Ground Source Heat Pumps
Ground source heat pumps use electricity to capture energy from the ground to heat your home and hot water. Ground source heat pumps use either shallow horizontal trenches or deep vertical boreholes. The choice depends on the space you have available.
Horizontal trenches are cheaper than drilling a borehole for a vertical system, but are usually only suitable for properties in rural areas because of the large amount of outdoor space required. A vertical system can cost more than £30,000 for installation alone, because it will require specialist machinery to drill the borehole. A horizontal ground source heat pump system needs a minimum of 700 square metres, while a vertical system needs at least two separate 25cm wide boreholes spaced at least six metres apart.
How do Ground Source Heat Pumps work?
Ground source heat pumps work in a similar way to air source heat pumps, but gather energy from the latent heat in the ground rather than the air. A mixture of water and antifreeze is pumped around a loop of pipe buried under the ground to absorb naturally occurring heat. The water mixture is compressed and goes through a heat exchanger, which extracts the heat and transfers it to the heat pump. The heat is then transferred to your home heating system. Reversible heat pumps can provide heating in the winter and cooling in the summer.
Ground Source Heat Pump efficiency
A well-installed ground source heat pump is supposed to deliver four kilowatt hours (kWh) of heat for each kWh of electricity it consumes – making ground source technology more efficient than air source. This ratio is called the coefficient of performance (CoP). The higher the CoP, the more efficient the system is – some guides and manufacturers quote CoP figures as efficiency figures meaning that a gound source heat pump with a CoP of 4 is listed as “400% efficient” but don’t be fooled, they can not make energy out of thin air! The CoP of a ground source heat pump can vary considerably depending on how well it is installed, how well sized it is in comparison to your home, how many radiators you have, how hot you keep them and also with variations in the weather. (Tip: consider installing smart radiator valves to easily shut off the heating in unoccupied rooms.)
A CoP of 4 or above is seen as good because it means that the cost of running the heat pump will be cheaper than a gas boiler, for the equivalent heat output since gas is currently about a third of the price of electricity.
HeatpumpMonitor.org is an open source project via which people share and compare heat pump performance data, so you can see a variety of installations and compare detailed statistics to see how performance can vary – although currently (March 23) there is only one ground source heat pump listed.
Advantages
The main advantage of ground source heat pumps is that they can be zero carbon, when powered by renewable electricity. So, if your electricity is supplied from solar PV, or via a guaranteed renewable energy tariff, replacing your gas boiler with a ground source heat pump can reduce your heating and hot water emissions to zero. You may also be able to reduce the costs of running a ground source heat pump by switching to one of the new smart tariffs.
Ground source heat pumps also qualify for the Boiler Upgrade Scheme, via which the UK government offers grants of £6,000.
When combined with solar PV, ground source heat pumps make a lot of sense because the renewable energy generated by the solar PV in the day can be used by the heat pump making you largely self-sufficient, presuming your PV system is big enough to power the heat pump.
Disadvantages
Ground source heat pumps are not cheap to install. A vertical system can cost more than £30,000 for installation alone.
Ground source heat pumps require external ground space, making them less suitable in inner-city locations and for flats than air source heat pumps.
Ground source heat pumps must be correctly sized and installed, to ensure they run efficiently and deliver a CoP of 4 and above. A badly installed or sized system which delivers a CoP of less than 4 may well leave you paying more than you would have paid when burning gas.
Installing a ground source heat pump will definitely increase your electricity bill – and since electricity is currently more than 3 times the price of gas in the UK any increase in electricity consumption can have a big impact on bills. It is amazing how many people forget to factor in the extra costs of electricity when considering a heat pump – and forget to monitor how much more electricity they are consuming after having one installed. Always take a baseline before installing a heat pump so you can calculate its true efficiency after it is installed.
Ground source heat pumps require a hot water tank so may not be suitable for flats and smaller homes.
The Installation works for a ground source heat pump can be considerable, depending on how much pipework and plumbing needs replacing.
Ground Source Heat Pump costs
The total cost of a ground source heat pump can vary considerably depending on the size of the house, how well insulated it is, how many radiators and how much pipework needs replacing. We estimate that the installed cost for a 3 bed home, with space for a horizontal system, should be between £14,000 to £19,000 although ground source heat pumps also qualify for a £6,000 grant from the Boiler Upgrade Scheme which will help reduce the costs. We have seen quotes for vertical systems exceed £60,000!
Running costs for a ground source heat pump can also vary dramatically, depending on the size of your home and the CoP your system delivers. We estimate that the running costs for a system which is delivering a CoP of 4, for a 3 bed home should be around £845/year, which is a saving of almost £500/year compared to heating your home and hot water with gas.
Is a Ground Source Heat Pump worth it?
If your motivation is to reduce your carbon emissions to zero then yes, a ground source heat pump is definitely worth it, providing you have enough outside space. However, the cost and disturbance of installation mean we do not recommend installing a ground source heat pump unless you’re building a new home, or need to replace an existing boiler.
Check out the other alternatives to gas boilers in our energy guide.
