Trane Tracer ZN.520 Manual page 75

Sequence of operation
Freeze avoidance
Freeze avoidance is used as low ambient temperature protection and is only
invoked when the fan is off. This includes unoccupied mode when there is no
call for capacity or any other time the fan is off. The controller enters the freeze
avoidance mode when the outdoor air temperature is below the freeze
avoidance setpoint (configurable). The controller disables freeze avoidance
when the outdoor air temperature rises 3°F above the freeze avoidance
setpoint.
When the controller is in freeze avoidance mode:
I All water valves are driven open to allow water to flow through the coil
I Fan is off
I Face bypass damper (when present) is at full bypass
I DX and electric heat are off
For additional coil protection, the Tracer ZN.520 zone controller disables
economizing and drives the outdoor air damper closed during freeze avoidance
mode.
Freeze protection (discharge air temperature low limit)
Freeze protection operation is invoked whenever the discharge air temperature
falls below the discharge air temperature low limit. During freeze protection
operation, the controller increases the heating capacity or decreases the
cooling capacity in order to raise the discharge air temperature above the limit.
If the discharge air temperature remains below the limit for three minutes, the
controller generates a Discharge Air Temp Limit diagnostic.
When the controller has generated a Discharge Air Temp Limit diagnostic:
I All water valves are driven open to allow water to flow through the coil
I Fan is off
I Face bypass damper (when present) is at full bypass
I DX compressor (when present) is off
I Electric heat (when present) is off
For additional coil protection, the Tracer ZN.520 zone controller disables
economizing and drives the outdoor air damper closed during a freeze
protection diagnostic.
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