Heat Pumps

Heat pumps work in the same way as a refrigerator does, the heat exchanger extracting heat from the surrounding atmosphere (ground, air or water) and transferring it into an enclosed space. The heat can then be distributed around a building via a central heating system. This can also be reversed with the heat taken from the enclosed space and ‘dumped’ outside to cool the building.

At a domestic level both air source and ground source heat pumps can be used to good effect.

Air Source

An source heat pump on a building takes heat from the air surrounding the building to heat the building. An air source heat pump takes up very little space, and can be fitted to a side wall or roof of a building, and will work in many situations including flats, though this sort of systems may require planning permission in conservation areas or on listed buildings. An air source heat pump can work at up to 400% efficiency, but for this to be a renewable solution the electricity used to power it must be from a renewable or green source (see green electricity).

Ground Source Heat

Ground source heat pumps extract heat from the ground through pipes that are buried either horizontally in a trench or vertically in a borehole. These pipes contain a mixture of water and anti-freeze which is pumped through, absorbing the heat from the ground and transferred to the heat exchanger which transfers this to the heat pump.

A heat pump can be intalled in domestic homes where there is either space to dig a trench (2m deep) or the ground is appropriate to bore a hole (15 – 150m deep). The heat pump is about the size of a fridge and must be as close to the loop as possible. In most situations planning permission is not required, but in some areas it is asked for.

A ground source heat pump uses electricity and can only be considered renewable if this electricity is from a renewable or green source. Though a ground source heat pump can provide all of the heating and hot water demands of a household.

The cost of installation of a system varies, but is around £1000 per kW, and an average system is between 3 – 7 kW.

It is also possible to use water as the heat source using either an open or closed loop system. In an open sourced system, water is extracted from wither the ground water table or a lake, and passed through filters before being pumped to the heat pump in the building being heated, and then returned to the water table or lake some distance from the extraction point. Alternatively, with a closed loop system, a collecting loop (like the slinkies used in a ground source system) is laid on the bottom of a lake or river, and the water in the heat exchanger system never intermingles with the body of water from which the heat is being extracted. The internal circuits are usually the same as for a Ground Source Heat Pump. Water source heat pumps are only rarely used in the UK, as both types normally require an extraction licence from the Environment Agency, and ground source pumps are usually at least as easy to design and install. However, if you own a large property with a lake, it may be worth seeking specialist advice on a water source heat pump, and undertaking some modelling to ensure that the water source temperature is not lowered by too much.