Operation and Maint.Manual VUD VUF VDF HCD SeriesRev6.doc
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OPERATION
The following are typical modes of operation for a classroom unit ventilator. Please refer to the
manual provided by the control’s contractor for a more specific controls sequence.
1. Unoccupied Condition
During the “unoccupied heat” mode (night set-back) space temperature is maintained by a
signal from the thermostat/controller to either the modulating control valve, modulating face and
bypass damper actuator or electric coil triac. The powered exhaust damper (if applicable) and
the outdoor air damper are fully closed, and the return air damper is fully open. The supply air
fan operates on call from the thermostat/controller.
2. Occupied Condition
The unit ventilator is switched to “occupied heat” by the thermostat/controller. In this mode a
signal is sent from the thermostat/controller to either the modulating control valve, modulating
face and bypass damper actuator or electric coil triac to maintain room temperature at set point.
The outdoor air damper is held at a minimum position in the heating mode. The supply air fan
runs continuously.
3. Economizer (up to 100% outdoor air)
The first stage of cooling is the “economizer” mode during which all stages of heating are off.
The outdoor air and the return air dampers modulate to maintain the room at the “economizer”
set point. If the mixed air falls to a programmed temperature (usually 52°F) the outdoor air
damper will modulate towards closed until the mixed air temperature rises again (typically to
55°F). in this “free cooling” mode the ability of the unit ventilator to provide sufficient cooling is
limited only by the outdoor air temperature and the total C.F.M. rating of the unit.
4. Cooling
The unit can be equipped with a split system DX evaporator coil that would work in conjunction
with a remote condensing unit (provided by others) or can be equipped with a chilled water coil.
The system maintains the “set temperature plus 5°F” and the cooling is cycled on call from the
thermostat/controller. In this mode the outdoor air damper will return to minimum position.
During night set-back the mechanical cooling and economizer should be locked out.
5. Freeze Protection
For units with a steam or hot water coil, some type of freeze protection is usually implemented.
This can be by either a low limit temperature control (autoreset or manual reset) or by using a
supply air sensor and programming from the controller. When a mixed air temperature is
determined to be too low, then the outdoor air damper will close, and the control valve will fully
open. In some cases, the fan will also shut down until the temperature returns to normal levels.
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