Instruction manual

McQuay OM 920 67
Operator’s Guide
Heat Cool
When the Control Mode is set to “Heat/Cool,” both cooling and heating operation are allowed
to operate as required to maintain the cooling and heating set points.
Auto
When the Control Mode is set to “Auto,” the heat/cool, cool only, heat only, and fan only
decision is determined by the network application mode parameter, which is set via a network
signal as described below. The NetApp Mode parameter has no effect on unit operation unless
the Control Mode is set to “Auto.”
Determining Cooling Status
Clg Status is a status item which indicates whether or not mechanical cooling is currently
allowed. If cooling is disabled, the reason is indicated. The current Cooling Status can be
viewed via the following path: Standard Menu/System/Clg Status.
The following are descriptions of cooling status states.
Enabled
Mechanical cooling is enabled if all the following are true:
• Cooling capability is provided
• Control mode is not set via the System Menu to fan only or heat only
• Control mode is set via the System Menu to auto and not disabled via a network command
• The outdoor air temperature (OAT) on air cooled units or the entering water temperature
(EWT) on water cooled units is high enough for operation
• Compressor operation is not disabled by an alarm condition
None
Cooling capability is not provided.
Off Ambient
The outdoor air temperature (OAT) on air cooled units or the entering water temperature
(EWT) on water cooled units is too low for operation.
The OAT becomes too low for operation when it drops below the OAT cooling lockout setting.
OAT becomes high enough for operation when it rises above the OAT cooling lockout setting
by more than 2°F (adjustable - OAT Diff).
Note – The OAT cooling lockout cannot be set lower than 40°F when the unit is equipped with an
evaporative condenser and should not be set lower than 50°F unless the unit is equipped
with low ambient capability. The EWT becomes too low for operation when it drops below
the minimum EWT set point. EWT becomes high enough for operation when it rises above
the minimum EWT set point by more than 2°F (adjustable - Econo Diff).
Off Alarm
The compressor operation is disabled by an alarm condition. This happens when either all
circuits are disabled by either a high or low pressure alarm, when applicable the water flow
switch alarm is active, when applicable sump pump fail alarm is active or applicable water
regulating valve alarm is active.
Off Network
Control mode is set via the keypad to auto and cooling is disabled via a network command.
Off Manual
Control mode is set to Fan Only or Heat Only via the System menu.