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Modulating outdoor/return air dampers
CNT-SVX07C-EN 45
ASHRAE Cycle 1 conformance
Tracer ZN521 applications with an outside air damper support ASHRAE
Cycle 1 conformance. ASHRAE Cycle 1 operation admits 100% outdoor
air at all times except during a warm-up cycle. A tri-state modulating
damper actuator is required for this operation. For Tracer ZN521
ASHRAE Cycle 1 conformance, configure the minimum position of the
economizer setpoint to 100% open during occupied periods. If the zone
temperature drops 3°F (1.7°C) below the active zone temperature set-
point, the Tracer ZN521 closes the outdoor air damper regardless of the
minimum position setpoint.
ASHRAE Cycle 2 conformance
Tracer ZN521 controller applications with modulating outside air damper
support ASHRAE Cycle 2 conformance. ASHRAE Cycle 2 operation
allows the modulating outdoor air damper to completely close when the
zone temperature drops 3°F (1.7°C) or more below the active zone temper-
ature setpoint. If the zone temperature rises to within 2°F (1.2°C) of the
active setpoint, the damper opens to the occupied or occupied standby
minimum damper positions. If the zone temperature is between 2°F and
3°F (1.2°C and 1.7°C) below the active zone temperature setpoint, the
damper modulates between the minimum position and closed.
If the discharge air temperature is between the discharge air tempera-
ture low limit and the discharge air temperature low setpoint, the
damper modulates between the minimum position and closed. If this situ-
ation is concurrent with ASHRAE Cycle 2 operation, the lesser of the two
setpoints will determine the actual damper position.
Economizing (free cooling)
Cooling with outdoor air, when the temperature is low enough to make
this possible, is referred to as economizing or free cooling. Tracer ZN521
controller applications with modulating outside air damper support econ-
omizing. The modulating outdoor air damper provides the first source of
cooling for the Tracer ZN521. The controller initiates economizing if the
outdoor air temperature is below the economizer enable point (config-
urable using the Rover service tool). If economizing is initiated, the con-
troller 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.