McQuay OM 920-1 83
Operator’s Guide
Exhaust Fan
A variable speed exhaust fan controlled by a VFD is provided for all Economizer units with
either Constant Volume or VAV Supply Fans and on 100% Outside Air units with VAV Supply
Fans. Either a constant volume exhaust fan or a variable speed exhaust fan controlled by a
VFD may be provided on 100% Outside Air units with Constant Volume Discharge Fans.
The exhaust fan is turned on when one of the following sets of conditions is true:
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 OA Position
• The Minimum OA Position is greater than 0%
• 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)
OR
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 OA Position
• The Minimum OA Position is greater than 0%
• The Exhaust Fan capacity is commanded to a value above the minimum value (Default =
5%) by a BAS for longer than the Minimum Exhaust Fan Start Time (Default = 120
seconds)
OR
• The Exhaust Fan is Constant Volume AND
• The Supply Fan has been commanded on for at least 4 seconds
The exhaust fan is turned off when one of the following sets of conditions is true:
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
• 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)
OR
Both of the following are True:
• The Exhaust Fan is controlled by a VFD
• The Exhaust Fan capacity is commanded to less than or equal to its minimum value (Default
= 5%) by the BAS for longer than the Minimum Exhaust Fan Stop Time (Default = 120
seconds)
OR
• The Supply Fan is Off
Whenever a variable speed exhaust fan is on, its capacity will be modulated using a VFD. The
speed of the fan will be either (1) modulated to maintain the building static pressure at a
desired value or (2) set at a fixed speed provided by a Building Automation System via a
network.
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