Instruction manual

McQuay OM 920 115
Operator’s Guide
Supply Fan Capacity Control (VAV)
The speed of a modulating Supply Fan is controlled by an analog signal provided to the VFD.
Supply Fan Capacity Control for a modulating fan is controlled to either:
• Maintain the duct static pressure at a desired value
OR
• Maintain a fixed speed based on a signal provided via a network.
The choice of control method, SF Cap Ctrl, may be set to Duct Press or Speed via the keypad.
After the Supply Fan is started, a speed signal of 25% is sent to the variable frequency drive
for the DSPCtrlDelay (Default=30 seconds) and the VFD will increase its speed to the larger
of 25% or it's internally programmed minimum speed. Control reverts to either duct pressure
or speed after the fan has been on for the duration of the DSPCtrlDelay time. The VFD speed
is not controlled below the minimum SAF speed setting (default 25%) while the fan is
operating.
Duct Static Pressure Control
The supply air fan speed is controlled by a VFD. The control parameter for the fan speed is the
duct static pressure setpoint. If the duct static pressure is below the duct static pressure
setpoint by more than ½ the deadband, the fan speed will increase. Likewise if the duct static
pressure is above the duct static pressure setpoint by more than ½ the deadband the fan speed
will decrease. Example - if the duct static pressure setpoint is 1.2" and the deadband is 0.1",
the duct static pressure must reach 1.14 before the fan will increase in speed. The Duct Static
Pressure setpoint may be set through the keypad or via a network signal. The active setpoint is
changed whenever either of these values changes so it equals whichever value was changed
most recently.
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.
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)