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VCCX2 Controller Technical Guide
Mechanical Failure Alarms
Mechanical Cooling Failure
Fixed Stage Cooling:
The Mechanical Cooling Failure Alarm is
generated if the Supply Air Temperature fails to drop 5ºF (within
a user-adjustable time period) from the temperature the supply
air was at when the cooling was activated and the Supply Air
Temperature is not within 5ºF of the setpoint. The alarm will be
cleared when the Supply Air Temperature drops the 5ºF and sets
the failure timer back to zero.
Variable Capacity Compressors (Utilizing RSMs):
This alarm
is not generated; instead, the user has to monitor the Compressor
Module Alarms for Mechanical Cooling issues.
This alarm is also not generated if using a modulating chilled
water valve.
Mechanical Heating Failure
For units with Fixed Stage Heating, the Mechanical Heating
Failure Alarm is generated if the Supply Air Temperature
fails to rise 5ºF (within a user-adjustable time period) from
the temperature the supply air was at when the heating was
activated and the Supply Air Temperature is not within 5 degrees
of Setpoint. The alarm will be cleared when the Supply Air
Temperature rises the 5ºF and sets the failure timer back to zero.
If using the PREHEAT-X, any alarm on the PREHEAT-X will
generate this alarm, also. This alarm is not generated if using a
modulating hot water valve.
Supply Fan Proof of Flow Interlock Alarm
A Proof of Flow Switch provides a 24 VAC wet contact closure
when the Supply Fan is operating. If this contact opens while the
fan is being called to run, all heating and cooling is disabled, the
Outdoor Air Damper closes (if it is not configured for Outdoor
Temperature Control or Space Control with high percentage
outdoor air), and a Fan Proving Alarm is generated. Fan Proving
needs to be configured for this alarm to occur. There is a 30
second delay for this alarm.
Return/Exhaust Fan Proving Alarm
There are configuration options for Return Fan Proving and
Exhaust Fan Proving Alarms. If the unit is configured for Return
Fan Proving, it is assumed that the Return Fan is wired in parallel
with the Supply Fan Relay. In this case, if the Supply Fan Relay
is energized, but the Return/Exhaust Fan Proving Binary Input
on the VCC-X EM1 Module does not see 24 VAC, a Return/
Exhaust Fan Proving Alarm will occur. If the unit is configured
for Building Pressure Control of the Exhaust Fan, if the Exhaust
relay is energized, but the Return/Exhaust Fan Proving binary
input on the VCC-X EM1 Module does not see 24 VAC, a Return/
Exhaust Fan Proving Alarm will occur.
Dirty Filter Alarm
A differential pressure switch is used to provide a 24 VAC wet
contact closure to indicate a Dirty Filter status. A Dirty Filter
Alarm is then generated. Dirty Filter needs to be configured for
this alarm to occur.
Emergency Shutdown Alarm
A 24 VAC wet contact input is available to be used when a
Normally Closed Smoke Detector, Firestat, or other shutdown
condition occurs. If this contact opens, it will initiate shutdown of
the VCCX2 and will generate an alarm condition. If an Occupied
relay is configured, it will remain energized. BACnet
®
override
commands are canceled if the Emergency Shutdown Alarm is
activated.
Failure Mode Alarms
High and Low Supply Air Temperature Alarms
If the Supply Air Temperature rises above the user-adjustable
High Supply Air Temperature Cutoff Setpoint, heating will be
immediately deactivated, and a High Supply Air Temperature
Cutoff Alarm will be generated. The fan will continue to run. This
mode and alarm will be canceled if the Supply Air Temperature
falls below the High Supply Air Temperature Cutoff Setpoint
by 5ºF.
If the Supply Air Temperature falls below the user-adjustable
Low Supply Air Temperature Cutoff Setpoint, all Mechanical
Cooling will be immediately deactivated. If, after 10 minutes, the
Supply Air Temperature is still below this setpoint, the fan will
be deactivated, the Outdoor Air Damper will close, and a Low
Supply Air Temperature Cutoff Alarm will be generated. This
mode and alarm will be canceled if the Supply Air Temperature
rises above the Low Supply Air Temperature Cutoff Setpoint
by 5ºF.
High and Low Space Temp Failure
When the Space Temperature rises above the Cooling Mode
Enable Setpoint plus the Control Mode High Alarm Offset
Setpoint for 30 seconds, the controller will generate a High
Control Temp Failure Alarm. When the Space Temperature drops
below the Heating Mode Enable Setpoint minus the Control
Mode Low Alarm Offset Setpoint for 30 seconds, the controller
will generate a Low Control Temp Failure Alarm. Both offset
setpoints are user-adjustable.
Preheat Low Temperature
This alarm indicates a Leaving Air Temperature Cutoff Alarm
condition which is activated if the Controlling Leaving Air
Temperature has dropped below 35ºF for more than two minutes.
The alarm will be disabled if after a fixed delay period the
Leaving Air Temperature has risen above 35ºF.
SEQUENCE OF OPERATIONS
Mechanical Failure and Failure Mode Alarms