Book - School of Science and Technology

648 Automatic controls and building management systems(a) Heating mode–+In heating mode the cooling damper is closed andair passes through the heating coil. Cooling pick-upis minimised by the cooling duct and damper design.Discharge(b) Bypass mode (No load)Heating coilFan–+In bypass mode both the heating and cooling dampersare closed, air passes through the filter and betweenthe coils. Any heating / cooling pick-up is minimised bythe damper and air flow design.Cooling coilLinear dampers(c) Cooling mode–+In cooling mode the heating damper is closed,directing air through the cooling coil. Heat pick-upis minimised by the damper design.Figure 22.29 Principles of operation for an air side control fan coil unit (Lennox)In the simple form of the true system, control of space conditions and in particularthose in internal zones of deep plan buildings is achieved by variation in the quantity ofair supplied rather than by any changes in temperature. This control, by room thermostatoperation upon electrical or pneumatic devices, acts to reduce the volume of conditionedair delivered either to a zone or an individual room where the load is at less than thedesign peak. Such devices may be integral to the actual terminal diffusers or incorporatedin regulators serving a number of such diffusers.Good practice suggests that the volume of air supplied to meet the designed full loadcannot be reduced under control by more than about 40% if room distribution difficultiesare to be avoided, unless variable geometry supply diffusers are used to maintain airvelocities. In consequence, control by terminal reheat may be necessary should localdemand for cooling fall below that level: hence, room or zone terminals may be suppliedfrom a dual-duct system or be themselves provided with reheat batteries supplied by hotwater or, rarely, with electrical resistance heaters. The degree of wasteful cooling and then