There are two main types of gas or electric heaters-storage cylinder and instantaneous-to provide running hot water in the home. Your choice will depend on how much hot water you use: if you require a constant supply in large quantities, a storage cylinder heater is probably the best, although large multi-point instantaneous gas heaters can also be used. If you need small quantities of hot water for relatively short periods of time, instantaneous heaters are the answer since they will provide an adequate supply. Available to run off either gas or day (or off-peak)electricity, storage heaters will give an ample supply of hot water. These era available to provide the whole house with hot water or to supply one particular area. If you intend to use one or more electric immersion heaters as the sole heat source for a simple, direct cylinder hot water system, it is best to install an Under Draining Board water heater instead of a conventional cylinder. UBD heaters are specially designed to heat water economically by electricity. They have a twenty-five gallon capacity, are very heavily insulated and, as the name suggests, fit under the draining board of the kitchen sink, close to the hot water draw-off point in most frequent use. They usually have two horizontal immersion heaters, one about a third of the distance from the top of the cylinder and the other near base. The upper heater is switched on all the time to provide an immediate source of hot water for hand basins and sinks. The lower heater should be switched on about an hour before larger volumes of water are required,such as for baths or household washing.UBD heaters must be supplied from the main cold water storage cistern and must have a vent pipe, as with other cylinder hot water systems.
OPEN OUTLET HEATERS: Designed for use with gas or electricity, open outlet heaters are usually of (one or two gallons) capacity and are installed immediately above the area they serve. This type of heater may, if the local water authority permits, be connected direct to the rising main or be supplied from the cold water cistern. When hot water is required, a control valve on the inlet side of the appliance is opened. Cold water flows into the base and hot water overflows down a stand-pipe, through an open outlet and into the sink. The stand-pipe lip is fitted with an anti-siphoning device which,when water has been drawn off, ensures the level within the heater is about half an inch below the lip to allow for the increased volume of heated water.
INSTANTANEOUS WATER HEATERS: The great advantage of this type of heater is that you heat only the water you are going to use immediately; there is no reserve of stored water which, however thoroughly the cylinder is
lagged, slowly loses its heat. There are draw backs to this method of water heating: the rate of hot water delivery is lower than that of a storage system and, in hard water areas, there is a tendency for scale to form. Also these heaters do not raise water to one particular temperature: they raise it through a range of temperatures as it passes through the heater. In cold weather you may find the temperature of the water at the outlet too low or the rate flow markedly reduced. Like the open outlet,instantaneous heaters may be connected direct to the rising main. Incoming cold water flows through a system so small bore copper tubing within the appliance and is heated either by a gas flame or by a powerful electric element as it flows.