Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSDigital Control of the two pipe heating/cooling fan coil unitshould above could be as Figure 33. The fans could be batchscheduled via one or more optimum start programs. Wallmodules could have a cooling setpoint knob (with softwarebounds) and a heating deadband. The entering watertemperature (from a plant sensor) and the outside air temperatureare shown for operator information. The heating/cooling plantcould be controlled similarly to that described in the paragraphHot Water Plant Considerations.The four-pipe heating/cooling fan coil unit control would beidentical except the valves would be sequenced through thedeadband.FOUR-PIPE HEATING/COOLING, SPLIT COILYear-round heating and cooling is possible at each unit with afour-pipe heating/cooling, split coil application (Fig. 34). A fancoil unit in one zone can cool while a unit in another zone heats.The control system comprises a room thermostat connected totwo valves, one controlling hot water flow and the other controllingchilled water flow. Valves can be two-way or three-way.When space temperature is below the thermostat setpoint,the hot water supply valve modulates open and hot water flowsthrough the heating coil. As space temperature increases, thehot water valve modulates closed. As space temperature risesabove setpoint, the thermostat signal starts to open the chilledwater valve. The room thermostat throttling range and valveactuator movement should be selected to provide a “deadband”between heating and cooling so that both valves are closed whenspace temperature is satisfied. The fan can be operated by alocal fan switch or a central time clock.FOUR-PIPE HEATING/COOLING, SINGLE COILFigure 35 shows the single-coil method for four-pipe heating/cooling. A thermostat controls two three-way valves to regulatethe flow of chilled or hot water into a single coil.3-WAYMIXINGVALVEDISCHARGEAIRTHERMOSTATCHILLEDWATERRETURNSUPPLYHOTWATER3-WAYMIXINGVALVE3-WAY MIXINGVALVERETURNSUPPLYTIMECLOCKPOWERDISCHARGEAIRCOOLINGCOILFANHEATINGCOILTHERMOSTATCHILLEDWATERSUPPLYRETURNHOTWATERRETURNSUPPLY3-WAYDIVERTINGVALVEFANTIME SWITCHCLOCKPOWERFANHEATING/COOLINGCOILFILTERRETURNAIRC3031Fig. 35. Four-Pipe Heating/Cooling Fan Coil Unit.FILTERFig. 34. Four-Pipe Heating/Cooling FanCoil Unit with a Split Coil.RETURNAIRC3030-1This system requires special three-way sequencing valves thatshut off all flow at the middle of the thermostat throttling range.On a fall in space temperature, with the chilled water supply andreturn ports close, the hot water return port opens 100 percent,and the supply valve hot water port modulates the hot water flowto maintain space temperature. As space temperature rises, thehot water supply and return ports close and flow through the coilis stopped. When space temperature rises above the thermostatsetpoint, the chilled water return port opens to 100 percent flowand the supply port modulates chilled water flow through the coilto maintain the space temperature. Both the chilled water and hotwater supply ports are closed in a deadband between heating andcooling operations. The control is similar to having sequencedmodulating two-way valves on the HW and CW supplies andsequenced two-position two-way valves on the returns. The fancan be operated by a local fan switch or a central time clock.ENGINEERING MANUAL OF AUTOMATIC CONTROL 396