Book - School of Science and Technology

572 Hot water supply systemsThe megaflo internal air gap systemOutletOutletFloatingbaffleHeatingelementWater(b)Before expansionColdAfter expansionHotFigure 20.10 (b) Integral air gap system detail (Heatrae Sadia)Unventilated models are also available and can be considered where the cold watersupply pressure and flow rate are suitable for the system. As with all unventilated waterheaters, hot water expansion is to be catered for. Unlike the small 10±15 litre multi-pointspreviously mentioned, only one method of expansion is generally used. This being theincorporation of an expansion vessel on the cold feed with the associated safety fittingasidentified in Option 2 for unventilated multi-points.However, variation to the expansion vessel is the incorporation of an internal air gapwithin the vessel itself utilising a float baffle to maintain the integrity of the air gap asshown in Figure 20.10(a) and 20.10(b).Local storage heaters are well insulated and are normally fitted with an enamelledjacket ready for mountingin an exposed position. In comparison with an instantaneousheater, at least ten times the space is needed for the same level of service but if sited in acupboard the difference may not be relevant. A suitably sized storage heater does howeveroffer the advantage of using off-peak current which is not possible with the alternativetype.To make use of the cheap off-peak rate, a storage unit should be fitted with two sets ofimmersion heater. One set, fitted near the bottom of the vessel, will charge the storeovernight and the other, fitted near the top, will be arranged to heat a smaller volume ofwater at the end of the day when the overnight charge has been exhausted. On a domesticscale, an existing120 litre cylinder (900 mm 450 mm diameter) might be converted byusing a dual heater as shown in Figure 20.11(a) but the arrangement as Figure 20.11(b) isto be preferred. Better still, for a new installation, would be a 210 litre cylinder(1450 mm 450 mm diameter) as in Figure 20.11(c). Based on published data, Table20.1 shows how energy consumption would arise from the alternative arrangements.Central systemsA central system will usually consist of a boiler or water heater in some form, coupled bycirculatingpipingto a storage vessel or vessels. The combination of the two will be soproportioned as to provide adequate service to the draw-off points to match the pre-