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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)..
Figure 101. Cascade zone control
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
and the cooling capacity is at a minimum, the
available heating capacity is used to raise the
discharge air temperature to the low limit.
•
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.
108
Calculated
Calculated unit
heating/cooling
setpoint
capacity
Difference
Measured
discharge air
temperature
®
TU service tool),
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.
Heating or Cooling Mode
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 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.
In heating or cooling mode, the controller maintains the
zone temperature based on the active heating setpoint and
the active cooling setpoint, respectively. The active heating
and cooling setpoints are determined by the occupancy
mode of the controller.
For 2-pipe and 4-pipe changeover units, normal heat/cool
operation will not begin until the ability to conduct the
desired heating or cooling operation is verified. This is
done using the entering water temperature sampling
function, for which a valid entering water temperature is
required. When neither a hard wired nor a communicated
entering water temperature value is present on changeover
units, the controller operates in only heating mode and
assumes the coil water is hot. The sampling function is not
used.
The entering water temperature sampling function is used
only for changeover applications and for information and
troubleshooting. It does not affect the operation of the
controller.
BCX-SVX002A-EN
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Troubleshooting
