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CNT-SVX07C-EN 35
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.