Chapter 5 Sequence of operations
•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.
Table 8. Valve control options
| Application | Tri-state | Two-position | Isolation |
| modulating | (two-position) |
| | |
| | | | |
| Hydronic/steam fan coils and blower coils | × | × | |
| | | | |
| Unit ventilators with valve control | × | | |
| | | | |
| Face-and-bypass unit ventilators | | | × |
| | | | |
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.
