Will Anderson: The Green House

Here's a curious thing. In the olden days when washing machines consumed energy and water like hyperactive Alsatians, they always had three pipes sticking out the back: one for the drain and two for the hot and cold water. After a decade of energy labels and consumer pressure, the electrical shops are now full of poodles with oh-so-meagre appetites, yet these efficient models have only two pipes out the back - the drain and the cold fill. So instead of taking water from your hot tank or combi boiler, the modern washing machine draws cold water and uses electricity to heat it up. If you heat your water with a climate-friendly solar panel or wood-burner, this switch to dirty electricity seems particularly galling.

Surely some mistake, right? Well, no. You see, Alsatians and poodles are different beasts in every way. Sometimes when you design with energy in mind, you reach a threshold that changes your whole approach. Let me explain.

A new, efficient washing machine will use much less water than an old model - around 40 litres - and will often heat this water to no more than 40C, thanks in part to biological detergents which work best at this temperature. This hot water, however, is only required for the very first part of the cycle because all the rinses are cold. So our total hot water demand is down to about 10l. But in practice, only half of thiswill come from the hot pipe (if there is one), because both hot and cold water are drawn for a 40C or 60C wash.

This takes us over the threshold. When I spoke to technicians at AEG-Electrolux (www.aeg-electrolux.co.uk) and Bosch (www.bosch.co.uk) - brands with cold-fill only machines - they both drew my attention to the same issue. If you are drawing very little water from your hot water tank, a lot of the water will actually be taken from the pipe between your tank and the machine. Consequently, the machine has to "reheat" this water because it will have lost its heat while standing in the pipe. What's more, when the fill has finished you will leave another pipeful of hot water behind. Heating the water once in the machine, at the point of use, is therefore more energy efficient.

This ostensibly energy-saving redesign is based on the assumption that our houses are poorly designed for energy, in particular that our hot water store or supply is often far from where we actually use hot water. It is certainly a reminder of the importance of keeping pipe runs short, narrow and insulated wherever possible. If you have short pipe runs, or use solar hot water, or insist on washing at high temperatures, you may still do better with a cold- and hot-fill washing machine.

The improvements in energy and water efficiency of washing machines over the last 20 years have been remarkable. However, they have also become slicker, quieter and more pleasant to use. So guess what? We use them more - on average 270 times a year. This is the bind that constrains the benefits of energy-efficient design: we do not curtail our energy use if we do things more often as well as more efficiently.

When you do use the machine, make the most of it, because a half load uses much more than half the water and energy of a full load. In fact, Japanese use cold water for all their washing, so perhaps our behavioural shift from the boil wash to 40C should be seen as nothing more than a stepping stone to a future that is cold, bright and carbon-free.

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* GREAT WEBSITE

For more information about washing machines see www.washerhelp.co.uk. It makes a good case for prioritising durability over energy rating.

w.anderson@independent.co.uk