In an empty warehouse in Oxfordshire sits a large block of ice, which pumps out enough heat to keep a nearby office warm. Yes, you read that correctly. Heat from a block of ice. This oversized ice cube is an essential part of a clever heating system called a heat pump, widespread in the rest of the world but almost unheard of in the UK. Until recently.

Heat pumps work just like fridges, by sucking heat from warm things and moving it elsewhere. Put your hand round the back of the fridge. Touch the black metal bits. Warm, aren't they? That's the heat pump at work, taking heat from the milk, cheese, yoghurt and beer inside, and letting it all escape round the back.

That's the thinking behind domestic heat pumps. There's heat everywhere, in every substance over -273C (absolute zero). The block of ice in the warehouse is kept at -10C, warm in comparison. Plenty of heat to be had from that.

The same can be said of your garden. Every day it absorbs energy from the sun, storing it up like a gigantic hotplate. Domestic heat pumps can also suck warmth from the ground (or even the air or water),concentrate it using the application of GCSE-level science, then put the heat to work in your radiators and hot water taps.

With fossil fuels becoming alarmingly expensive, this environmentally friendly and low-cost alternative to gas central heating is finally coming into its own in the UK. It is incrediblyeffective, capable of achieving 400% efficiency - giving out more energy (typically 3 to 4 kilowatts) than the householder puts in to run it (typically 1KW). By comparison, an average gas boiler works at 90% efficiency at best.

According to Professor David Reay, of Heriot-Watt University, an expert on heat pumps, little can be said against them. Variants that extract heat from outside air perform less well in cold weather, just when the heat is needed most.

Strong argument

Ground source heat pumps overcome this problem. "If we are serious about global warming, we need to put our money where our mouth is," he says. "As energy from fossil fuels generally becomes a more precious commodity, the argument in favour of heat pumps gets ever stronger, and the return on investing in them can only improve."

While renovating their rural home a few miles outside Dorchester, Tim and Tessa Ferguson found the oil-fired heating system far too costly to run. Last year, they installed a heat pump and expect it to pay for itself within seven years. The estimated annual running cost will be £250, compared to about £1,000 for the oil system.

"This system operates all day, every day. We have plenty of hot water and the house is a comfortable temperature all year round," says Tim Ferguson. "It operates continuously, so the fabric of the house itself warms up, turning the building into one giant storage heater."

The system cost about £8,500 to install, but the Fergusons received a rebate of £1,500 from the government, which has been running a grant scheme to encourage more people to switch to such renewable energy sources.

The heat pump's success is partly due to its simplicity. A 150-metre length of plastic pipe buried a metre under the Ferguson's garden is filled a mixture of water and anti-freeze, circulated by a simple electric pump. As it moves around the circuit, the water warms up just a few degrees, which is enough to make a difference.

When the water arrives at the house, its temperature has been raised compared to that on the outgoing side of the circuit by four or five degrees celsius. It goes into the heat pump, a box about the size of a fridge-freezer. Here, the heat from the water is conducted to a sealed circuit containing refrigerant, warming it up. (Imagine putting a loop of hose connected to the hot water tap into your fridge. As the hot water cools, the fridge heats up.) The heated refrigerant in turn is pushed through a compressor (powered by electricity). Compressing the refrigerant creates more heat (up to 50C in a typical home with radiators in every room), and this heat is transferred once more to a third circuit - the one supplying hot water for use round the home.

The only cost is the electricity needed to power the compressor and three or four electric pumps that keep each circuit flowing. There's a chance that one of these essential parts might fail, but they are usually good for many years.

Constant heat

Ground-source heat pumps, where a length of pipe is laid horizontally into the ground, or a 15-centimetre borehole drilled about 50m down, can be installed just about anywhere. The heat continues to flow in winter, even if the ground is frozen. "Seasonal differences are very small," says Tony Bowen, president of the Heat Pumps Association, the UK trade body. "If you use a vertical borehole instead of an array of tubes buried in the ground, there's no discernible difference."

The Fergusons bought their system from Ice Energy, whose Oxfordshire office is heated by the aforementioned ice cube. Managing director Andrew Sheldon points out that 97% of all new buildings in Sweden are powered by heat pumps. "The energy ministry wants to eventually eliminate fossil fuel burning," he says. In the UK, the Office of the Deputy Prime Minister is reviewing options. "Housing developers are likely to be driven to use ground source heat pumps; the argument in their favour is very strong," he adds.

So if heat pumps are such a great idea, why haven't they caught on before? "Gas has been cheap, and the British are capital-averse," sighs Tony Bowen. "As a nation, we are bad at investing in low long-term running costs."

The Fergusons clearly buck the trend. "We'll save a fortune in fuel bills, and we hope we've added to the value of the property just by installing it."

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