Table of Contents
Advertisement
measurement accuracy (flow ring,
transducer, and UCM controller) during
the occupied mode should be +/- 8% 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
If a flow sensor failure is reported
(open, short, fails calibration, or is out
of the normal range of 5% to 110% of
box cataloged), the UCM will immedi-
ately 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 occu-
pied cooling and heating set point.
VAV-SVX01B-EN
Sequence Of
Operations
Communications Failure
A fifteen-minute failure in communica-
tions between the UCM and Tracer
Summit will cause the UCM to:
— 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 10
o
F 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
+10
o
F (zone temperature < supply air
temperature < zone temperature +
10
o
F), 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.
27
Advertisement
Table of Contents
