Air source heat pumps
The standard choice for most homes in Tunbridge Wells. An outdoor unit takes heat from the air and feeds your radiators, underfloor heating and hot water.
Air sourceWhy ground source outperforms air source
Air temperature in Kent swings from below zero to the high twenties. Ground temperature a metre or two down sits at roughly 8 to 12°C all year. A ground source heat pump therefore works against a much smaller and far more stable temperature difference, which is why it typically achieves a seasonal efficiency of 4 or better against 3 to 3.5 for air source.
The practical consequence is that ground source performs best precisely when you need it most, in the depths of January, when air source efficiency is at its lowest.
Horizontal loops or boreholes
Horizontal ground loops sit in trenches around 1.2m deep. They are much cheaper than drilling, but they need land: as a rough guide, two to three times the floor area of the house. Plenty of properties around Langton Green, Hildenborough, Sevenoaks and the Weald villages have it. The garden is disturbed during installation and reinstated afterwards.
Boreholes are drilled vertically, typically 90 to 150m deep, and take up almost no surface area. They cost more because of the drilling rig and the geology, but they suit properties without the land and generally give slightly better performance. Around here the Wealden clay and greensand geology is workable but does affect the drilling rate, and therefore the price.
What it costs
Horizontal loop systems typically run £20,000 to £28,000 before grant. Borehole systems run £25,000 to £35,000 and upwards depending on depth and how many are needed. The Boiler Upgrade Scheme pays £7,500 towards either, exactly as for air source, and that includes water source and shared ground loop arrangements.
The honest position is that ground source rarely pays back against air source on cost alone for an average house, because the grant is the same amount for a much more expensive installation. It makes sense where you have the land, where you are heating a large property, where running cost over decades matters more than capital cost, or where an outdoor unit would be unacceptable visually or on noise.
Groundworks and access
Trenching needs machine access to the garden. Drilling needs a rig with roughly three metres of headroom and a route in. We assess both at survey, because access is the single most common reason a ground source scheme turns out not to be viable on a site that otherwise looks ideal.