A unitventilator is a fan system dedicated to one area. They normally run continuously during occupied mode providing ventilation, heating and cooling. On day mode PE1 and PE2 close their contacts causing the fan to run, allowing a signal through the solenoid valve to modulate the fresh air damper and the heating control valve. The signal from the thermostat goes through the low limit to both the mixing damper and the control valve. As the thermostat signal starts to rise, first the split-range, fresh air motor drives from 3# to 6#, opening to the minimum ventilation requirement; then the heating valve drives from full heating to no heating. If the thermostat requires cooling the fresh air motor will restart to drive from 9# to 12# attaining 100% outside air. The low limit will over-ride the thermostat’s demand for cooling if the supply air temperature decreases to the 55°F setting of the low limit. On night mode PE1 and PE2 open their contacts and the unit shuts down. The thermostat changes to the night set point. When the room cools to the night setting the unit is duty cycled via PE3. The fresh air damper is completely closed and the heating valve is open on night mode. The thermostat may have an indexing switch, allowing manual day mode operation of the room at times that are normally night mode. There are many control arrangements used on unitventilators; however, each should address safety, comfort and energy requirements. UNITVENTILATOR CONTROL VALVE LOW LIMIT 55°F FAN PE3 EP1 NO VALVE DAY/NIGHT 4# TO 8# THERMOSTAT COIL SPLIT-RANGE ACTUATOR 3# TO 6# THEN 9# TO 12# NO NC PE1 PE2 RETURN OUTDOOR AIR AIR V PE1 EP1 OFF ON PE2 M PE3 7.60 D/N
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