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Flow Sensor
The flow control is pressure independent utilizing the VariTrane flow-sensing ring. The flow ring
provides one of the most accurate differential pressure flow measurements in the industry by
averaging pressure differentials across 16 sensing points. These sensing points, arranged in a ring
configuration to compensate for various inlet duct configurations, provide a signal accurate to
within +/-5% of cataloged CFM provided there is 1½-inlet diameters of straight ductwork upstream
of the VAV box. The pressure differential signal from the flow ring is then converted to an electrical
signal, which is utilized in the control algorithm of the unit controller. It is recommended that the
recalibrate sequence be initiated on a weekly basis. Assuming 1½ inlet diameters of straight duct
work and proper system recalibration, total system flow measurement accuracy (flow ring,
transducer, and UCM controller) during the occupied mode should be +/- 5% of cataloged airflow
over the typical operating range of 20% to 100% of unit cataloged airflow.
Failure Modes
Each UCM has diagnostic capabilities, which allow it to sense various failure conditions. This
diagnostic capability aids in maintenance and trouble shooting of the system. Along with reporting
these failure conditions, the UCM will follow a pre-programmed operating sequence designed to
maintain zone comfort during a failure. The failure conditions and backup operating sequences are
explained below.
Temperature Sensor Failure
Failure of a zone temperature sensor (open or below low limit of 0°F (-17.8°C) will cause fan and heat
outputs to be disabled and airflow to be controlled to the minimum cooling flow set point. If the
measured temperature fails above the high limit (short or above 100°F (37.8°C), the airflow shall be
controlled to the maximum flow set point.
Flow Sensor Failure.
of the normal range of 5% to 110% of box cataloged), the UCM will immediately revert to a position-
based, pressure dependent control algorithm. This allows full operation of the unit and maintains
the ability to control the zone temperature. If a flow sensor has failed, it is periodically checked for
proper operation. If found to be operating properly, the UCM will be switched to the standard
pressure independent control. Otherwise, position control is used as long as the flow signal
remains out of range.
Local Thermostat Set Point Failure
Failure of a local thermostat set point adjustment potentiometer will cause the UCM to revert to the
edited occupied cooling and heating set point.
Communications Failure
A fifteen-minute failure in communications between the UCM and Tracer Summit will cause the
UCM to:
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VAV-SVX01C-EN
If a flow sensor failure is reported (open, short, fails calibration, or is out
Operate in the occupied mode (provided that no occupancy sensor is present on input TB4-1).
Set the IAQ multiplier to 1.0.
Disable the control offset.
Use the control action as determined by the auxiliary sensor. The auxiliary temperature is compared
to the zone temperature. If the supply air temperature is 10ºF (5.5°C) greater than the zone
temperature, then the control action will be heat. If the supply air temperature is less than or equal
to the zone temperature, the control action will be cool. If the supply air temperature is between the
zone temperature and the zone temperature +10ºF (5.5°C)(zone temperature < supply air temperature
< zone temperature + 10ºF) (5.5°C), the control action remains the same and the UCM controls to the
minimum flow set point. If an auxiliary sensor is not installed and the VAV unit is not under Tracer
Summit control, the UCM will retain the last control action in effect.
Sequence of Operations
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