92 UNT-SVX07B-EN
Start-Up
The controller uses the measured space temperature, the active heating setpoint, and discharge
air temperature, along with the control algorithm, to determine the requested heating capacity of
the unit (0%–100%). The outputs are controlled based on the unit configuration and the required
heating capacity.
Unit diagnostics can affect the Tracer ZN520 controller operation, causing unit operation to be
defined as abnormal. Refer to the Troubleshooting section for more information about abnormal
unit operation.
The heating output is controlled based on the heating capacity. At 0% capacity, the heating output
is off continuously. Between 0% and 100% capacity, the heating output is controlled according to
modulating valve logic (modulating valves) or cycled on (two-position valves). As the load
increases, modulating outputs open further and binary outputs are energized longer. At 100%
capacity, the heating valve is fully open (modulating valves) or on continuously (two-position
valves).
The Tracer ZN520 fan output(s) normally run continuously during the occupied and occupied
standby modes, but cycle between high and off speeds with heating/cooling during the unoccupied
mode. When in the occupied mode or occupied standby mode and the fan speed is set at the high,
medium, or low position, the fan runs continuously at the selected speed. Refer to the
Troubleshooting section for more information on abnormal fan operation.
When the unit’s supply fan is set to auto, the controller’s configuration determines the fan speed
when in the occupied mode or occupied standby mode. The fan runs continuously at the
configured heating fan speed or cooling fan speed. For all fan speed selections except off, the fan
cycles off during unoccupied mode.
The economizer outdoor air damper is never used as a source of heating. Instead, the economizer
damper (when present) is only used for ventilation; therefore, the damper is at the occupied
minimum position in the occupied mode. The damper control is primarily associated with occupied
fan operation.
Fan Mode Operation
For multiple fan speed applications, the Tracer ZN520 controller offers additional fan configuration
flexibility. Separate default fan speeds for heating and cooling modes can be configured. The fan
runs continuously for requested speeds (off, high, medium, or low). When the fan mode switch is
in the Auto position or a hardwired fan mode input does not exist, the fan operates at the default
configured speed. See Table 21 for default fan configuration for heat and cool mode. During
unoccupied mode, the fan cycles between high speed and off with heating and cooling fan modes.
If the requested speed is off, the fan always remains off.
During dehumidification, when the fan is on Auto, the fan speed can switch depending on the error.
Fan speed increases as the space temperature rises above the active cooling setpoint.
Additional flexibility built into the controller allows you to enable or disable the local fan switch
input. The fan mode request can be either hardwired or communicated to the controller. When both
Table 21. Fan configuration
Auto Fan Operation Fan Speed Default
Heating Continuous Off
Low
Medium
High
Cooling Continuous Off
Low
Medium
High
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