Trane Tracer ZN.520 Manual page 70

If the zone temperature drops 3ºF below
the active setpoint, the Tracer ZN.520 zone
controller closes the outdoor air damper
regardless of the active minimum damper
position.
Economizing is possible during the
occupied, occupied standby, unoccupied,
and occupied bypass modes.
68
Sequence of operation
ASHRAE Cycle 2 conformance
The Tracer ZN.520 zone controller conforms to the ASHRAE cycle 2 by allowing
the modulating outdoor damper to completely close when the space
temperature drops 3°F or more below the effective setpoint. When the space
temperature rises within 2°F of the effective setpoint, the damper is 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 only available on units equipped with a
modulating outdoor air damper.
Economizer operation
With a valid outdoor air temperature (either hardwired or communicated),
Tracer ZN.520 zone controller uses the modulating economizer damper as the
highest priority source of cooling. Outdoor air is used as a source of cooling
capacity before hydronic cooling; this function is known as economizing.
Economizer operation is only possible through the use of a modulating damper.
The controller initiates the economizer function if the outdoor air temperature is
cold enough to be used as free cooling capacity. If the outdoor air temperature
is less than the economizer enable setpoint (absolute dry bulb), 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 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 damper position.
CNT-SVX04A-EN