Instruction manual

McQuay OM 920 63
Operator’s Guide
In the Start Up state, the optional Fan Operation input is turned on to allow shut off dampers
to be opened before any the Supply Fan is turned on. A Fan Op/VAV output may be selected as
either the Fan Operation output or the VAV output via the keypad. The parameter is adjustable
via the following path: Extended Menus/Unit Setup/VAV/FanOut. The outdoor dampers
remain closed.
The Supply Fan is turned on when the unit enters the Recirculation state. The supply fan in
VAV units is controlled as described in the Supply Fan Capacity Control section. The outdoor
dampers remain closed.
A separate Morning Warm-up State is not provided, but an edited ZeroOATime is used to keep
the Outside Air damper closed when the unit first starts. The parameter is adjustable via the
following path: Extended Menus/Timer Settings/Zero OA Time.
The Minimum OA Position is set to zero as long as the as the fan has been on for less than the
ZeroOATime. This allows the Return Air type units to cool down the space with mechanical
cooling or to warm up the space with the dampers closed. If the ZeroOATime is set correctly,
the OA dampers will be open only during occupied periods. When Optimum Start is used Zero
OA Time is set equal to the time to occupancy when the unit starts so that the OA dampers
will open at occupancy time.
Neither heating nor cooling is provided when the unit is in the Fan Only state. The outdoor
dampers are opened to the minimum position in this state when the Fan On Time exceeds the
Zero OA Time.
In the other four states, temperature is controlled as describe in the appropriate sections of this
document. These states are Minimum DAT, Heating, Economizer, and Cooling. The outdoor
dampers are opened to at least the minimum position in these states when the Fan On Time
exceeds the Zero OA Time.
Off Operating State
In the OFF operating state there are five different reasons that explain the reason for the Off
status. If the controller is set to off for any reason the unit is shut down. The fans are off, the
outside air dampers are closed and any VFD's are driven to 0%. Cooling and heating are
disabled.
OFF Manual
The unit operating state is Off Manual when the Control Mode is set to OFF via the keypad.
The Control Mode can only be changed via the System menu.
OFF Manual Control
The unit operating state is Off manual control when the controller is set to manual control via
the Manual Control menu.
OFF Network
The unit operating state is Off Network when the control mode is set to Auto via the keypad
System menu, and the network Net App Mode is set to off.
OFF Alarm
The unit operating state is Off Alarm when an active alarm of the “fault” type has the unit
shutdown.