Heat pumps – air, ground and water source

Heat pumps make full use of heat naturally stored in the ground, water and even the air to reduce the amount of fossil fuels we need to burn to heat or cool our buildings.

The ground is continually soaking up and retaining warmth from the sun. Heat pumps extract that heat and use it to pre-heat water for space and water heating so reducing the amount of gas, oil or electricity consumed. 

Fridge
A heat pump works like a fridge in reverse. While a fridge takes heat out of the food stored inside and releases it into the room, the heat pump extracts heat from cold surroundings. It then brings this heat up to a temperature sufficient for central heating (between 55 and 60degC).

This works in the summer as well as in the winter 24 hours a day. As it is a low temperature heat source – unlike conventional central heating, which operates at high temperatures – larger heating surfaces such as underfloor heating and low surface temperature radiators are the best way to extract maximum efficiency.

Slinky
Depending on the plot size and ground conditions, ‘slinky’ loops can be buried a metre or two below the surface in horizontal trenches or in vertical boreholes drilled down to around 80 metres.

The higher the temperature of the heat source, the more efficiently the heat pump will operate, but the key thing is having a constant temperature. The earth around UK buildings is on average a constant 10degC, which is ideal. 

The heat is collected by polyethylene pipes filled with a water and anti-freeze mixture that extract about 50 watts of heating energy per metre in a borehole or one kilowatt per 25 square metres with the horizontal coils. This temperature is ideal as a pre-heated source of water for heating or as a cool water source for cooling in summer.

Groundworks
Air source heat pumps are not as efficient, but are easier to apply as they do not require any groundworks and can be used in properties without much surrounding land. They extract heat from the air and can be installed either inside or outside.

Open loop water source heat pumps can be the most efficient of all as they use heat extracted from a body of water. In this case the water from the source itself is pumped directly through the system. This source water is also proving highly effective in air conditioning systems and is particularly well suited for chilled ceiling systems providing, in theory, full building cooling with no mechanical systems at all.

Issues to consider:

Ground source heat pumps require adequate land for the ‘slinky’ approach or a borehole and groundworks can be expensive and disruptive.

Air source systems are less efficient and can be unsightly as they are relatively large units installed either inside or outside the property.

Water source heat pumps require a standing body of water and also care must be taken to ensure components are rust and corrosion proof.