Book - School of Science and Technology

618 Piping design for central hot water supply systemsdesign solution is, as for other building services, the introduction of intermediate plantrooms at intervals over the height of the building. Plant for the hot water system mightthus be disposed as shown in Figure 21.11.Unvented systemsSome of the comments made under previous headings will not apply to systems fed from amains pressure service pipe and, similarly, some of the problems associated with thetraditional design approach will no longer arise. These, however, are succeeded by otherdifficulties particular to the unvented system.The most important side effect, in the context of piping design, is the necessity to ensurethat the various safety devices fitted to the storage vessel will remain `drowned' at alltimes, that is to say that they must always remain responsive to water temperature. It is, inconsequence, necessary to take particular precautions to prevent a hot water cylinderfrom being drained either by leakage or by draw-off from a circulation below the level ofthe vessel.The flow connection from the cylinder, being at the top, does not commonly presentany difficulties and drainage via the cold water feed may be prevented by arranging for itto approach the cylinder at a level above the various sensors and fitting a vacuum reliefvalve. Thus, the residual problem rests with the secondary return connection and this maybe solved by either of the two methods shown in Figure 21.12.Circulating pumpsIn instances where a pump is required to assist in the secondary circulation it may befitted, as was the case for a heating circuit pump, in either the flow or return pipework,Figure 21.13(a) and (b), to achieve the required hydraulic performance, however, thelatter is recommended.With the circulation pump positioned on the flow, the quantity of water flow to servethe draw off at peak simultaneous demand is generally many times greater than thatAnti-vacuumvalvesSecondaryflowAnti-vacuumvalveSecondaryflowLowreturnconn.Highreturnconn.(a)(b)Figure 21.12 Unvented systems: piping arrangements to prevent drainage