UNT-SVX07B-EN 91
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Cooling Operation
The heating and cooling setpoint high and low limits are always applied to the occupied and
occupied standby setpoints. During the cooling mode, the Tracer ZN520 controller attempts to
maintain the space temperature at the active cooling setpoint. Based on the controller’s occupancy
mode, the active cooling setpoint is one of the following:
• Occupied cooling setpoint
• Occupied standby co
oling setpoint
• Unoccupied cooling setpoint
The controller uses the measured
space temperature, the active cooling setpoint, and discharge air
temperature along with the control algorithm to determine the requested cooling capacity of the
unit (0%–100%). The outputs are controlled based on the unit configuration and the required
cooling capacity. To maintain space temperature control, the Tracer ZN520 cooling outputs
(modulating hydronic valve, two-position hydronic valve, or outdoor air economizer damper) are
controlled based on the cooling capacity output.
The cooling output is controlled based on the cooling capacity. At 0% capacity, all cooling capacities
are off and the damper is at minimum position. Between 0% and 100% capacity, the cooling outputs
are controlled according to modulating valve logic (modulating valves) or cycled on (2-position
valves). As the load increases, modulating outputs open further and binary outputs are energized
longer. At 100% capacity, the cooling valve or damper is fully open (modulating valves) or on
continuously (and 2-position valves).
Unit diagnostics can affect fan operation, causing occupied and occupied standby fan operation to
be defined as abnormal. Refer to the Troubleshooting section for more information about abnormal
fan operation.
The Tracer ZN520 controller operates the supply fan continuously when the controller is in the
occupied and occupied standby modes, for either heating or cooling. The controller only cycles the
fan off with heating and cooling capacity in the unoccupied mode.
The economizer is used for cooling purposes whenever the outdoor temperature is below the
economizer enable setpoint and there is a need for cooling. The economizer is used first to meet
the space demand, and other forms of cooling are used if the economizer cannot meet the demand
alone. See modulating outdoor air damper operation for additional information.
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.
Discharge Air Tempering
The discharge air tempering function enables when cold outdoor air is brought in through the
outdoor air damper, causing 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 ZN520 controller attempts to maintain the space temperature at
the active heating setpoint. Based on the occupancy mode of the controller, the active heating
setpoint is one of the following:
• Occupied heating
• Occupied standby heating
• Unoccupied heating
During dehumidification in the heating mode, the controller adjusts the heating setpoint up to the
co
oling setpoint. This reduces the relative humidity in the space with a minimum of energy usage.
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