Book - School of Science and Technology

154 Indirect heating systemsColdwaterfeedVent pipeIndirectcylinderRadiators inbedrooms15 mmBedroom15 mm20 mmDiningroomLivingroomGravitycirculationto HWSBoilerPump20 mmKitchenHall15 mmFigure 6.8 Small-bore hot water circuits (generally obsolete)While the principles applied follow those noted above, small-bore systems, as tentativelyintroduced for commercial premises some 50 years ago, were later adapted to domesticuse. The number of radiators which may be carried by a single circuit is limited by pumpcapacity and by the size of pipe used, commonly 15 mm copper. Where the system is tosupply a large number of radiators, further ladder loops are added in parallel with thefirst, as shown in Figure 6.8.Single-pipe circuits are generally confined to conventional low temperature hot watersystems since the temperature decay would not be acceptable if the water were only warmat the point of origin. They have been applied to medium temperature systems using anunorthodox arrangement as referred to later under the hybrid system heading.The two-pipe circuitThis arrangement has, in most respects, come to supersede the single-pipe configurationand has an inherent logic as far as parallel circuits are concerned, as shown in Figure6.9. Flow and return mains originate from the boiler plant and each main or sub-circuitconsists of branches from them. Each branch conveys an appropriate quantity of waterto and from whatever heat emitting terminals are connected to it. In an ideal world, allcirculating pipework would be insulated perfectly, and the water inlet temperature ateach terminal would be exactly the same as that leaving the boiler. Similarly, thetemperature of water returned to the boiler would be exactly the same as that leavingeach terminal. In practice, heat output from the pipework ± heat loss in this context ±will reduce the temperature at the various inlets to a level which will vary, roughly,according to how distant each is from the boiler. Likewise, the water returned to theboiler will be at a lower temperature than that at which it leaves the outlets of thevarious terminals.In terms of hydraulic balance, systems arranged as shown suffer from a number ofdifficulties. It will be obvious from the diagram that the most distant heat emitter isdisadvantaged, by comparison with that nearest to the boiler, in this respect. Theproblem will be reduced if either the heat emitters, or the final sub-circuit pipework