VCES-DDC-IOM-1C
22
Table D3: DDC Troubleshooting Guide
Problem Cause Solution
Unit is not ventilating.
Occupancy contact open.
Check contact closure between field wiring terminals 304 and
305 (see wiring schematics in the VHC-36, 42 and 50 IOM for
wiring details).
Check the BacStat shows ‘OV’ for occupied ventilation.
Blowers are overloaded.
BacStat will flash ‘OL’ in the upper left corner.
Check motor overload trip indicator on overload relay (located
below contactor). If it has tripped, press the reset button.
Check for proper voltage at output of overload relay (T1, T2, T3)
to confirm that the supply and exhaust blower motors are running.
Low limit thermostat tripped.
BacStat will show ‘LL’ for low limit. Low limit sensor is supplied
by others.
Supply air sensor not hooked
up.
BacStat will show a temperature in the top right corner in the
range of −59 to −50. Hook up the sensor that was supplied with
the unit.
Unit is in recirculation/exhaust
defrost mode.
BacStat will flash ‘dF’ in the upper left corner. Wait for defrost
mode to finish its timed sequence (minimum of six (6) minutes).
Unit is not recirculating (for
units with a recirculation
damper).
Unoccupied recirculation
contact open.
Check contact closure between field wiring terminals 305 and
306 (see wiring schematics in the VHC-36, 42 and 50 IOM for
wiring details).
Supply blower not running.
BacStat will flash ‘OL’ in the upper left corner.
Check motor overload trip indicator on overload relay (located
below contactor). If it has tripped, press the reset button.
Unit will not stop ventilating. Occupancy contact closed.
Check contact for break between field wiring terminals 304 and
305 (see wiring schematics in the VHC-36, 42 and 50 IOM for
wiring details).
Air from supply diffusers too
cold.
Imbalance of supply and
exhaust air.
Check filters and heat exchanger for blockage.
BacStat will flash ‘SF’ or ‘EF’ to indicate supply or exhaust filters
are dirty.
Check balance of airflows.
Post heat required. Install post heat module.
Wheel failure.
BacStat will flash ‘CF’ in the upper left corner if wheel failure is
detected (units with wheel rotation board).
Unit makes an annoying
noise.
Blower wheel out of align-
ment.
Remove the motor/blower assembly. Adjust the blower assembly.
Enthalpy wheel freezing.
Imbalance of supply and
exhaust air.
Check filters and heat exchanger for blockage.
Confirm supply filter needs replacing.
Confirm exhaust filter needs replacing.
Check balance of airflows.
Check for operation of damper actuators.
Defrost mode not function-
ing.
Confirm that the unit is in defrost mode. BacStat II will flash ‘dF’
in upper left corner. See defrost sequence of operation in the
Frost Control section of this document.
Enthalpy wheel not running.
Unit is in free cooling. Confirm that the unit is in free cooling mode.
Unit is in recirculation defrost. BacStat will flash ‘dF’ in upper left corner.
Wheel failure.
Units with wheel rotation sensor.
BacStat will flash ‘CF’ indicating wheel rotation failure. Check
relays and verify that motor is getting power.
Drive motor capacitor failure.
Check capacitor connections. Check motor operation with a new
capacitor.
Drive pulley.
Check for drive belt derailment off of drive pulley or failure.
Check for securely fastened pulley on motor shaft.
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