System Diagnostics and Performance
System Diagnostics and Performance tests commissioning and troubleshooting with measured data: temperature split, static pressure, airflow, refrigerant charge, and electrical draw. The exam wants a sequence, not a guess.
Establish baseline
Run the system to steady state. Measure return and supply dry-bulb temperatures; a typical cooling temperature split is 16–22°F. Measure external static pressure at the proper locations and compare to the blower performance table. Verify airflow before judging charge. Then use superheat or subcooling per the metering device and record amp draw.
Interpret, don't guess
A high split often means low airflow, not overcharge. A low split can mean low load, high airflow, or undercharge. High total static with normal split points to duct restriction. Confirm with blower table and filter pressure drop.
Worked example
A 3-ton system has return 75°F, supply 57°F, split 18°F, but total external static is 0.90 in. w.c. against a 0.50 rating. The split looks acceptable, yet airflow is below design. Inspect the filter, coil, and duct restrictions; correct static before charging adjustments.
Traps
- Adjusting charge before airflow.
- Measuring split at the wrong location.
- Using dry-bulb alone to diagnose charge.
- Ignoring blower table and nameplate data.