Most modem hot water systems rely on a boiler to supply both hot water and central heating, but there are still many properties where hot water is supplied instantaneously through a multipoint gas or electric heater, or a single-point unit such as the familiar Ascot.
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| Instantaneous Water Heaters |
In fact, the modem combination boiler is essentially a multipoint unit with the addition of a central heating function. Hot water is supplied on demand, often temporarily cutting off the heat to the central heating in the process. However, there are many instances where the use of a water heating unit, separate from a central heating system, will provide an economical and practical solution to a hot water need. This is particularly the case where the ‘point-of-use’ is remote from the rest of the hot water system. The most common point-of-use electrical unit is, in fact, the instantaneous electric shower , and exactly the same installation criteria apply to
the over- or under-sink water heater as to the electric shower. The only difference with the under-sink unit is the need for special taps and valves to deal with the expansion of water. In the over-sink water heater, expansion takes place through the spout, which must always remain open, the water flow being controlled on the inlet side. In the case of under-sink heaters, control is provided by the tap on the outlet, which prevents expansion in that direction.
There are gas instantaneous water heaters available, but since any work on gas appliances may only be carried out by a competent CORGI-registered installer, they are outside the scope of this book.
Sink-based, point-of-use electric water heaters are designed for hand washing and similar activities, and units range in power from about 3kW to over 7kw, the latter being a typical power rating for an electric shower. The electrical installation also follows the rules that apply to electric showers.
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