Book - School of Science and Technology

Central systems 577To HWScylinder(gravity)Outdoor thermostatFlow thermostatFlueTo heating circuitsCalibratorPumpMixingvalveBoiler(from rear)Injector teeFigure 20.15 Combined heatingand hot water supply systemCombination systemsReference to boilers so described has been made in Chapter 10 but it is the provision ofhot water output to a combined system which is considered here. Where both heatingandhot water are supplied from a single boiler, it is unlikely that both systems will requiretheir full share of output simultaneously, even in mid-winter. For the remainder of theyear, as stated above, the boiler will be too large, with attendant penalties.Despite this situation, however, interaction between the two systems may be such thatresponse by the boiler to demand from either will appear sluggish. The introduction ofsome form of thermal store between the boiler and the two varyingdemands should thusimprove not only operatingefficiency but also response rate. Research has producedresults at domestic level which show promise, but comprehensive application data forsystems of any significant size are not available.The design concept used for one make of domestic unit is illustrated in Figure 20.16, theheat source being either a gas fired circulator of the generic type shown in Figure 20.7 or aconventional boiler. The output of the heat source is pump-circulated to the store whichhas a capacity, varied to suit the size of house served, of between 120 and 200 litre. Fromthe store, a quite separate circulation is directed to the heatingsystem and this feature isturned to advantage when, following a night-time shut down, restarting the heating pumpbrings the radiators to store temperature in a matter of minutes.The two coils immersed within the store have external fins to aid heat transfer and areconnected in series. Since their water content is small, they are permitted to be suppliedwith a cold water feed from the incoming main supply with the result that outflow pressure ismore than adequate to serve a shower head. Provision is made for the hot water dischargeto be blended with cold water so that the final supply is at a safe temperature for children.In effect, the store acts as a batch-production instantaneous heater able to provide a hot