44
Packaged and Split Rooftop Ventilator
© 2021 Mitsubishi Electric US, Inc.
Premi
Sys
Always have a completed Pre-Start-Up Checklist and Start-Up Checklist prior to requesting parts or service information.
Symptom
Possible Cause
Corrective Action
High suction
pressure
Excessive load on evaporator coil.
Check for high entering wet bulb temperature, check for
excessive air flow.
Compressor is unloaded.
(digital scroll)
Check digital scroll controller signal and solenoid valve.
Expansion valve sensing bulb not
secured to suction line.
Check the thermal expansion valve, ensure bulb is
insulated. Check superheat. If superheat is high, then
valve is choking refrigerant flow.
• Check bulb for contact.
• Adjust valve for superheat ~10°F.
• Replace valve powerhead or valve.
Thermostatic expansion valve.
Overfeeding.
Check bulb location and clamping. Adjust superheat.
Replace expansion valve power head.
Room load too large.
Reduce the load or add more equipment.
Overcharged.
Check pressures and subcooling.
High
discharge
pressure
Thermal expansion valve setting.
Check thermal expansion setting and calibrate
superheat / subcooling.
Air inlet to condenser coil dirty or
obstructed.
Check for proper clearances and possible air recirculating.
Clean coil.
Condenser fan motor defective.
Check condenser fan motor.
Too much refrigerant.
Check subcooling. Remove excess refrigerant.
Non-condensable in system.
Remove non-condensable from system.
Dirty condenser coil.
Clean condenser coil.
Condenser fan not running or running
backwards.
Check electrical circuit and fuse. Check fan cycling
controls.
High load conditions.
Add more equipment or reduce load.
Low suction
pressure
Refrigerant undercharge/loss of
refrigerant charge.
Check pressures and subcooling.
Blower running backward.
Confirm blower rotation. If reversed, interchange any two
wires from 3-phase disconnect.
Low entering air temperature.
(Low load conditions).
Check entering air wet bulb conditions.
Refrigerant leak.
Check system for leaks. Repair leaks and add refrigerant.
Evaporator dirty or iced-up, or airflow
restricted.
Clean the coil. Check fan operation. Check airflow.
Plugged liquid line filter-drier.
Replace filter-drier, check psi across filter.
Improper hot gas bypass setting (if
equipped).
Check setting and correct as required.
Expansion valve defective, superheat
too high or valve too small.
Adjust valve for proper superheat or replace the
expansion valve if too small or defective.
Moisture in system, check sight glass.
Reclaim refrigerant, check for leaks, recharge.
Troubleshooting – Packaged DX Refrigeration Circuit
