Water Heating

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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. It is the function of this section to help in reviewing the choices available and deciding whether you need to consider renewing your system.

An average family of four uses somewhere between 185 and 315 gallons 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. Some of these points are also discussed in two other sections in the guide: WATER and APPLIANCES.

The first point to understand about water heating is the difference between instantaneous 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; storage systems keep hot water available all the time, such as cylinders heated by electric immersion heaters or by the central heating boiler.


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


• Coal


In fact, if you are heating all your water electrically, replacement of your system should be a high ecological priority. The problem is that 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 can begin to find alternative ways of carrying out their functions.

At the top of the list is solar energy, which has many characteristics opposite 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 nonpolluting and can complement a storage system that is run by a central heating system boiler in the winter.

An assessment of the relative merits of gas, oil, and coal has already been made at the beginning of this section (Google “sources of energy”).


Storage heater (left) and instantaneous heater (right)

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 collector 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 best 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 multipoint 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 environmental costs of heating water with electricity, and this would include the shower units that have become so popular. It is quite possible to run showers from either multipoint or combination boilers if the plumbing is arranged correctly. The use of mini-electric heaters for warming water to wash hands is 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, which prevents waste when there is intermittent demand for hot water. Storage systems will cool down after a bath or shower so that it then takes time for the water to heat up again. The main disadvantage of an instant system is that only a limited amount of water can be drawn off at any one time.

Some gas combination boilers take longer than others to fill a bath, especially if hot water is being drawn off from elsewhere in the house. However, gas-heated instantaneous hot water is generally the most appropriate system for a small house and could also be appropriate for a larger one if the conditions are right (see the bulleted lists above).

Storage Systems

A storage system depends on keeping water hot in a tank or cylinder. The water is heated in the cylinder itself; this can be done in several different ways. An electric immersion heater, acting like an oversized electric kettle, is one way.

Alternatively, a “calorifier” or heat exchanger, often in the form of a coiled tube, is used to transfer heat from water outside the cylinder: most commonly in a boiler. Other ways this water can be heated are through the back-boiler of a solid fuel fire, a boiler in a wood stove, or of course almost any central heating boiler. If you use your central heating boiler, you will require a programmer that can control the two systems together.

Needless to say, the electric immersion heater should be a water heater of last resort; if at present you rely entirely on this form of heating, consider replacing it with a multipoint or a combination boiler.

The most important point of all if you are using a storage cylinder is the insulation. Cylinders lose heat continuously, day and night, and this loss can account for a large amount of energy usage during the year. Most cylinders come already insulated with foam. If you are ordering one, ask for an extra layer of foam. There is also nothing to stop you packing more insulation around the cylinder once it is installed.

Traditionally, storage cylinders required a cold water storage cistern. However in recent years water regulations have allowed storage cylinders to be fed direct from the mains, doing away with this requirement. This is called an unvented system and is widely used in continental Europe.

Storage cylinders and cisterns are very heavy indeed when filled with water. It is thus important that these are well supported in two ways:

• The structure on which the tank stands, and

• The cylinder or tank itself needs to be supported properly across its base to prevent rupture.

The main advantage of storage cylinders is that they allow multiple usages at the same time. There is also the possibility of using heat-recovery technology to recover heat from waste hot water and waste hot air. The Eco House in Leicester uses a heat pump to recover heat from waste hot water, and in some Swedish super-insulated houses heat is recovered from ventilation exhaust air to meet all the water-heating needs.

Hot-Water Cylinder Thermostat

This is a small thermostat that maybe fitted to the outside of the hot-water cylinder (not to be confused with an immersion heater). Setting it at, say, 50 C (122 °F) will ensure that the water in your cylinder is only heated when the water temperature drops below that setting. It avoids the problem of your tap water becoming scalding hot. It also reduces the unnecessary operation of your boiler.


A little is included here on solar water heating as it directly relates to this section. Solar heating systems in Britain can't provide all domestic hot water needs around the year, because of our latitude. However, they can be used to preheat water before it enters a conventional cylinder heated by a gas boiler, for instance. This system complements the boiler, which is doing most of its work in the winter, whereas a properly installed solar water heater should provide all summer hot water requirements.

There are two basic types of solar water heaters: direct and indirect. In direct systems, the heat transfer liquid passes straight to a hot water tank. In indirect systems there is no direct contact between the transfer fluid and the hot water system. The solar-heated water/fluid is circulated from the storage tank through a heat exchanger coil immersed in the hot water cylinder. More information about solar water heating systems is given in the SOLAR ENERGY section.


As with space heating, conservation is cheaper than the cost of the energy itself, or of a new installation. There are two basic means of conservation. First, you can heat the water with an energy-efficient appliance; second, you can conserve energy by:

• Insulating the hot water pipes and cylinder storage tank.

• Reducing the temperature at which hot water is stored by using a cylinder thermostat and setting it at the lowest temperature that will give you the hot water you need.

• Saving on the actual amount of water used by having spray fittings installed on taps for washing hands. It is also worth using an adjustable water-saving shower head, since it is then possible to vary the amount of water being delivered according to how much hot water is available.

• insulating the bathroom or shower to reduce heat loss while bathing and showering.

• Recycling the waste hot water through a heat pump to extract the heat before it is lost down the drain.

• Turning off your storage heater if you are going on vacation.

Water conservation when using appliances is discussed in the APPLIANCES section.


• If you heat all your water by electricity, consider choosing an alternative system from the various options outlined in this section.

• Using the criteria listed above, decide whether an instantaneous or a storage system is better suited to your household.

• If you are buying a new heating appliance, consider one that releases the least amount of CO emissions and consult CHOICE OF ENERGY SOURCE. A gas appliance is likely to be the most efficient choice in this respect. If you are combining your central heating with your water heating, then read about gas appliances in the SPACE HEATING section.

• Undertake as many of the hot water conservation measures as you can. In particular:

• Insulate your storage cylinder well. Remember, it is losing heat all the time.

• Use less hot water where possible either by shorter or less frequent use or by using constricting flow nozzles for showering or hand-washing.

• Consider installing a solar collector for your summer water-heating needs.

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