Trane Tracer ZN521, Installation And Operation Manual

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CNT-SVX07D-EN 37
Sequence of operations
Two-pipe operation
For two-pipe applications, the Tracer ZN521 can be configured as heating only, cooling only, or
heat/cool changeover. The coil can be used as the primary heating source and/or the primary
cooling source. If present, an electric heating element can be used only as the primary heating
source (instead of hydronic or steam heating). A changeover unit requires a valid entering water
temperature value—either communicated or hard-wired—to operate properly (see
“AI1: Entering
water temperature,” p. 16 and “Entering water temperature sampling function,” p. 35 ).
Four-pipe operation
For four-pipe applications, the Tracer ZN521 can be configured as heat/cool or heat/cool
changeover. The main coil can be used as the primary cooling source or the primary heating/
cooling source. The auxiliary coil can be used only as the primary heating source, not as a second
stage of heating. During normal operation, the controller never uses the main coil and auxiliary coil
simultaneously. A changeover unit requires a valid entering water temperature value—either
communicated or hard-wired—to operate properly (see
“AI1: Entering water temperature,” p. 16
and “Entering water temperature sampling function,” p. 35 ). Electric heat control is not available
on four-pipe applications.
Modulating outdoor/return air dampers
The Tracer ZN521 operates the modulating outdoor/return air dampers according to the following
factors:
• 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 Rover service tool for
occupied and occupied standby modes and for low-speed fan operation. A controller can also
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. See
Table 9 and Table 10 for more information about how the
controller determines the position of the modulating outdoor air damper.
Table 9. Modulating outdoor air damper position setpoint determination
Occupancy
BAS-communicated
setpoint Fan speed Active minimum setpoint
Unoccupied Any value Any value 0% (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