VAV Controller—Variable Air Volume (VAV) Controller 81
The following table indicates symptoms and corrections for typical control
system malfunctions (primarily in pressure independent applications). We
recommend that you use HVAC PRO for Windows Commissioning mode
during all troubleshooting procedures.
Table 34: VAV Controller Troubleshooting Symptoms and Corrective Actions
Symptom Possible Cause Corrective Action
VAV Controller CFM
readings are not within
10 percent of balancing
contractors readings or
flow readings are not
repeatable.
Velocity Pressure
Transducer out of calibration
by more than 20 percent.
Damper does not seal.
Box Multiplier Constant
and/or Box Area parameters
are not set correctly.
One or both airflow pickup to
DPT tubing connections are
crossed, plugged, or open.
The ductwork configuration
upstream of the airflow
pickup is disturbing the air
flow across the pickup.
Command Auto Zero mode using HVAC PRO for Windows
and read the offset value for the DPT sensor. If the value is
greater than 20 percent of the sensor range, you need to
replace or manually calibrate the transducer.
Override damper closed and verify.
Verify the numbers displayed in the HVAC PRO for Windows
parameters screen against the information on the box, or check
the OEM Reference Manual (FAN 638) for the recommended
numbers per box type.
Check that the high or upstream side of the pickup connects to
the DPT high side, and the downstream side of the pickup
connects to the DPT low side. Check that the tubing is not
plugged and that the test taps are capped.
Turns, transitions, and flex duct should be no closer than
one duct diameter upstream from the airflow pickup. The
takeoff should be at least three duct diameters upstream from
the pickup.
No change in the CFM
value while you are
manually overriding the
damper by changing the
temperature setpoint or
issuing a direct override
to the damper command.
Damper linkage is slipping or
linked incorrectly.
Main system is not supplying
enough air.
Inspect the linkage while the actuator is being commanded to
ensure tight connection.
If the VAV box actuator is linked correctly and you cannot
obtain the maximum CFM setpoint, check the Central System
Static Pressure to ensure enough primary air is being provided
to properly balance the system.
The damper actuator
cycles excessively.
The Supply Deadband
and/or Heating/Cooling Prop
Bands are too narrow.
Electrical noise is present on
zone temperature signal.
Check the Supply Deadband value to ensure that it is greater
than (48*(box inlet area))ft
2
/(4.5*(box inlet area))m
2
.
If the cycling persists after changing the supply deadbands,
check the heating and cooling prop bands to see if they are
greater than 4°F/-15.5°C. If not, increase the prop band values
to at least 4°F/-15.5°C and add an integration timer of 400.
Diagnose with HVAC PRO for Windows data graphing. If
present, reroute wire or use twisted pair and/or shielded cable.
Series fan is not running.
Unit is not in Occupied
mode.
Unit in initial startup.
Check HVAC PRO for Windows parameter table to ensure
occupied status shows on.
Before the series fan is commanded on, the damper fully
closes to ensure the fan does not turn backwards.
103F104F105FTroubleshooting
the VAV
Controller with
HVAC PRO for
Windows
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