Donnachadh McCarthy: The Home Ecologist

The latest craze in hi-tech heating sounds great – but the figures simply don't add up

At last month's Eco-build show at Earl's Court, a few stands were promoting ground-source heat pumps. Heralded as a planet-friendly way to heat homes, they work by harnessing heat energy under the ground. But the figures don't always add up.

One metre below the surface, the temperature is a fairly constant 10C. By laying pipes beneath your garden, and pumping a liquid through them to a heat exchanger, this heat is captured and ratcheted up to the level required for central heating or hot water.

On paper, it looks like free energy. Sadly, there is a catch: the pumps run on electricity. The Ground Source Heat Pump Association states that for each unit of electricity used, you get three to four units of heat into the building. But much of that electricity comes from power stations, where nearly two-thirds of the energy is lost through cooling towers. Further energy is lost as the electricity comes down the wires to our homes. The truth is that, in most cases, you end up putting in three units of electricity to get out three units of heat. So, despite having invested anything up to £15,000, you could just be breaking even or making only a small reduction in your CO2 emissions.

It gets worse: in reality, heat pumps sometimes do not achieve their performance claims. One organisation we spoke to said its system is only delivering 2.3 units of heat per unit of electricity. Others reported even worse performance. This means those systems could be producing more CO2 emissions than a modern combination gas-boiler.

Liz Reason, of AECB, the sustainable building association, says: "The decision to invest in a ground source heat pump is made with the best of intentions – but it may be carbon illiterate. Homeowners may invest substantial sums of money, only to see their CO2 emissions increase."

A spokesperson for the Ground Source Heat Pump Association says that they are widely used in eco-conscious nations like Sweden. But CO2 emissions from Swedish electricity are far lower than in the UK, because more of it comes from "clean", renewable sources there.

For homes attached to the gas mains, the heat pumps are usually a waste of money. Most experts agree that if a large source of surplus renewable electricity is available from, say, a medium-sized wind turbine or a micro-hydro system, then they could make sense – but this would cost £10,000 to £20,000.

Sadly, like the halogen lighting adorning too many of the stands at Eco-build, ground-source heat pumps are not yet the golden bullet for climate change.