Book - School of Science and Technology

Chapter 20Hot water supply systemsThe provision of hot water for baths, showers, basins, sinks and other points of draw-offmay be dealt with either locally or from a central system. In this respect it echoes the`direct' and `indirect' methods of providingheatingservice.Local systems use either gas or electricity as a heat source, the heater being placed nearto the point of consumption. In the case of a central system, the water is heated at someconvenient position, which may be relatively remote from the point of consumption, anddistributed as may be appropriate by pipework. The heat supply to a central system maybe either solid, liquid or gaseous fuel or electrical energy.Pipework lossesIn order to provide a satisfactory level of service to the user, a central system of any sizemust be arranged to circulate hot water through the distribution pipework. This circulation,by thermo-syphon or by pump, will continue winter and summer although meanswill normally be provided to stop it when the buildingis unoccupied. Heat will be lostfrom this circulatingpipework, although insulated, and this loss will persist whether ornot any water is drawn off. The ratio between the energy expended in heating up the watersupply from cold and that lost through the circulating system represents the efficiency ofthe system. It is of prime importance, therefore, that the distribution system be kept ascompact as is practicable.Choice of systemFor a single isolated draw-off point, some type of local heater would be the obviouschoice and, equally, a central system would be preferred for a hotel havingbathroomsclosely planned both back-to-back and vertically on several floors. As a generality, mostinstitutional buildings, such as hotels, hospitals and educational establishments (whetherplanned for efficiency or not) are likely to be best served by a central system of some sortbecause this will, perforce, make available a bulk reserve supply of hot water to deal withpeak demands and will at the same time avoid dispersal of those scarce maintenanceactivities which may be available.In the case of many other applications, however, the choice is less easy to make and asingle building could quite well be served by a mixture of systems. As a simple example,consider a centrally heated eight-storey building consisting of a dozen lock-up shops atground floor level with six floors of offices above, all occupied by a single tenant, and alarge penthouse flat leased separately.