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BCXC-SVX01B-EN 51
Start-Up
Note: Unit diagnostics can affect fan operation, causing occupied and occupied standby fan
operation to be defined as abnormal. Refer to “Troubleshooting,” p. 72 for more information
about abnormal fan operation.
Economizer Cooling (Tracer ZN520 Only)
The economizer provides cooling whenever the outdoor temperature is below the economizer
enable setpoint and there is a need for cooling. The economizer operates to meet the space
demand, with other forms of cooling enabling when the economizer cannot meet the demand
alone. See economizer air damper operation for additional information.
DX Cooling (Tracer ZN520 only)
The controller does not use both the DX compressor and the economizer at the same time. This
prevents problems where the entering air temperature is too low for the evaporator coil to operate
as designed, which leads to compressor short cycling due to low discharge air temperatures.
Discharge Air Tempering (Tracer ZN520 Only)
Cascade cooling control initiates a discharge air tempering function if:
• the discharge air temperature falls below the discharge air temperature control low limit,
• all cooling capacity is at minimum, and
• the discharge control loop determines a need to raise the discharge air temperature.
The controller then provides heating capacity to raise the discharge air temperature to its low limit.
The discharge air tempering function enables when cold, fresh air is brought in through the fresh
air damper and causes the discharge air to fall below the discharge air temperature control low
limit. The controller exits the discharge air tempering function when heat capacity has been at 0%
for five minutes.
Heating Operation
During heating mode, the Tracer™ ZN controller attempts to maintain the space temperature at the
active heating setpoint. Based on the controller’s occupancy mode, the active heating setpoint can
be:
• occupied heating,
• occupied standby heating (Tracer ZN510 or ZN520 only), or
• unoccupied heating.
Note: Unit diagnostics can affect the controller operation, causing unit operation to be defined as
abnormal. Refer to “Troubleshooting,” p. 72 for more information about abnormal unit
operation.
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 (ZN520 only) or cycled with 2-position valves. As the load increases,
modulating outputs open further and binary outputs are energized longer. At 100% capacity, the
modulating valve is fully open (Tracer™ ZN520 only) or on continuously with 2-position valves.
Economizer Damper (Tracer ZN520 Only)
The economizer damper option is never used for as a source for heating, but only 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.
Dehumidification (Tracer ZN520 only)
During dehumidification, the Tracer™ ZN520 controller adjusts the heating setpoint up to the
cooling setpoint. This reduces the relative humidity in the space with a minimum of energy usage.
The controller uses the measured space temperature, the active heating setpoint, and discharge
air temperature (Tracer™ ZN520 only) along with the control algorithm, to determine the requested