Book - School of Science and Technology

566 Hot water supply systemswith a depth of perhaps only 60±80 mm. Electrical loadings are high, as from 3 kW to10 kW, and since usage of water is immediately adjacent it follows that particular caremust be taken in providingelectrical protection and good earthingfacilities.In most applications instantaneous heaters, gas or electric, are designed to be connecteddirectly to the cold water main supply. Where supply via a cold water storage cistern isenvisaged, a static head of at least 10 m, equivalent to a pressure of about 98 kPa, isnecessary. The rate of water flow from all such heaters is restricted and, in the case of multipointunits, it should not be assumed that a good supply may be obtained from more thanone tap at a time. In the case of single point units, water control is normally on the inletconnection and outlet is via an inconvenient swivel spout, permanently open to atmosphere.Storage heaters (on-peak)For single point draw-off, local storage heaters are available in the capacity range ofabout 7±70 litres and are normally provided with 3 kW electrical heatingelements.Provided that the storage capacity is not more than 15 litres this type may be connecteddirectly to the cold water main supply. In this application, the simplest form of heater isthe free-outlet or non-pressure type illustrated in Figure 20.3. It is the cold water inletFigure 20.3 Non-pressure-type electric water heater (Heatrae Sadia)