Siemens 125-1957 owner manual Basic Operation

Basic Operation

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: DO 2 is not commanded by the fail-safe mode; all other DOs can be affected.

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. 9