Issue:
Troubleshooting the pressure transducer on the Symbio 210, UC210, UC400, UC600, VAV UCM, VV550,
CCPII or CCP III communicating bypass.
VV551 air flow sensor
MP580, MP581 pressure transducer
Kavlico pressure transducer
Honeywell pressure transducer
VAV box air flow sensor
Resolution:
The differential pressure sensor part number
X13790831-02
SEN01465
Pressure transducers are responsible for reading the airflow across the flow ring and converting it to a DC
voltage.
This is read at the P1 input of the UC.
The signal voltage is converted to the airflow reading reported by the UC.
The range to pass air flow sensor calibration on a VAV is 0.25 +/- 0.06 VDC with no flow across the flow ring.
The valid range for VAV operation is 0.25 to 1.75 VDC.
1.75 VDC = 2.0 inches water column pressure differential across the flow ring.
When a VAV fails calibration or the pressure transducer fails, the VAV box will use pressure dependent
operation.
To determine whether or not the transducer has failed, perform the following steps:
1. Check the supply voltage to the UC.
Valid range is 20.4 to 27.6 VAC.
Verify the UC is grounded.
The supply voltage should not change if your negative test lead is touching the ground terminal of the UC or a
chassis ground.
2. Check the input voltage to the transducer between the green and red wires of the connector at the P1 input
of the UC.
The valid range is 5.00 +/- 0.25 VDC.
3. Remove the high and low tubes from the transducer (to simulate no flow).
Read the transducer signal voltage between the green and the black at the P1 input.
The voltage should be between 0.19 and 0.31 VDC with no flow.
4. Replace the tubes on the transducer.
Check the transducer tubes and the flow ring to make sure that they are positioned correctly and properly
attached.
With flow going across the flow ring, read the transducer output voltage between the green and the black
wires.
The difference between this voltage and the zero flow voltage should be 0.10 VDC minimum (1 inch water
column = 1.0 VDC).
The voltage should increase as flow increases.
If a 0.100 volts DC minimum is not present, check the transducer tubing with a Magnehelic differential pressure
gauge.
Compare the reading to the delta pressure (Delta P) chart located on the VAV box.
Ensure that there are no clogs or plugs.
In order to see if the flow ring is clogged or leaks, check with a Magnehelic differential pressure gauge to see if
there is flow going across the flow ring.
5. If the transducer checks out, initiate an air flow sensor calibration.
Check to see if any flow sensor diagnostics are present after calibration completes.
If any flow sensor diagnostics are present, disconnect the tubes from transducer and initiate calibration again.
Calibration should always pass with the tubes disconnected.
When calibration is complete (heat cool mode reports heat or cool, not calibrate), reconnect the tubes before
the air valve reaches 75% open.
You should read flow.
If this process works, there is likely a mechanical issue causing calibration to fail.
The likely causes are: damper not closing tightly and air leaking through preventing the zero flow reading,
loose or leaking tubes, clogged flow ring ports, or an exhaust fan or other fan causing a negative air flow during calibration.
If the flow sensor fails calibration, it will never read air flow.
Kavlico pressure transducer top, low pressure port is on top
IN is 5 VDC from the VAV controller
GND is common from the VAV Controller
OUT is the VDC signal back to the VAV controller representing the differential pressure
All VAV air flow pressure transducers follow this standard.