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There are three important elements to a GSHP:

1) The ground loop. This is comprised of lengths of pipe buried in the ground, either in a borehole or a horizontal trench. The pipe is usually a closed circuit and is filled with a mixture of water and antifreeze, which is pumped round the pipe absorbing heat from the ground.

2) A heat pump. This has three main parts:

The Evaporator - (e.g. the squiggly thing in the cold part of your fridge) takes the heat from the water in the ground loop;
The Compressor - (this is what makes the noise in a fridge) moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit;
The Condenser - (the hot part at the back of your fridge) gives up heat to a hot water tank which feeds the distribution system.

3) Heat distribution system. Consisting of under floor heating or radiators for space heating and in some cases water storage for hot water supply.

What options are available? - The ground loop can be:

‘Horizontal loops’

Piping is installed horizontally in trenches. The depth of the trenches will vary according to the design and soil characteristics, but is generally 1.5 – 2m deep. Horizontal loops require much more surface area than vertical loops. Around 200m of pipework is generally required for a single dwelling.

‘Vertical loops’

Most commercial and institutional projects using GSHPs use ‘Vertical loop’ systems. The advantage of a vertical loop system, which consists of pipe inserted into vertical bore holes, is less space is required. Holes are spaced at around 5m intervals and can vary between 15m and 60m according to the design and soil characteristics.

‘Slinky coils’

The ‘Slinky’ is a variation of the ‘Horizontal loop’. Slinky coils are flattened coils of overlapping piping, which are spread out and laid either horizontally or vertically. Their ability to focus the area of heat transfer into small volume reduces the length of the trenches and hence the quantity of land needed. A 10m long trench laid with a ‘Slinky’ coil will typically supply 1kW of heating load.

How much does it cost?

Capital

The installed cost of a GSHP ranges from about £800-£1,200 per kW of peak heat output, excluding the cost of the distribution system. Trench systems are cheaper so tend to be at the lower end of this range.

Running

The efficiency of a GSHP system is measured by the coefficient of performance (CoP). This is the ratio of units of heat output for each unit of electricity used to drive the compressor and pump for the ground loop. Typical CoPs range from 2.5 to 4. The higher end of this range is for under-floor heating, because it works at a lower temperature (30-35 °C) than radiators.

Based on current fuel prices, assuming a CoP of 3-4, a GSHP can be a cheaper form of space heating than oil, LPG and electric storage heaters. It is however marginally more expensive than mains gas. If grid electricity is used for the compressor and pump, then an economy 7 tariff usually gives the lowest running costs.

Ground source heat pumps and your home

What to keep in mind when considering a ground source heat pump.