Ground source and air source heating can form part of the route to low carbon living. They operate using the same principle as a refrigerator but in reverse. The ground source heat pump circulates fluid in pipes under the ground. The ground in UK one metre down remains at about 10 degrees celsius most of the year. A compressor uses this heat to heat the house and provide hot water and returns the fluid to the ground loop a few degrees cooler.

A full explanation is provided by the Heat Pump Association.

A ground source system has the potential to multiply the energy used to drive the compressor by 6 to heat the house. This would be a coefficient of performance (COP) of 6. Most suppliers do not quote the COP for the systems they install. This is probably because this would depend on the quality of the installation and the nature of the ground. However improving the COP of installations must be in the national interest and is also most valuable in saving money. The COP of each installation cannot easily be measured but the inclusion of a water meter in the ground loop enables the energy reaching the house to be calculated from the input and output ground loop temperatures. This could promote a real competition for the most efficient heat pump installation.

Air source heat pumps draw heat from the outside air but cannot achieve as high a COP. They are cheaper than ground source heat pumps and rather easier to install so are more popular.

The distribution method for the energy gained is most important. If an existing system of radiators is used the heat pump will have to be run at a higher output temperature which will result in lower efficiency. A system of underfloor pipes is ideal. The large heated area will enable the system to run at a lower temperature.

Many methods are on the market for the pipe work, templates, and aluminium distributor trays. Some are very costly even before the installation cost is considered. Underfloorheating1 are leading UK specialists in radiant water underfloor heating and wall heating. Their methods are more cost effective than most we encountered and were used for our Marchwood Eco-house renovation.

It now appears (2011) that there is a wide divergence in efficiency between the best and worst heat pumps. A recent study of 83 installations by the Energy Saving Trust shows many ground source installations to be worse than air source and German user data supports this.

It is vital that the reasons are found and the best performance obtained in future.

Sources

Heat Pump Performance  07/01/11

Heat Pump Variability  11/01/11