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Chapter 5 Sequence of operations
42 CNT-SVX07C-EN
• The exhaust output is de-energized if the outdoor air damper position
drops 10% below the Exhaust Fan/Damper Enable Setpoint.
• If the Exhaust Fan/Damper Enable Setpoint is less than 10%, the
exhaust output is energized if the outdoor air damper position is at
the setpoint and de-energized at 0.
Valve operation
The Tracer ZN521 zone controller supports one or two tri-state modulat-
ing or two-position valves, depending on the application (see Table 8). The
controller opens and closes the appropriate valve(s) to maintain the active
zone temperature setpoint at the heating setpoint in heating mode or the
cooling setpoint in cooling mode (see “Cascade zone control” on page 38).
For face-and-bypass applications, one or two isolation valves are
controlled.
Modulating valve operation
The Tracer ZN521 supports tri-state modulating valve control. Two
binary outputs control each valve: one to drive the valve open and one to
drive the valve closed. The stroke time for each valve is configurable
using the Rover service tool. The controller supports heating, cooling, or
heat/cool changeover with a single valve/coil for two-pipe applications.
The controller supports cooling or heat/cool changeover with the main
valve/coil and heating only with the auxiliary valve/coil for four-pipe
applications. The controller moves the modulating valve to the desired
positions based on heating or cooling requirements.
Modulating valve calibration
Calibration of modulating valves is done automatically. During normal
operation, the controller overdrives the actuator (135% of the stroke time)
whenever a position of 0% or 100% is requested. as part of Tracer ZN521
normal operation. At power-up or after a power outage, the controller first
drives all modulating valves (and dampers) to the closed position. The
controller calibrates to the fully closed position by overdriving the actua-
tor (135% of the stroke time). Then, the controller resumes normal
operation.
Table 8. Valve control options
Application
Tr i- st ate
modulating
Two-position
Isolation
(two-position)
Hydronic/steam fan coils and blower coils × ×
Unit ventilators with valve control ×
Face-and-bypass unit ventilators ×