Startup
118
UNT-SVX07E-EN
4-pipe applications supports cooling or heat/cool
changeover with a main valve/coil and heating only with
an auxiliary valve/coil.
Modulating Outdoor/Return Air Damper
The controller operates the modulating outdoor/return air
dampers based on the following:
•
Occupancy mode
•
Outdoor air temperature (communicated or hard wired
sensor)
•
Zone temperature
•
Setpoint
•
Discharge air temperature
•
Discharge air temperature setpoint
The minimum position for an outdoor air damper is
configurable using the Tracer TU service tool for both
occupied mode and occupied standby mode and for low-
speed fan operation. A controller can receive a BAS-
communicated outdoor air damper minimum position.
A BAS-communicated minimum position setpoint has
priority over all locally configured setpoints. When a
communicated minimum position setpoint is not present,
the controller uses the configured minimum position for
low fan speed whenever the fan is running at low speed,
regardless of the occupancy state. Refer to
Table 59
and
Table 60
for more information about how the controller
determines the position of the modulating outdoor air
damper.
Economizing (Free Cooling)
Cooling with outdoor air (during the times when the
temperature is low enough to allow) is referred to as
economizing (free cooling). The controller and
applications with modulating outside air damper, support
economizing. The modulating outdoor air damper
provides the first source of cooling for the controller.
The controller initiates economizing if the outdoor air
temperature is below the economizer enable point
(configurable using the Tracer TU service tool). If
economizing is initiated, the controller modulates the
outdoor air damper (between the active minimum damper
position and 100 percent) to control the amount of outdoor
air cooling capacity. When the outdoor air temperature
rises 5°F (2.8°C) above the economizer enable point, the
controller disables economizing and moves the outdoor
air damper back to its predetermined minimum position,
based on the current occupancy mode or communicated
minimum outdoor air damper position. If an outdoor air
temperature value is not present, economizing is disabled.
Modulating Outdoor Air Damper
The controller supports two-position outdoor air damper
actuators. However, a modulating outdoor/return air
damper actuator can be used for two-position control.
Two-position control can be achieved by not providing an
outdoor air temperature (neither hard wired nor
communicated) to the controller, and by setting the
damper minimum position (using the Tracer TU service
tool) to the desired value, typically 100 percent.
Table 59. Modulating outdoor air damper position setpoint determination
Occupancy
BAS-communicated Setpoint
Fan speed
Active Minimum Setpoint
Unoccupied
Any value
Any value
0 percent (closed).
Occupied
Occupied bypass
Occupied standby
Valid
Any value
BAS-communicated
Occupied
Occupied bypass
Occupied standby
Invalid
Low
Occupied low fan minimum
Occupied
Occupied bypass
Invalid
Medium/high
Occupied minimum
Occupied standby
Invalid
Medium/high
Occupied standby minimum
Table 60. Relationship between outdoor temperature sensors and damper position
Outdoor Air Temperature
Modulating outdoor air damper position
Occupied or Occupied Bypass
Occupied Standby
Unoccupied
No or invalid outdoor air temperature Open to occupied minimum position
Open to occupied standby minimum
position
Closed
Failed outdoor air sensor
Open to occupied minimum position
Open to occupied standby minimum
position
Closed
Outdoor air temperature present and
economizing possible (Refer to section,
“Economizing (Free Cooling),” page 118
)
Economizing; damper controlled
between occupied minimum position
and 100 percent
Economizing; damper controlled
between occupied standby
minimum position and 100 percent
Open and economizing during
unit operation; otherwise
closed
Outdoor air temperature present and
economizing not possible (Refer to
section,
“Economizing (Free Cooling),”
page 118
)
Open to occupied minimum position Open to occupied standby minimum
position
Closed
