Book - School of Science and Technology

616 Piping design for central hot water supply systemsventing and drainage are considered as the design is developed. It is worth emphasis thatthe presence of air, and the necessity to dispose of it, are perennial problems with any hotwater system. More practical difficulties arise in use from air venting, or the lack of it,than from almost any other cause.Various specific system arrangements can be identified and these are illustrated inFigure 21.10, the two fundamental sub-categories being up-feed and down-feed, withand without high level storage. It should be noted here, however, that although the use ofhigh level hot water stores may provide a solution to a variety of problems, access to themand maintenance of them may create other difficulties which must not be overlooked.Up-feed systemsFor the sake of easy comparison, item (a) in the figure shows a simple up-feed system asinstanced also by the example considered earlier in this chapter. This is probably the mostcommon arrangement but has a disadvantage in that the longest runs of pipe very oftenserve the draw-off points subject to the least pressure head. Air venting at the top of eachriser is sometimes possible but otherwise reliance must be placed upon release of airthrough the highest tap of each riser which, in that case, must be above the end of thecirculation, as shown.In order to cater for periods when draw-off is heavy and supplies to the upper floorsmight tend to be reduced, an up-feed system with subsidiary high level storage at the topof each riser may be used, as shown in item (b) of the diagram. The assumption is madethat a peak demand will not last for more than a minute or so and that the high levelstores will not be emptied before the demand ends. The sizing of outflow pipework forsuch a system may be simplified by ignoring the top one or two floors and selecting sizesfor the piping to serve the lower floors only.Down-feed systemsThe least satisfactory of such arrangements is as item (c) of the diagram, and arises frombuilding arrangements which require that both the outflow and the return main are runnot far below the cold water cistern. At times of heavy demand, the outflow main will beemptied of water, air will be admitted through the vent and a spluttering spasmodicsupply will result. This configuration should be avoided except in circumstances where itis possible to fit the cistern very much higher than the circulating mains.To overcome this problem in other circumstances, a method has been evolved wherebya single hot water store is introduced at the top of the rising outflow main, as shown initem (d) of the diagram. Here again, the assumption is made that supply to the upperfloors derives from the high level store, where the effective travel is no more than the pipelength between that store and the points of draw-off. For the lower floors, it is assumedthat supply will be assisted by flow through the return piping.Capacity of high level storesThis is necessarily based upon yet a further assumption, in this case by allowing for a 2±3minute supply to those draw-off points which have been taken to be fed from the highlevel store. Thus, if a supply of 2 litre/s has been assumed to be fed downwards, the storecapacity would be 360 litre. Since the high level stores are auxiliary to the main storagecylinder, the capacity of the latter should not be reduced.