Electric instantaneous water heaters are a relatively new development but the suspicion with which early models were received has now been largely dispelled. They have the advantage of quick and simple installation in virtually any situation where a water and a power supply are available.
They are particularly useful in providing a hot water supply for wash basins in w.c. compartments remote from the main house hot water system and in making possible the provision of a shower in situations where this would otherwise be impossible.
The great advantage of instantaneous water heating is that only water actually used is heated. There is no stored water slowly losing its heat and requiring more fuel to restore it. A multipoint gas water heater can therefore be particularly useful to a working husband and wife who require hot water only for relatively short periods in the morning and evening.
A limitation of this kind of appliance that is not always appreciated is the fact that it docs not, by its nature, raise a given volume of water to a required temperature. It raises the water that passes through it through a range of temperatures. Open outlet heaters are small storage heaters designed for direct connection to the rising main. They may be used over sinks and wash basins where there is no cylinder system or where the use of the storage cylinder would involve an unacceptably long distribution pipe or dead leg.
The essential feature of the open outlet heater is the position of the control valve. This must be on the inlet side-no r on the outlet-of the appliance. When hot water is required the inlet control is opened. Cold water flows in to the base of the appliance, displacing stored hot water which overflows through an internal stand-pipe connected to the outlet spout. Modern variations of this kind of appliance may be installed under, instead of over, the sink or wash basin. These/must still comply with the essential requirement of a controlled inlet and a free outlet.