Book - School of Science and Technology

642 Automatic controls and building management systemsof the upper floors, it might well be desirable to zone the upper floors separately from thelower ones. Further zoning may be required depending on the elevational treatment,leading to four or more zones for the building.With the increasing cost of energy/environmental awareness and the reducing cost ofterminal controls, such coarse control is rarely in practice used alone.Individual room controlPresent good practice is to be achieved by control of the heat emitter in each room. Inbuildings where there is significant variation in room loads, due perhaps to machinery oroccupancy levels, individual room control will be required to enable uniform temperaturesto be maintained. The alternative is, of course, to assume that the occupants cancontrol the output from heat emitters manually, dependent upon conditions at any onetime: in practice, however, this is seldom satisfactory because openable windows are mostoften used as the control device with the consequent waste of energy.A combination of compensator control plus individual room control has considerablemerit, since the zone compensation provides a limit to energy use (by reducing standinglosses from the distribution system) and the room controls give good control of comfortdue to a capability to off-set local heat gains. Clearly, with such a combination systemzoning may not be critical and larger areas may be served from each zone, resulting ininitial cost savings.Whereas compensated temperature circuits may be required for systems which serveradiators, radiant panels and natural convectors, it may be undesirable to serve heaterbatteries from these circuits as their load characteristics may differ from other loads. Inthe case of fan convectors, care has to be taken to avoid only mildly heated air feeling coolto occupants due to the effects of `wind chill'. A constant temperature circuit is requiredfor systems serving the primary heat source to hot water service storage calorifiers.BoilersControl of the output of the boiler is achieved by control of the burner unit (for gas or oilfired boilers). Burners may have, on/off, high/low or fully modulating burner control.Unless boilers are designed to work satisfactorily in a condensing mode, with thesubsequent formation of acids within the boiler, the output of the boiler should becontrolled to ensure that it runs at a constant high temperature. Heating water, to loadsrequiring lower temperature water can then be mixed as described earlier.Where boilers are designed to work satisfactorily should condensing occur with theresulting greater overall thermal efficiencies, then systems should be designed for thewater temperatures through the boiler to drop to as low as possible under changing loadconditions.The control of the boiler will usually be under the influence of a time switch and frostand optimum start control, as described previously. In other than very small systems,multiple boilers would be installed, which provide standby in case of failure of one unitand allow each unit, when in use, to operate closer to full output and peak efficiency,rather than on low load with consequent inefficiency.Boilers are commonly provided by the manufacturer complete with integral safety andthermostatic controls. Multiple boiler installations are piped in parallel and controlled insequence; more boilers operating as the load increases. Provision should be made for thesequence to be varied to give equal use of all units through the life of the plant. Although