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Summary: How ground source heat pumps heat water using geothermal energy and water pipes buried under the ground
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A ground source heat pump is a pump which pumps water and anti-freeze around a loop of pipe (Ground loop) which is buried in your garden. As the water is pumped round the ground loop the heat from the ground, called geothermal energy, warms the water which can then heat underfloor heating systems, hot water and radiators.
The Ground Source Heat Pumps were invented about 60 years ago so they are not new, but it has taken this long to refine them into a marketable, efficient product that is capable of replacing modern day systems. A heat pump for a domestic property is usually about the same size as a big fridge and some units can be used for cooling as well as heating.
For some idea of how a heat pumps works, consider the fridge in your home. It sucks in heated air from the fridge body, takes the heat out and expels it through a vent, as waste. With a heat pump, heat is not expelled, it is used to warm water and spaces and it is the colder air that is expelled.
Obviously sinking, or burying a ground loop in your garden requires some disruption to the lawn and the size and length of the trench depends on the size of your house and garden. The ground loops are usually buried at about 1.8m deep but in really small gardens a bore hole can be dug up to about 90 metres deep to take the pipes.
A typical Ground Source installation for a 2 bed detached house would need (at least) two trenches about 12 inches wide and about 45m long. The trenches also need to be about 15 feet (5m) apart. The trenches will ideally be level to avoid making the pump work too hard and the wetter the ground, the more heat is conducted. If the ground is particularly hard, the trench can be shallower (minimum 4 feet or 1.2m) and the coils of the ground loop can be laid flat in the trench rather than the usual upright loop. This however is not as efficient.
Obviously you need to be able to get a digger or other excavation equipment into your garden!
Heat pumps can heat water to about 55 degrees centigrade. For optimum efficiency and energy saving it is best not to heat water beyond this temperature or the energy used is greater than the energy gained and your money is wasted. This is why geothermal heat pumps are ideal to heat your home using pumped underfloor heating. The warm air simply fills the room from the floor and heat is generated everywhere because of the floor area.
Radiators have a very small surface area so to heat a home the water has to be heated to considerable temperatures. To use heat pumps to heat radiators efficiently, the radiator size would need to be increased dramatically to accommodate the lower water temperature. The same benefits would be realised but the cost would be much greater both in financial terms and wasted energy.
As well as reducing your heating bills, a ground source heat pump will save a huge amount of energy and the Carbon dioxide (CO2) savings per year are huge.
Using ground source heat instead of electricity for heat can not only save you about £1000 per year on your heating bills but it can save up to 7 tonnes of CO2. This is assuming you use your ground source heat for heating some of your water and all of your house.
Most modern heat pumps include integral, automatic weather compensation. When the temperature outside drops the heat increases. This is governed to gain maximum efficiency even at the highest temperatures.
The modern gas central heating boilers are rather more efficient that their oil and water counter-parts and your savings from Ground source heating will be smaller but you can still expect to save about £400 pounds a year on your bills and over a tonne of CO2 !
The effectiveness of ground source heat in the home is dependant on how well your home is insulated. The heat from a GS pump is less than conventional heating methods and more of the heat will need to be contained. If your home is free from draughts and well insulated you can install a smaller GS system to heat your home.
Depending on the size of your property a GS heating system can cost anything between £6,000 and £12,000. The more inefficient your existing heating, the faster you will get your money back.
For every kilowatt of electricity used to run a heat pump to heat water to 35 degrees centigrade, 4 kilowatts of energy are produced. This makes GS systems very efficient indeed. The term Coefficient of Performance (COP) is used to measure the efficiency of a heat pump. The COP describes the ratio of heat output to energy input so the higher the figure, the more efficient the system. When choosing a system and taking everything into consideration including draughts, insulation etc, ideally one should look for a COP of at least 3.5 to gain an advantage over modern day gas condensing boilers.
Grants of up to £1200 are available to install domestic ground source heat pumps and websites such as the Energy Savings Trust can 
help with this. It is worth noting that grants will not be available for pumps which cool as well as heat. To receive a grant for the installation of a GS system you need to comply with rules on carbon emissions from buildings as outlined by the Department of Energy and Climate Change
There is also an organisation called The Ground Source Heat Pump Association. This organisation lists the following as one of its principle aims, and they are there to help the consumer with information on the website and by telephone
"To promote the highest standards of integrity, professionalism and technical competence within the Ground Source Energy Industry to ensure consumer confidence." (gshp.org.uk 05.06.09)
In Scotland the rules are slightly different and grants can be obtained under the Scottish Community and Household Renewables Initiative.
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