There are several different ways to heat water for domestic use. You will probably already have a system installed and be more or less pleased with its performance.

An average family of four uses 700—1200 litres of hot water a week. The energy needed to heat this water often amounts to about one fifth of the total energy used (and it can be a higher proportion in a well insulated house). Besides choosing the right source of energy, there are ways of using less water more effectively.

The first point to understand about water heating is the difference between instant systems and storage systems. These are the two basic approaches, and they are suitable for different circumstances. Instant systems are those that heat water as required, such as gas multi-points or electric showers; and storage systems keep hot water available all the time, such as cylinders heated by electric immersion heaters or by the central heating boiler.

Choice of energy source
In addition to these two main systems there is the matter of energy source. Here there is a choice between the following, given in order of ecological soundness:

• Solar energy
• Gas
• Oil
• Coal
• Electricity

In fact if you are heating all your water electrically, replacement of your system should be a high ecological priority. The problem is that very many people do heat their water electrically, because of its convenience; even if we don’t heat our main supply of hot water electrically we use washing machines, dishwashers, showers and kettles that do. Once we become aware of the environmental impact of all this usage we have a choice about how often we use these appliances and also begin to find alternative ways of carrying out their functions.

At the top of the list is solar energy, which has many characteristics oppo¬site to those of electricity. It is intermittent and dependent on the seasons and the weather. It is expensive to install and cheap to run. It requires much care in its design, installation and operation. However it is essentially unpolluting and can complement a storage system that is run by a central heating system boiler in the winter.

Choosing between an instant and a storage system
The choice between an instantaneous system and a storage system will depend on a number of factors: the size of the house, the number of people using the system, how frequently the system is used and how uneven the usage is. The choice of an instantaneous system is indicated if:

• The house is small.
• The number of people using the system is small.
• The system is used unevenly, sometimes requiring large amounts of water but most frequently small amounts.
• There is little requirement for multiple use i.e. when two or three people want to use hot water at the same time.
• There is no space for a storage cylinder.
The choice of a storage system is indicated if:
• The opposites of the above are true.
• There exists an efficient boiler that can also be used to heat a storage cylinder.
• There are plans to install a solar panel or a heat recovery system with a heat pump.

Going through the above list and finding out how many indicators you have for each system should give you a good idea of which one is most suited to your needs.

Instantaneous systems
Instantaneous systems are essentially appliances which heat the water as it passes through, producing hot water on demand. The heating is switched on or ignited when water begins to flow through the unit. There is usually a minimum flow that needs to take place before the heating is activated, to avoid overheating.

Two types of instant system are available: gas and electric. Gas units consist of multi-point heaters and combination boilers that heat water for central heating as well. Electric systems include those that heat water for showers, and miniature units that heat small amounts of water for hand-washing at remote locations. Much has been said about the heating of water with elec¬tricity and this would include the shower units that have become so popular. It is quite possible to run showers from either multi-point or combination boilers if the plumbing is arranged correctly. The use of mini-electric heaters for warming water to wash hands are useful only at the end of long pipe runs where more energy would be wasted running the tap than heating the water electrically.

The main advantages of instant systems are that (a) there is no need for a storage cylinder, which is constantly losing heat; and (b) only the water that is required is heated,

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