Diagnostics, Testing, and Verification
Core concepts
Diagnostics require systematic measurement: pressures, temperatures, superheat, subcooling, amp draw, voltage, and airflow. Understand how to interpret temperature split (return minus supply air), delta T, and static pressure. Know common failure modes: low charge, restricted metering device, dirty condenser, restricted airflow, compressor valve failure, and reversing valve malfunction. Familiarize yourself with electrical diagnostics-capacitors, contactors, and compressor windings-and with using a manifold gauge set and digital psychrometer.
Traps in questions
Watch for symptoms that overlap: low suction pressure can mean low charge or a restriction. High superheat with normal subcooling suggests a restriction; high superheat with low subcooling suggests low charge. High head pressure may result from dirty condenser or overcharge. Do not overlook airflow as a root cause. Questions may confuse delta T with superheat.
Worked example
Return air at 75°F and supply at 60°F gives a 15°F temperature split, within a typical 15–20°F range. If split is low, check airflow and charge. Rule of thumb: verify airflow and filter condition before condemning refrigerant charge.
Study tip
Build a symptom-to-cause matrix and practice with real gauge readings.