Pipe dreams

Eight weeks into his self-build project and the foundations - and heat pumps - are going in. Will Anderson explains how this mysterious technology uses warmth from the earth

Why do cats sit on fridges? Fitted kitchens have banished this nesting site from the domestic routine of most cats, but I remember, as a child, the imperious look of Moppet, sitting high up on the fridge-freezer in a chilly Edinburgh kitchen. It seemed a strange choice to me, given her fondness for lying dangerously close to the gas fire.

Why do cats sit on fridges? Fitted kitchens have banished this nesting site from the domestic routine of most cats, but I remember, as a child, the imperious look of Moppet, sitting high up on the fridge-freezer in a chilly Edinburgh kitchen. It seemed a strange choice to me, given her fondness for lying dangerously close to the gas fire.

Cats are, of course, smarter than little boys. They understand that in order to keep the inside of a fridge cold, heat regularly has to be pumped out of it. This heat is concentrated and released at the back of the fridge, creating a warm draught for furry hides above.

There are four cats in our current family, all experts in finding the warmest corner of our south London flat. Little do they know, but two blocks away at our Clapham building plot, a piece of kit is currently being installed that will make our entire house feel like the top of a fridge-freezer.

The ground has been well-prepared: the last two weeks on-site have been dominated by earth-movers, carving a tentative house footprint in the clay. Our four young (male) neighbours have been awestruck. Little Lucas is so gripped by the events on the other side of his garden fence that he bursts into tears if "Mr Digger" stops for a fag. Happily, Mr Digger - otherwise known as Steve - has not dug up anything other than soil, gravel and clay. There are no signs of Roman remains, unexploded bombs, Victorian wine cellars or anything else that might obstruct our piled foundations. We are using piles, rather than strip foundations, in order to protect the roots of our beloved tree and the house, should the earth move beneath it. But we're getting some added value from the piling rig: four 25 metre boreholes. The pipe that we're threading through each borehole will take the heat out of the ground and into our house in exactly the same way that heat is pumped out of a fridge.

A "ground source heat pump" is highly unusual in central London, although the invertebrates in London Zoo have enjoyed one for some time. This is not because the technology is unproven - they are installed in most new homes in Sweden - but because gas is cheap.

The renewable energy under your back garden is there for the taking but you still need electricity to pump it out. Although a heat pump will generate about three times as much heat energy as the electrical energy you put in, electricity costs three times the price of mains gas. Not surprisingly, the UK heat pump market is concentrated where mains gas is not available.

We are installing a heat pump because we care about environmental costs as well as our monthly bills. The carbon emissions generated by a heat pump vary depending on how the electricity is generated but over the year a ground source heat pump will be greener than even the most efficient gas boiler. In order to achieve our "zero carbon" goal, our heat pump will be driven by our roof-top solar power station.

Sixty-two per cent of the energy consumed by households goes on space heating, so it's a good place to begin any domestic eco improvements. Your first priority should always be to stop the heat escaping: an afternoon spent draught-proofing will, I promise, transform your life. Improving insulation is rarely a top DIY priority, but even if you can't see the result, you will feel it.

If you are ready for a new heating system, get advice about the most energy efficient and climate-friendly options for your circumstances. If you're after a new gas boiler, make sure it's a condensing boiler that extracts energy from its own exhaust gases. If you're reliant on oil, coal or electricity, it's certainly worth considering a heat pump. If the capital cost seems a little steep, remember that heat pumps have low maintenance costs and a very long working life.

Wood, the fuel of choice for almost the entire history of humanity, remains a highly ecological fuel because growing and burning wood does not result in any net increase in atmospheric carbon dioxide. Automated wood-pellet boilers are the most advanced carbohydrate burners and, like ground source heat pumps, attract government grants. In an exceptionally energy efficient home, "casual" heat sources can also be important.

In The New Autonomous House, ecological architects Brenda and Robert Vale contemplate populating their bedroom with 15 cats to meet their small heat demand. A cat produces about 15W of heat, so we can rely on a steady output of about 65W (including an extra 5W for alpha-male Trevor). This is significant, though we know from our nightly experience of being pinned to the bed that the cats value our heat more than we will ever value theirs.

Hopefully, when Tree House is complete and occupied, Trevor and family will no longer feel the need to cling to their guardians but instead stretch their considerable bellies over the warm floors, enjoying Moppet's top-of-fridge experience in every corner of the house.

Contact Ice Energy Heat Pumps on 01865 882202 or www.iceenergy.co.uk. For details of grants for ground source heat pumps and wood-pellet boilers call 08702 430930 or visit www.clear-skies.org.

HOW TO BE ENERGY-EFFICIENT

The Government wants to reduce CO2 emissions by 60 per cent by 2050. As half of this cut has to come through energy efficiency measures, you shouldn't find it too difficult to get advice on the matter. Call 0800 512 012 to speak to your local energy efficiency advice centre.

This is national Energy Efficiency Week, with special events organised around the country designed to encourage smarter use of energy and raise awareness of the adverse effects of energy, such as climate change. To find out what's happening in your area, or for practical information about energy efficiency, call the Energy Saving Trust helpline on 0845 727 7200. The Trust also provides extensive advice on its website, www.est.org.uk, including information about the grants available from the Government and from energy suppliers.