pump are elligible to receive a large portion (or in some cases, all) of the installation costs back from the RHI (see below).
Air source heat pumps are as noisy as an air conditioner, but planning permission ensures the external unit won’t be sited anywhere that might disturb your neighbours. In contrast, ground source heat pumps can operate almost silently.
Time to install
If there’s an existing flue available, an air source heat pump can be up and running in a matter of days depending on the installer. On the other hand, a ground source head pump may take a number of weeks to lay the pipes underground.
The Renewable Heat Incentive (RHI) is a government-funded scheme to encourage homeowners to switch to renewable heating systems. You receive quarterly payments from the RHI for 7 years; after which you may have recouped enough funds to cover the cost of the heat pump installation (subject to performance).
As the RHI is relative to the installation cost, the rate for air source systems is currently at 10.18p/kWh, whereas the ground source RHI is 19.86p/kWh.
All in all, there’s very little in it to say which is the better of the two. If you can afford it, we recommend getting a ground source heat pump for peace of mind. They run almost silently, most of the system is buried underground, and they maintain a relatively high efficiency all year round.
However, if you’re looking for a quick fix to a broken boiler or can’t bring yourself to uproot your entire garden, go for an air source heat pump. An air source system can be up and running in a matter of days, yet it’s still a renewable heating system that gives off no harmful gases locally.
 Sourced from CIBSE Guide C
 Figures correct at the time of writing (Oct 2017)