Installation and Maintenance Manual

Startup
100 UNT-SVX07D-EN
Timed Override Control (UC400)
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 (UC400)
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
di
scharge 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 (UC400).”).
• 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.
Important: T
his is not a normal operating mode. The
source of the invalid zone temperature
needs to be corrected to restore normal
operation.
Discharge Air Tempering (UC400)
If the UC400 controller is in cooling mode, cascade zone
control initiates a discharge air tempering function when:
• The discharge air temperature falls below the
di
scharge air temperature low limit setpoint
(configurable using the Tracer TU service tool)
• All cooling capacity is at a mi
nimum. 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
a
ir 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.
Heating or Cooling Mode (UC400)
The heating or cooling mode can be determined in one of
two ways:
• By a communicated signal from a BAS or a peer
controller
• Automatically, as determined by the UC400 controller
A communicated heating signal permits the controller to
only heat and a communicated cooling signal permits the
controller to only cool. A communicated auto signal
allows the controller to automatically change from heating
to cooling and vice versa.
Difference
Active zone
temperature
setpoint
Calculated
discharge air
temperature
setpoint
Calculated unit
heating/cooling
capacity
Measured
zone
temperature
Measured
discharge air
temperature
Difference
UNT-SVX07_-EN.book Page 100 Friday, April 27, 2012 9:40 AM