Siemens 125-1957 Owner's Manual page 19

If the room temperature sensor input to the Unit Vent Controller fails or the LTDT equals ON,
the controller goes through the following shutdown sequence:
•
Outdoor air damper is closed.
•
Heating is full ON (except electric, which is OFF).
•
Cooling is full OFF.
•
Face-bypass damper is open to face.
•
Fan is OFF.
•
Auxiliary radiation is OFF.
•
Two-position heating valve actuator is open.
•
Two -position cooling valve actuator is closed.
NOTE:
If the discharge air temperature sensor fails, the following conditions occur:
•
If the last valid value is greater than 150 degrees, the heat is turned off, the outdoor
air damper is closed and the fan is turned on.
•
If the sensor does not come back within 10 minutes or if the last valid value is less
than 150 degrees, the controller shuts down.
If a failed sensor returns or if the LTDT turns off, normal control resumes.
Analog and digital outputs cannot be commanded when the controller is in fail-safe mode;
however, failed points may be overridden, allowing the controller to return from fail-safe
mode. In this instance, room temperature control is not possible.
Siemens Building Technologies, Inc.
DO 2 is not commanded by the fail-safe mode; all other DOs can be affected.
Basic Operation
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