Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSCEILINGTHERMOSTATCOILDIFFUSERFANMOTORDOWN-BLOWUNIT HEATERVALVEHOT WATEROR STEAMSUPPLYTHERMOSTATSome larger industrial gas-fired unit heaters have two stages:two-position (low fire) and modulating (to high fire). Theseunits are controlled by room thermostats designed to sequencethe two-position and modulating stages on a decrease in spacetemperature.Oil-fired unit heaters operate similarly to gas-fired unit heatersand include safety controls designed for oil burners.UNIT VENTILATORSFig. 18. Down-Blow Unit Heater.GAS- OR OIL-FIRED UNIT HEATERC3028The gas-fired unit heater (Fig. 19) is used when a central hotwater or steam system is not available. In a gas-fired unit heater,a gas burner heats a heat exchanger while a fan forces air acrossthe exchanger to warm the space. The two-position controlsystem comprises a thermostat, solenoid gas valve, safety cutoutswitch, pilot safety switch, and fan switch.FAN SWITCH(MAY BE COMBINATION FANAND LIMIT CONTROLLER)FLUES SHEATEXCHANGERSAFETYCUTOUTSWITCHTHERMOSTATGENERALA unit ventilator consists of dampers, a filter, a fan, a heatingand/or cooling coil, and the necessary controls (e.g., a valveand damper actuator). Unit ventilators use outdoor air,recirculated or return air from the space, or a mixture of both.Unit ventilators are designed for many capacities and are usedin areas where occupancy density indicates a need for controlledventilation (e.g., classrooms, conference rooms). A unitventilator control system varies heating, ventilating, and cooling(if available) while the fan runs continuously.Figure 20 shows a “blow-through” unit ventilator . Dampersat the bottom of the unit control the amounts of outdoor air andreturn air brought into the unit. The air passes through the filtersection and enters the fan section, where the fan blows it acrossthe coil.WALLDISCHARGEAIRFANBURNERGASSUPPLYGAS-FIREDUNIT HEATERSOLENOIDGAS VALVESPILOTSAFETYSWITCHC3024COILFANFig. 19. Gas-Fired Unit Heater.When space temperature falls below the thermostat setpoint,the thermostat contacts close to energize the gas valve. If thepilot safety switch indicates the pilot burner is lit, the gas valveenergizes. The burner warms the heat exchanger and the fanswitch turns on the fan. The burner operates until spacetemperature warms to setpoint. If unacceptably hightemperatures occur in the heater (e.g., during fan failure), thesafety cutout switch closes the solenoid gas valve. After theroom thermostat shuts off the burner, the fan continues to rununtil the heat exchanger cools.OUTDOORAIRFILTEROA, RADAMPERSRETURNAIRC3037Fig. 20. Blow-Through Unit Ventilator.ENGINEERING MANUAL OF AUTOMATIC CONTROL 387