The prime uses of energy are for lighting, household appliances, electronic equipment, cooking, water heating and space heating. Of these groups, the first three require electricity whilst the last three groups can use gas or other fuels.

Heating by combustion

The very common heating source is natural gas which is burned in a boiler. The heat is transferred to air, or more likely water, flowing through the heat exchanger. The heated water is distributed to for example the radiators, where heat is given off by conduction to the air and the heated air then circulates by convection around the room. The cooler water passes back to the boiler where it is reheated.

Similarly, oil, coal or wood, other types of gases or even biomass may be used in other types of boilers which will burn the fuel and heat the water in a similar way. The decision as to which fuel to use will depend on its availability and cost. In terms of limiting climate change, it is important to consider the efficiency of combustion and resulting environmental pollution arising from the combustion process. The biggest pollutant is carbon dioxide which is the principal gas responsible for global warming whilst other pollutants which may be harmful include carbon monoxide and nitrous oxides. While wood and biomass are renewable energy sources, oil and gas have been created over millions of years and are being used up faster than they can be created.

Heating using electricity

Electricity is also used for heating homes. Then no chimney is required as the energy flows into the house by wire and is converted to heat through the resistance of an electrical element. A more efficient form of electrical heating is by a heat pump which concentrates the low grade heat energy available in the air, ground or water. Electricity is then only used to concentrate the heat not to produce it.

Remember that electricity is only a carrier of energy and is produced somewhere else! Most often, the same fossil fuels mentioned above, are also used to generate electricity, a process which also results in environmental pollution; this time in a central location rather than distributed in individual homes.

Direct heating using the sun

This may be done by passive means such as conduction or convection (Trombe wall) or actively through solar thermal collectors. As air or water is heated directly, the only environmental impact is that of distributing the heat around the house.

criteria	economics
installation	advice