Temspec Unit Ventilator IOM 2023-07-05
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OPERATION
Typical Modes of 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.
Unoccupied Heating/Cooling
During Unoccupied heating/ cooling the outdoor air damper, wheel air damper (if applicable) and
powered exhaust damper (if applicable) are fully closed. The powered exhaust fan (if applicable) and
energy recovery wheel (if applicable) are off. The space is maintained at a setback temperature by
modulating the control valve actuator or face and bypass damper actuator or outdoor condensing unit
or electric heat as needed. The supply fan should only run when heating or cooling is required.
Occupied Heating/ Cooling
During Occupied heating/ cooling for standard Unit Ventilators (VUD, VUF, VDF, HCD) the outdoor
air damper will modulate to the minimum required position. If the unit has powered exhaust, the
powered exhaust damper will open, and the powered exhaust fan will turn on. The space temperature
is maintained by modulating the control valve actuator or face and bypass damper actuator or outdoor
condensing unit or electric heat as needed. The supply fan will run at all times.
If the unit is equipped with an Energy Recovery Wheel, the wheel air damper will open, the wheel will
start spinning and the powered exhaust fan will start. The economizer damper will remain closed. The
space temperature is maintained by modulating the control valve actuator or electric heat as needed.
The supply fan will run at all times.
Economizer (up to 100% outdoor air)
Economizer mode is activated when the controller compares the Outdoor Air Enthalpy to the Mixed
Air Temperature and determines that the outdoor air is cool and dry enough to properly cool the
space. During this mode the outdoor air damper and return air damper will modulate to maintain the
space temperature.
If the unit is equipped with an Energy Recover Wheel, the wheel will stop, and the wheel air damper
will close. When the economizer damper opens, the exhaust air damper will open, and the powered
exhaust fan will run at high speed for 100% powered exhaust operation.
Dehumidifying Mode (VGB/HGB Series)
During occupied cooling, the Dehumidifying AHU can enter a dehumidifying mode. In this mode
(normally when the outdoor air is above 55°F) water is diverted to the outdoor coil to cool and remove
moisture from the entering outdoor air.
Freeze Protection
For units with a water coil, some type of freeze protection is usually implemented. This can be by
either a low limit temperature control (auto reset or manual reset) or by using an air sensor and
programming from the controller. When the 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. The standard hot water actuator is fail
open.
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