Will Anderson: The Green House

From the kettle to the bathtub - water can boost the eco-factor of all our homes
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Why is the sea always cold on British beaches even on a summer's day? I remember being asked this question in a geography lesson by Mr Watt (whose name was a joy to 14-year-olds) and all 30 of us failed to come up with a convincing answer. Does the warm water at the top of the sea mix with cold water below? No. Do the waves carry the heat and dump it on the land? No. Are cold ocean currents to blame? No.

The answer to Mr Watt's question is simply that water has a remarkably high heat capacity. This property defines how much heat a material can "hold", so the higher the heat capacity of a liquid or solid, the more heat is needed to raise its temperature. Water has a much higher heat capacity than concrete, brick, oil, wood, steel, glass and sand. So the same level of solar radiation can create a hot beach next to a cool sea. So ends the physics lesson.

Water's heat capacity has a huge effect on the environment. It helps keep the climate of Britain relatively stable, as the sea dampens annual temperature swings. As Mr Watt pointed out, Edinburgh has the same latitude as Moscow but does not suffer from the latter's severe winters and blistering summers. But a high heat capacity also means that a lot of energy is needed to produce hot water for daily domestic needs.

Kettles use far more energy than televisions, even though they are turned on for much less time. A modern kettle draws 3,000W compared to 100W for a TV, so the two minutes it takes to boil the kettle consumes the same energy as an hour in front of the box. The familiar eco-tip to only fill the kettle with as much water as you need is not mere penny-pinching.

Showers are recommended as an eco-choice because they use far less water than baths, but they consume far less energy too (unless they are the powered variety). But in the winter, you can make the energy in a bath go much further. When you get out, don't empty all that precious hot water. Let it stand overnight and the heat will be released into the air, helping to warm the house.

The worst domestic hot-water guzzlers are hot tubs, spas and swimming pools. If you want to install a profligate mod con such as this, work out how it will affect your energy bills first. And only install a private heated swimming pool if you expect to win gold in 2012.

The cleaner the fuel you use to heat your water, the better. A solar thermal panel is at the top of the list, electricity is at the bottom (as always, thanks to its high carbon intensity). The simplest form of a solar thermal panel, a "flat plate collector", is a large black sheet of metal with a water pipe running through it. The metal has a low heat capacity and heats up very quickly, but its high conductivity ensures that this heat is rapidly absorbed by the energy-hungry water pumped through the pipe. This heat is then transferred once again through the metal coil in the hot-water cylinder.

There is always one more trick in the energy-saving book. Another lifetime ago, I lived with English missionaries in north India whose frugality was positively Galilean. Although they drank huge volumes of tea, they always kept a Thermos flask next to their kettle so that any surplus hot water from the boil could be kept until the next brew was demanded. I do my best to fill my kettle to the right level, I have not yet adopted this energy-saving technique. I am reliably informed that twice-boiled water does not make good tea - and good tea is one mod con that even I am unwilling to sacrifice.


Install foil behind your radiators and reflect their heat back into the room. Especially important on exterior walls. £6.99 per roll from www.screwfix.com.


If you missed the march on Saturday, there is still plenty of political pressure you can bring to bear to tackle climate change. See www.stopclimatechaos.org.