A question we often get asked at Green Square is: “Which is better a ground source heat pump or an air source heat pump? I have heard a ground source is more efficient.”
As with all renewables, the answer depends on your circumstances. At Green Square, because we provide a full range of renewable technologies, we take the time to talk to our customers, look at their situation and recommend the best technology for them and their home, whether heat pumps, solar or biomass.
Heat pumps are ideal for well insulated houses, especially if they have underfloor heating, and therefore tend to work best for customers with modern or new-build properties.
Heat pumps work more efficiently, the lower the flow temperature of the water they produce. With UFH in a well insulated house that can be as low as 35C for the heating. In such a case, because the heat pump is only raising the incoming mains water by about 25C then it is “easy work” and they can achieve efficiencies of 400%. That means that for every 1KW of energy supplied the heat pump produces 3KW of renewable energy – in effect meaning the running cost is just a quarter that of an electric boiler which is 100% efficient. So, if your electricity bill is 12p per KWh you are in effect paying 3p per KWh to run the heat pump, which is cheaper than oil or gas.
As the heat pump has to work harder that efficiency (known as the Seasonal Co-efficient of Perfomance or SCOP) starts to drop. In a house that is not particularly well insulated the efficiency may drop to 300% or below. Once that happens it is time to consider biomass or wood pellet boilers as the best option for your heating.
Biomass stoves and boilers are more like oil or gas boilers and produce much higher flow temperatures, at about 70-80C and are therefore more suitable to older, less well-insulated houses with traditional radiators and a high heat loss.
So, back to the question. Assuming you have a suitable property, which system is best for you – air source or ground source? Again it depends on your circumstances.
Ground source heat pumps
Tend to be slightly more efficient over the year and therefore cheaper to run.
GSHPs gather heat from the soil, which is heated during the summer months by the sun. The temperature below ground is more stable over the year typically about 8C at about 1-2m down which is the depth that shallow ground pipework is laid.
Where this pays dividends is in the winter. While the temperature of the soil is about 8C, the temperature of the air in winter, for an air source heat pump, might be 2C. Conversely in the summer months while the temperature of the soil is still 8C the temperature of the air may be 20C and the air source is more efficient.
However, the ground source wins out over the year because when we need heating the most – in the winter – it will run more efficiently than the air source, typically 20% more efficiently over the year.
GSHPs are not visually intrusive once installed. The heat pump is housed within the building and so is not exposed to the weather and should be reliable and have a long life-span. They are therefore silent and unobtrusive in respect of any neighbours.
More expensive to install
Whereas an air source heat pump is a single unit, relying on the air as its “fuel”, a ground source heat pump needs the associated pipework laid in the ground to access its “fuel”. This can make the installation a fair bit more expensive than an air source. For new builds the trench work can be done at the same time as other utilities are laid, saving on cost, but for retrofits it normally involves the cost of a digger as well as the cost of the ground equipment and labour. In some circumstances, where space is limited, boreholes may be preferred but again the cost of the drilling equipment can be high.
The costs can be offset by payments under the Renewable Heat Incentive (RHI) which is available to those installing renewable heating systems – a bit like the Feed in Tariffs (FITs) for solar PV. See section below on RHI
Land is needed to lay the pipeworks. On a very rough estimate you need three times the footprint of your house for shallow ground works. If space is a premium then boreholes can be used but that can be expensive and you would need access for the machinery.
The installation can cause disruption
As mentioned for new builds there may not be any additional disruption but for retrofits, diggers and trenching can make a mess of a prized garden.
Air source heat pumps
They are cheaper and easier to install
An air source heat pump can be half the cost of a ground source installation and once a suitable site is chosen, the install can be fairly quick with minimal disruption.
They are not quite as efficient as aground source and will therefore cost slightly more to run:
Despite not being as efficient as a ground source, a well designed air source heat pump can still provide all your heating and hot water at less cost than fossil fuel heating. At 300% efficient (a SCOP of 3) your 12p per KWh electricity bill is in effect 4p per KWh still cheaper than a KWh of gas or oil.
They do have a visual impact and, although quiet, do make some noise:
An air source heat pump sits outside the house against a wall. Ideally this would be outside a utility room but wherever sited they do have a visual impact. They also do hum very quietly which may be noticeable.
Renewable Heat Incentive
The Government offers payments over seven years to people who install renewable energy heating systems, under the Renewable Heat Incentive or RHI. This is available for both air source and ground source heat pumps and helps to cover the cost of the installation, making it competitive with installing a new fossil fuel boiler.
The tariff is normally paid on the KWh figure deemed on your home’s Energy Performance Certificate (EPC) and takes account of the efficiency of the system you install. The tariff for the ground source heat pump is roughly twice that of an air source heat pump – reflecting the fact that a ground source system may cost twice as much to install.
For more information see: www.ofgem.gov.uk/environmental-programmes/domestic-rhi