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 8kW system costs £6,400-£9,600 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 2.5 to 4. The higher end of this range is for under-floor heating, because
it works at a lower temperature (30-35ºC) 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
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
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.
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