Heat pumps
Ground source heat pumps use a buried ground loop which transfers heat from the ground into a building to provide space heating and, in some cases, to pre-heat domestic hot water. As well as ground source heat pumps, air source and water source heat pumps are also available.
The benefits
The system does not require any external fuel and is designed to heat a whole building.
The efficiency of a ground source heat pump 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. Average CoP is around 3-4 although some sytems may produce a greater rate of efficiency. This means that for every unit of electricity used to pump the heat, 3-4 units of heat are produced, making it an efficient way of heating a building. If grid electricity is used for the compressor and pump, then you should consult a range of energy suppliers to benefit from the lowest running costs, for example by choosing an economy 10 or economy 7 tariff.
How it works
There are three important elements to a ground source heat pump:
- 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 around the pipe absorbing heat from the ground.The ground loop can be:
- Vertical, for use in boreholes
-Horizontal, for use in trenches
Spiral, coil or 'slinky', also for use in trenches - A heat pump - In the same way that your fridge uses refrigerant to
extract heat from the inside, keeping your food cool, a ground source heat
pump extracts heat from the ground, and uses it to heat your home.
A ground source heat pump has three main parts:
-The evaporator, (e.g. the squiggly thing in the cold part of your fridge) absorbs the heat using the liquid 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. - Heat distribution system - This consists of under floor heating or radiators for space heating and in some cases water storage for hot water supply.
Is it suitable for my home?
You should consider the following issues if you're thinking about installing a ground source heat pump. An accredited installer will be able to provide more detailed advice.
- You will need space outside your house for the ground loop.
- The ground will need to be suitable for digging a trench or borehole.
- What fuel is being replaced? If it's electricity, oil, LPG or coal the payback will be more favourable than gas. Heat pumps are a good option where gas is unavailable.
- The type of heat distribution system. Ground source heat pumps can be combined with radiators but these will normally be larger than with standard boiler systems. Under floor heating is better as it works at a lower temperature.
- Want to further reduce your home's carbon dioxide emissions? Purchase a green electricity tariff, or install solar PV or some other form of renewable electricity generating system to power the compressor and pump.
- Is the system for a new building development? Combining the installation with other building works can reduce costs.
- Have you installed insulation measures? Wall, floor and loft insulation will lower your heat demand and make the system more effective.
Air and water source heat pumps
Air and water source heat pumps use air or water respectively. They do not rely on a collection system and simply extract the heat from the source at the point of use.
Air source heat pumps can be fitted outside a house or in the roof space and generally perform better at slightly warmer air temperatures. Water source heat pumps can be used to provide heating in homes near to rivers, streams, lakes and lochs for example.
Costs and savings
A typical air and ground source heat pump 6 - 15kW system costs £8,000 - £12,000 (not including the price of distribution system).
This can vary with property and location. On average you can save £400 - £800 on your heating bills and 2 - 8 tonnes of C02 per year, dependent on the type of fuel being replaced.