Book - School of Science and Technology

568 Hot water supply systemswhich is the point of control, discharge of hot water being by displacement as the coldsupply is admitted. The storage is always open to atmosphere, normally via a swivelspout, and, as the contents expand on heating, a drip may occur at the outlet. Toovercome the inconvenience of a swivel arm, the outlet may be piped to a specialcombined cold water inlet hot outlet tap as Figures 20.4(a) and (b).Over the recent years followingthe modification of the water bye laws the use of mainsfed unventilated water heaters have become popular. For local multi-point use, up to 3outlets, an electric storage heater of 10±15 litres capacity can be chosen.These units adopt the option of two methods to accommodate hot water expansion asshown in Figure 20.5. Option 1 is to allow the expanded heated water to reverse flow upthe cold feed supply pipe. Providingthat the nearest cold water draw off on the coldsupply pipe is a minimum of 2.8 m, for 10 litres storage capacity, and 4.2 m for 15 litrestorage capacity distance away from the heater.Option 2 is for the storage heater's cold feed supply adjacent to the heater to be fittedwith safety fittings incorporating a suitable sized expansion vessel, check valve, pressurerelief and a combined pressure and temperature relief valve on the heater.For multi-point local use, as might arise in each of a number of dispersed toilets in anoffice building, both gas fired and electrical storage heaters are available, on-peak and offpeakin the latter case. Packaged gas fired units are, in effect, cylinders incorporating astainless steel heat exchanger, direct fired as shown in Figure 20.6, and are insulated andcased in sheet metal. Such heaters come with various storage capacity and heat input andhave a typical water capacity of about 80 litres and a heat input of 8 kW: the products ofcombustion must be discharged to outside via a conventional or balanced flue arrangement.Diagram 1For inlet water pressure up to 4.1 bar (60 psi)Factory-fittedtemperature andpressure relief valveHot ColdTo drain (waste)Nearest colddraw-offCold water mainsService valve(fixed jumper or1/4 turn ball type)Multipoint10 litreor 1.5 litreheater2.8(10 l) or 4.2(15 l) metresis required for expansionin pipeworkDiagram 2For inlet water pressure up to 4.1 bar (60 psi) whereexpansion down the mains is not possibleFactory-fittedPressureNearest coldrelief Expansiontemperature and (expansion) vessel draw-offvaluepressure relief valveCheckHot ColdValveService valveT pieceCold water mainsMultipoint10 Litreor 1.5 litreheaterTo drain(waste)To drain(waste)Figure 20.5 Hot water expansion detail 10±15 litre heaters (Heatrae Sadia)