118 McQuay OM 920-1
Operator’s Guide
Exhaust Fan - Building Static Pressure Control
An Exhaust fan driven by a variable frequency drive is started based on building static
pressure. A constant volume Exhaust Fan is turned on through the same output as the Supply
Fan. An external Fan Delay Relay is used to provide a delay between startups if required.
A VAV exhaust fan is turned on when all four of the following are true:
• The Exhaust Fan is controlled by a VFD
• The OA Dampers are at least open to the Minimum Exhaust OA Position (default 5%)
• The SAF capacity is above the Minimum Exhaust SAF capacity (default 10%)
• The building static pressure is above the building static pressure setpoint by more than the
deadband for longer than the Minimum Exhaust Fan Start Time (Default = 120 seconds)
A VAV exhaust fan is turned off when the Supply Fan is turned off or when all three of the
following are true:
• The Exhaust Fan is controlled by a VFD
• The building static pressure is below the building static pressure setpoint by more than the
deadband OR the OA Dampers are open less than the Minimum Exhaust OA Position
(default 5%). OR the SAF capacity is below the Minimum Exhaust SAF capacity (default
10%)
• The Exhaust Fan capacity is at or below its minimum value (Default = 5%) for longer than
the Min Exhaust Fan Stop Time (Default = 120 seconds)
Exhaust Fan - Speed Control
When speed control is selected, the fan operates at the larger of its minimum speed or a value
provided via a connected network or the keypad/display. This value reverts to zero when the
unit is off and remains at that value after startup until a non zero value is written via the
network or the keypad/display.
The exhaust fan is turned on when all four of the following are true:
• The Exhaust Fan is controlled by a VFD
• The OA Dampers are at least open to the Minimum Exhaust OA Position (default 5%)
• The SAF capacity is above the Minimum Exhaust SAF capacity (default 10%)
• The Remote Exhaust Fan capacity is commanded to a value above the minimum value
(Default = 5%) by a BAS for longer than the Minimum Exhaust Start Time (Default = 120
seconds)
The exhaust fan is turned off when the Supply Fan is turned off or when both of the following
are true:
• The Exhaust Fan is controlled by a VFD
• The Remote Exhaust Fan capacity is commanded to less than or equal to its minimum value
(Default = 5%) for longer than the Minimum Exhaust Stop Time (Default = 120 seconds)
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