valent VX Series - Troubleshooting - Refrigeration Circuit (Continued); Low Discharge Pressure Troubleshooting; Compressor Short Cycle Troubleshooting; Compressor Oil Loss Troubleshooting

Dedicated Outdoor Air Unit 37
Always have a completed Pre-Start-Up Checklist and Start-Up Checklist prior to requesting parts or service information.
Symptom Possible Cause Corrective Action
Low
discharge
pressure
Insufficient refrigerant charge.
Check subcooling, check for leak. Repair leak and add
refrigerant.
Defective or improperly adjusted
expansion valve.
Check superheating and adjust thermal expansion valve.
Low suction pressure. See “low suction pressure”.
Faulty condenser temperature controls.
(Condensing fan cycle switch).
Check condenser controls and reset to obtain desired
condensing temperature.
Compressor
short cycles
Thermostat location or controls
malfunction.
Check thermostat, check heat anticipator setting.
Improper refrigerant charge. Check subcooling, verify superheat.
Defective high or low pressure control. Check high or low pressure switch.
Poor air distribution. Check ductwork for recirculating.
Low airflow at evaporator coil. Check blower operation and airstream restrictions.
Incorrect unit selection (oversized). Contact factory.
Compressor
loses oil
Refrigerant leak. Check system for leaks. Repair leaks and add refrigerant.
Short cycling. Check low pressure control settings.
Refrigerant flood back.
Check thermal expansion valve setting. Check for
refrigerant overcharge. Check crankcase heater
operation.
Reheat flush cycle inadequate. Contact factory.
Not enough
capacity
or lack of
performance
Refrigeration undercharged. Check subcooling. Adjust charge, if necessary.
Dirty filter or evaporating coil. Check filter, coil and airflow.
Dirty or clogged condensing coil. Check coil and airflow.
Air or other non-condensables in
system.
Check equalized high side pressure with equivalent
outdoor temperature.
Restriction in suction and liquid line. Check for restrictions in refrigerant circuit.
Control contacts stuck. Check wiring.
Excessive load. Add more equipment or reduce room load.
Liquid line
is frosted
or wet
Restriction in liquid line.
Clear restriction upstream of point of frosting. (Note:
liquid line frosting in normal on ASHP units).
Suction line
is frosting
Insufficient evaporating coil airflow.
Check airflow, check filters, check VFD control signal for
proper operation.
Malfunctioning or defective expansion
valve.
Check bulb of thermal expansion valve.
Frost on
evaporating
coil (Note:
Normal on
ASHP units
in heating
mode).
Hot gas bypass valve not functioning
properly (if equipped).
Check valve. If defective, replace.
Manual hot gas bypass valve closed (if
equipped).
Open valve.
Low load or airflow. Increase airflow, check filters.
Troubleshooting – Refrigeration Circuit