Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSTWO-PIPE HEATING/COOLINGThe flow of medium through a fan coil unit can be controlledin two ways. One method uses a two-way valve to control theflow of steam or hot or chilled water. The second method, shownin Figure 31, uses a three-way valve to control hot or chilledwater flow.DISCHARGEAIRHOTWATERORCHILLEDWATERRETURNSUPPLY3-WAYMIXINGVALVEPIPE-MOUNTEDSENSORDISCHARGEAIRHEATING ORCOOLING COILTHERMOSTATHOTWATERRETURN3-WAYMIXINGVALVEHEATINGCOILTHERMOSTATTIMECLOCKPOWERFANHOTWATERSUPPLYFANFILTERRETURNAIRC3032FANSWITCHPOWERFILTERRETURNAIRC3033Fig. 31. Two-Pipe Heating Fan Coil UnitUsing Three-Way Mixing Valve.The three-way valve provides constant flow in the supplyand return lines and minimizes pressure fluctuations at the valve.A modulating thermostat controls the three-way valve toregulate water flow through the coil. In a heating application,when the thermostat senses space temperature below thethermostat setpoint, the valve is positioned to allow full flowthrough the coil. As space temperature rises, the thermostatcontrol signal modulates the valve to decrease the amount offlow through the coil and increase the bypass flow around thecoil. At the upper end of the thermostat throttling range, thevalve is in the coil bypass position, eliminating flow throughthe coil. The fan runs continuously or is stopped as spacetemperature rises above setpoint. Cooling-only systems operatein a similar way.TWO-PIPE HEATING/COOLING, SINGLE COILFigure 32 shows a two-pipe fan coil application that uses asingle coil for heating and cooling with seasonal changeover.Control for this application requires a heating/cooling roomthermostat that reverses its action from a remote heating/coolingchangeover signal. One method of automatic changeover is toinstall a pipe-mounted sensor that switches thermostat actionwhen it senses a change in supply medium between hot andchilled water. Because it cannot offer simultaneous heating andcooling capability, this system can cause problems duringintermediate seasons when hot water is in the system andcooling is needed, or vice versa.Fig. 32. Two-Pipe Heating/Cooling Fan Coil Unit.The room thermostat opens the coil valve on a fall in spacetemperature during the heating cycle. In the cooling cycle,chilled water in the pipes causes the pipe-mounted sensor toreverse the thermostat action and the thermostat opens the valveas space temperature rises. The fan can be operated from a localswitch or a central time clock.The surface area of the coil is large to accommodate coolingrequirements. Therefore, when a single coil is used for bothheating and cooling, the hot water supply temperature shouldbe lower. Typical hot water supply temperatures range between90 and 140F.FANFILTERRA7746ONWALL MODULE7575CONTROLPROGRAM 2.5OUTDOOR AIR55Fig. 33. Digitally Controlled Two-PipeHeating/Cooling Fan Coil Unit.TEMPERATURECOOLINGSETPOINTHEATINGDEADBANDM10533ENGINEERING MANUAL OF AUTOMATIC CONTROL 395