32
McQuay OM 751
Description of Operation
Unoccupied Operation
During unoccupied mode, the IA fan typically remains off and cycles with calls for heating
and cooling.
Cycle Fan
The UVC is provided with a Fan Cycling Configuration variable that can be used to force the
IA fan to cycle with calls for heating and cooling during the occupied, standby, and bypass
occupancy modes. When the fan is off, the OA damper is closed. McQuay recommends using
this feature only when it is acceptable that normal ventilation is not required.
When the IA fan is set to cycle, the UVC is configured to continue fan operation for a time
period after heating or cooling is complete.
Off Delay
When the UVC is placed into off mode or shutdown mode, the UVC is configured to continue
fan operation for a short time period and then shutdown.
Outdoor Air Damper Operation
The UVC is configured for an OA damper operated by a floating-point actuator. The OA
damper actuator contains a spring that ensures the OA damper is closed upon loss of power.
The floating-point actuator is driven by the UVC using two binary (Triac) outputs. The OA
damper typically is open to the current minimum position during the occupied and bypass
occupancy modes and closed during the unoccupied and standby occupancy modes.
A Triac output is best tested under load using a 24 V relay for verification. To verify:
1
Put a relay across the Triac outputs.
2
Cycle the power.
3
Verify the relay’s closed contacts during calibration.
Minimum Position
The UVC is configured to maintain three OA damper minimum positions based on the
operation of the IAF fan. This allows each unit to be field configured to provide the amount of
fresh air required to the space at each of the three IA fan speeds.
Table 17: Default OA damper minimum positions
Note –
If the CO
2
Demand Controlled Ventilation (DCV) option is used, the UVC only uses the IA
fan high speed OA damper minimum position regardless of fan speed. The DCV function
adjusts the OA damper above this minimum as needed to maintain CO
2
set point.
Economizer Operation
The economizer function is used by the UVC to determine if the OA is adequate for
economizer (primary) cooling. When both the economizer and mechanical cooling are
available, the economizer is used as primary cooling and the UVC adds mechanical cooling
only if the economizer is not adequate to meet the current cooling load (e.g., the OA damper
reaches 100% and cooling is still required).
The UVC supports three economizer functions:
•
Basic (default)—Temperature Comparison Economizer
IAF speed
Without CO
2
With CO
2
High
20%
5%
Medium
25%
5%
Low
30%
5%
