Trane VUV Installation, Operation And Maintenance Manual page 84

Start-Up
configured (continuous or cycling). The outdoor air damper
closes when the fan is OFF. The controller remains in occupied
bypass mode until either the CANCEL button is pressed on the
Trane zone sensor or the occupied bypass time (configurable
using the Tracer® TU service tool) expires. The temperature
setpoints can configured as local (hard wired), communicated,
or stored default values using the Tracer® TU service tool.
Timed Override Control
If the controller has a timed override option (ON/CANCEL
buttons), pushing the ON button initiates a timed override on
request. A timed override on request changes the occupancy
mode from unoccupied mode to occupied bypass mode. In
occupied bypass mode, the controller controls the space
temperature based on the occupied heating or cooling
setpoints. The occupied bypass time, which resides in the
controller and defines the duration of the override, is
configurable from 0 to 240 minutes (default value of 120
minutes). When the occupied bypass time expires, the unit
transitions from occupied bypass mode to unoccupied mode.
Pushing the CANCEL button cancels the timed override
request. In addition, it will end the timed override before the
occupied bypass time has expired and transition the unit from
occupied bypass mode to unoccupied mode.
If the controller is in any mode other than unoccupied mode
when the ON button is pressed, the controller still starts the
occupied bypass timer without changing to occupied bypass
mode. If the controller is placed in unoccupied mode before the
occupied bypass timer expires, the controller is placed into
occupied bypass mode and remains in this mode until either
the CANCEL button is pressed on the Trane zone sensor or
the occupied bypass time expires.
Zone Temperature Control
The controller has three methods of zone temperature control:
• Cascade zone control—used in the occupied, occupied
bypass, and occupied standby modes. It maintains zone
temperature by controlling the discharge air temperature to
control the zone temperature. The controller uses the
difference between the measured zone temperature and
the active zone temperature setpoint to produce a
discharge air temperature setpoint. The controller
compares the discharge air temperature setpoint with the
discharge air temperature and calculates a unit heating/
cooling capacity accordingly (refer to the illustration
below). The end devices (outdoor air damper, valves, and
so on) operate in sequence based on the unit heating/
cooling capacity (0–100 percent).
84
Figure 81. Cascade zone control
Calculated
Active zone
discharge air
temperature
temperature
setpoint
Difference
Measured
zone
temperature
If the discharge air temperature falls below the discharge
air temperature low limit setpoint, (configurable using the
Tracer® TU service tool), and the cooling capacity is at a
minimum, the available heating capacity is used to raise
the discharge air temperature to the low limit (refer to the
following section,"Discharge Air Tempering," p.
• Simplified zone control— if discharge air temperature
failure occurs, then simplified zone controls runs. In the
unoccupied mode, the controller maintains the zone
temperature by calculating the required heating or cooling
capacity (0–100%) according to the measured zone
temperature and the active zone temperature setpoint.
The active zone temperature setpoint is determined by the
current operating modes, which include occupancy and
heat/cool modes.
• Discharge air temperature control— is the backup mode
that runs only if there is not valid zone temperature. In this
mode, the active space temperature setpoint is used as the
discharge air temperature setpoint.
Important: This is not a normal operating mode. The
source of the invalid zone temperature
needs to be corrected to restore normal
operation.
Discharge Air Tempering
If the controller is in cooling mode, cascade zone control
initiates a discharge air tempering function when:
• The discharge air temperature falls below the discharge air
temperature low limit setpoint (configurable using the
Tracer® TU service tool)
• All cooling capacity is at a minimum. The discharge air
tempering function allows the controller to provide heating
capacity (if available) to raise the discharge air
temperature to the discharge air temperature low limit
setpoint.
• The cold outdoor air is brought in through the outdoor air
damper and when the damper is at (high) minimum
position. This causes the discharge air temperature to fall
below the discharge air temperature low limit setpoint.
Calculated unit
heating/cooling
setpoint capacity
Difference
Measured
discharge air
temperature
84).
UV-SVX005G-EN