System Performance, Commissioning, and Diagnostics
What the exam expects
This domain ties everything together. You must verify charge using the method the manufacturer specifies - subcooling for TXV systems, target superheat for fixed-orifice systems - and confirm the result with airflow, delta-T, and static pressure. Weighing in charge by scale is the most reliable method when line-set length or a factory holding charge makes other methods unreliable.
Commissioning means documenting what you measured: ambient and indoor conditions, suction and liquid pressures, line temperatures, superheat, subcooling, TESP, CFM, temperature split, and combustion readings on the heating side. Diagnostics is a process: gather symptoms, form a hypothesis, test, and confirm - not swapping parts.
Traps
- Charging by suction pressure alone, especially in low ambient.
- Checking charge with a dirty filter or blocked coil.
- Confusing a low delta-T caused by low airflow with a low charge.
- Not allowing the system to stabilize before reading.
- Skipping the heating-side combustion check at commissioning.
Worked example
Use the target superheat formula for fixed-orifice systems: target SH = (3 x indoor wet bulb - 80 - outdoor dry bulb) / 2. With 63°F indoor wet bulb and 95°F outdoor dry bulb: (189 - 80 - 95) / 2 = 7°F target superheat. Set the charge so measured superheat matches that target, then verify delta-T and airflow before signing off.