Although we may not know it heat pumps are very familiar to us - fridges and air conditioners are two examples. Ground source heat pumps (GSHP) transfer heat from the ground into a building to provide space heating and, in some cases, to pre-heat domestic hot water.

For every unit of electricity used to pump the heat, 3-4 units of heat are produced. As well as ground source heat pumps, air source and water source heat pumps are also available.

How does it work?

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:

1)       borehole;

2)       straight horizontal - trench costs less than a borehole, but needs more land area;

3)       spiral horizontal (or 'slinky coil') - needs a trench of about 10m length to provide about 1kW of heating load.

How much does it cost?

Installation

A typical 6 - 8kW system costs £7,300-£11,800 plus the price of connection to the distribution system. This can vary with property and location.

Running costs
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 3 to 4 although some systems may produce a greater rate of e. The higher end of this range is for under-floor heating, because it works at a lower temperature (30-35ºC) than radiators. If grid electricity is used for the compressor and pump, then you should consult a range of energy suppliers to benefit from the lowest running costs, for example by choosing an economy 7 or economy 10 tariff.

Ground source heat pumps and your home

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

• The type of heat distribution system. GSHPs can be combined with radiators but under-floor heating is better as it works at a lower temperature.
• Is there space available for a trench or borehole to accommodate a ground loop?
• Is the ground suitable for digging a trench or borehole?
• What fuel is being replaced? If it's electricity, oil, LPG or any other conventional fossil fuel the payback will be more favourable. Heat pumps are a good option where gas is unavailable.
• Want to be 100% renewable? Buy green electricity, or install solar PV or some other form of renewable electricity generating system to power the compressor and pump.
• Need a back-up heating system?
• Is there also a cooling requirement?
• Is the system for a new building development? Combining the installation with other building works can reduce costs.
• Can you incorporate insulation measures? Wall, floor and loft insulation will lower your heat demand.