System Troubleshooting & Diagnostics
Diagnosing high/low pressure conditions, restricted lines, non-condensables, and using gauges and instruments.
- Diagnose high head and low suction pressure conditions
- Identify symptoms of restrictions, non-condensables, and flash gas
- Use compound gauges, thermistors, and diagnostic instruments correctly
- Explain pressure switch operation, cutouts, and differentials
Lesson 1
High Head & Low Suction Pressure Diagnosis
Systematic Troubleshooting
Refrigeration troubleshooting starts with reading suction pressure and discharge (head) pressure on the gauge manifold. Abnormal pressures indicate specific system problems. The Red Seal exam heavily tests your ability to match symptoms to causes.
High Head Pressure Causes
Dirty condenser - reduced heat rejection
Condenser fan failure - no airflow
Overcharge - excess refrigerant in condenser
Non-condensables - air or nitrogen in system
Recirculated air - condenser in enclosed space
Low Suction Pressure Causes
Low charge - insufficient refrigerant
Restricted metering device - blocked TXV or cap tube
Dirty evaporator - reduced heat absorption
Evaporator fan failure - no airflow
Iced evaporator - blocked by frost
High Head Pressure Diagnosis
When head pressure is abnormally high, the system works harder, uses more energy, and may trip the high-pressure cutout. The most common cause is a dirty condenser coil. Check condenser cleanliness and airflow first. If the condenser is clean, check for overcharge by verifying subcooling - high subcooling with high head pressure indicates overcharge.
Low Suction Pressure Diagnosis
Low suction pressure means the evaporator is not absorbing enough heat. This could be a refrigerant-side problem (low charge, restriction) or an air-side problem (dirty coil, fan failure, iced coil). Check superheat to differentiate:
- High superheat + low suction = low charge or restricted metering device
- Low superheat + low suction = poor airflow across evaporator
Superheat Tells the Story
If suction pressure is low and superheat is high, the evaporator is starved for refrigerant (charge or restriction problem). If suction pressure is low and superheat is low, the evaporator has enough refrigerant but cannot absorb heat (airflow problem).
High head pressure is most commonly caused by a dirty condenser or condenser fan failure. Low suction pressure with high superheat indicates low charge or a restriction. Low suction with low superheat indicates an airflow problem.