Chapter 5
Sequence of operations
The Tracer ZN521 zone controller will operate to maintain the zone tem- perature setpoint. This chapter discusses many of the operational sequences used by the controller to accomplish this goal.
Power-up sequence
When 24 Vac power is initially applied to the Tracer ZN521 zone control- ler, the following sequence occurs:
1.The green status indicator LED turns on (see “Interpreting LEDs” on page 56).
2.All outputs are controlled off. All modulating valves and dampers close, and the face-and-bypass damper calibrates to bypass (when present).
3.The controller reads all input local values to determine initial values.
4.The random-start timer begins (see “Random start” on page 35).
5.If a hard-wired zone-temperature value is not detected, the controller begins to wait for a communicated value. (This can take several min- utes [15-minute default] and occurs concurrently with the remainder of the power-up sequence.)
6.The random-start timer expires.
7.The power-up control wait function begins automatically if the config- ured power-up control wait time is greater than 0 seconds. When this function is enabled, the controller waits for the configured amount of time (from 0 to 120 seconds) to allow a communicated occupancy request to arrive. If a communicated occupancy request arrives, nor- mal operation can begin. If a communicated occupancy request does not arrive, the controller assumes stand-alone operation.
8.Normal operation begins assuming no diagnostics have been generated.
Random start
Random start is intended to prevent all units in a building from energiz- ing at the same time. The random-start timer delays the fan and any heating or cooling start-up from 5 to 30 seconds. If neither heating nor cooling is initiated, or if fan operation is not required during the delay, the random-start timer will time-out.
