Book - School of Science and Technology

Feed cisterns 591the store is cold and will close progressively as the temperature rises. Since the temperaturedifference between the primary medium and the stored water will decrease as thevalve closes, sluggish recovery of the last few degrees of storage temperature is inevitable.Any form of on/off control is to be preferred for this particular application.Feed cisternsFor a conventional hot water supply system, a cold water storage cistern is required tosupply the hot water storage vessel and replace the water drawn off at taps, etc. A cisternis necessary also, as in the case of a heatingsystem, to accommodate the increase involume as the water content of the storage vessel and pipework is heated. This expansiontakes place via the feed pipe connectingthe cistern to the storage vessel and not, as somepublications assert through the vent pipe!To prevent the expanded hot water enteringthe cold water storage cistern with thepossible consequence of contamination by the legionella bacterium. Hospital technicalnotes advise that consideration should be given to installing a non-return valve on thecold feed adjacent to the hot water storage vessel, together with an increase in the openvent pipe size.The cold water feed pipe from the cistern (or pressurisingequipment as noted later) tothe storage vessel should, ideally, be so connected to the latter that mixing of the coldsupply with the hot water stored is prevented and disturbance to stratification of the latteris minimised. A number of devices has been produced from time to time to achieve thisend but with only limited success.In order that an adequate pressure be provided at the points of use, the cistern shouldbe sited as far above the highest such point as is practicable: in a building of anysignificant size, a purpose built tank room having easily cleaned surfaces, and not usedas an overflow store, is to be preferred.It is common practice to provide cold water storage for one day's interruption of watermains supply. However, when it is known that the mains supply is reliable and the watersupply company's reaction time for re-instatement of supply is short, considerationshould be given to reducing the water storage quantity.Circumstances sometimes arise where either a requirement for the limitation of buildingheight (e.g. a development near an airfield) precludes the provision of an elevated cisternor, more rarely, where the incomingmains pressure is not sufficient to produce anSecondaryconnectionsBleed valveSmall borespill pipeColdwatersupplyPrimaryconnectionsHot waterstorage vesselNon returnvalvePressure pump(in duplicate)StoragecisternFigure 20.26 Pump pressurization for hot water secondary supply