BAS-SVX48H-EN
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allowed to open to the occupied or occupied standby minimum damper positions. When the
space temperature is between 2ºF and 3ºF below the setpoint, the damper is set between 0 and
the minimum damper position. If the discharge air temperature is between the discharge air
temperature low limit and the discharge air temperature low setpoint, the damper modulates
between closed and the minimum position. When this function and ASHRAE Cycle 2 are both
active, the minimum of the two control points is chosen. The ASHRAE Cycle 2 operation is
available only on units equipped with a 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 UC400 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 (configured 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%) 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.
Outdoor Air Damper Control With Varying Fan Speed
To ensure proper ventilation as the fan speed changes in AUTO fan mode, the UC400 Controller
is configured with two (2) outdoor air damper position setpoints for occupied operation. As the
fan speed changes to LOW or HIGH, the damper adjusts position to maintain proper ventilation.
NNoottee:: The damper position does not adjust when the fan speed changes to medium.
With the fan operating at maximum speed, the Design Minimum OA Damper Position at
Maximum Fan Speed setpoint (bubble 11 on graph) is set in order to bring in the required amount
of outdoor airflow. With the fan operating at minimum speed, the Design Minimum OA Damper
Position at Minimum Fan Speed setpoint (bubble 22 on graph) is set in order to bring in the same
required amount of outdoor airflow. As the fan speed varies between minimum and maximum
speeds, the position of the outdoor air damper is adjusted in proportion to the changing fan
speed as shown below.
Figure 20. OA Damper Adjustment
SSeeqquueennccee ooff OOppeerraattiioonn
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