Unit Operation
Automatic Outdoor Air Damper Adjustment
The Tracer ZN.520 controller is ca- pable of using different minimum outdoor air damper positions for high and low fan speeds, which are configurable. This allows the venti- lation rate to be maintained when switching between fan speeds.
Note: The typical reduction in airflow between high and low fan speeds is 25%, however, if precise control is desired, an air balancer should be consulted.
Economizer
Operation
Note: When there is a request for 0% or 100% actuator position, the actuator will overdrive the actuator 135% to help maintain calibration.
With a valid outdoor air tempera- ture (hardwired or communicated), Tracer ZN.520 uses the modulating economizer damper as the highest priority source of cooling.
The controller initiates the econo- mizer function if the outdoor air temperature is cold enough to be used for free cooling capacity. If the outdoor air temperature is less than the economizer enable set- point (absolute dry bulb), the con- troller modulates the outdoor air damper between the minimum po- sition and 100% to control the amount of cooling capacity. When the outdoor air temperature rises 5°F above the economizer enable setpoint, the outdoor air damper returns to its minimum position and mechanical cooling is initiated.
ASHRAE Cycle I
ASHRAE Cycle I admits 100% out- door air except during the warm up cycle or the unoccupied mode. To set up the Tracer ZN.520 controller for ASHRAE Cycle I conformance, set the occupied outdoor air damp- er minimum position to 100%.
ASHRAE Cycle II
The Tracer ZN.520 controller con- forms to ASHRAE Cycle II by allow- ing the modulating outdoor air damper to maintain a minimum position during the occupied mode unless economizing is enabled or the space temperature falls more than 3°F below the effective set- point.
If the space temperature falls more than 2°F below the setpoint, the outdoor air damper will begin to close. When the space temperature hits 3°F below the setpoint, the outdoor air damper is completely closed.
Figure 22: ASHRAE cycle 2
DX Operation
Tracer ZN.520 supports direct ex- pansion (DX) compressor opera- tion for cooling only.
The controller does not allow both the DX compressor and economiz- er to operate at the same time. This prevents problems where the en- tering air temperature is too low for the evaporator coil to operate as designed.
Tracer ZN.520 also includes a fixed minimum on/off time of 3 minutes for compressor operation to pre- vent short cycling of the compres- sor.
Electric Heat
Operation
The Tracer ZN.520 controller sup- ports 1 or 2 stages of electric heat control.
To control space temperature, electric heat is cycled to control the discharge air temperature. The rate of cycling is dependent on the load in the space and the temperature of the incoming fresh air.
Baseboard Heat
Operation
The Tracer ZN.520 controller can be configured to support one stage of baseboard heat. A 2 position valve or an electric heat contactor will be energized at 2.5°F below the effective heating setpoint. The baseboard heat is
The unit will provide heating ca- pacity as required regardless of the presence of baseboard heat. If the unit can not maintain the space temperature, the baseboard heat will come on as a secondary stage.
If baseboard heat is to be used as the primary source of heating, the discharge air high limit should be set to a moderate temperature.
This will allow the ventilation air to be tempered and avoid a “drafty” feeling. The baseboard heat will then operate as the heating capaci- ty for the space.
In the UNOCCUPIED mode, the baseboard heat and unit heating capacity are brought on simulta- neously as needed to maintain the unoccupied setpoint.
Exhaust Fan
Operation
The exhaust fan is coordinated with the unit fan and outdoor air damper operation. The exhaust fan output is energized only when the unit fan is operating and the out- door air damper position is greater
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