Catalogue

CAB-PRC001-EN 35
Controls
addition, the outdoor air damper remains closed, unless economizing. The controller always uses
the stored default setpoint values (configurable using the Tracer TU service tool), regardless of the
presence of a hard wired or communicated setpoint value.
Timed Override Control
If the UC400 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 UC400 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 UC400 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).
If the discharge air temperature falls below the disc
harge 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.
• Simplif
ied 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.
• Disc
harge 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.
Difference
Active zone
temperature
setpoint
Calculated
discharge air
temperature
setpoint
Calculated unit
heating/cooling
capacity
Measured
zone
temperature
Measured
discharge air
temperature
Difference
CAB-PRC001-EN.book Page 35 Tuesday, May 8, 2012 10:11 AM