Trane CNT-SVX07C-EN User Manual

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.

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 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.