Commissioning, Maintenance and Troubleshooting
This domain tests the process of proving a system works as designed and the diagnostic order used when it does not.
Core rules
- Commissioning starts from the owner's project requirements and basis of design, then pre-functional checklists, then functional performance testing, with a test and balance report and an issues log. Seasonal testing verifies heating and cooling modes.
- Maintenance includes filter change, belt tension, coil cleaning, condensate management, lubrication, economizer inspection and combustion analysis: oxygen, carbon dioxide, carbon monoxide, stack temperature and draft.
- Troubleshoot in order: verify the call, verify airflow, then refrigerant charge, then controls and safeties. Airflow first prevents misdiagnosing charge.
- Refrigerant diagnosis pairs superheat with subcooling. Undercharge gives high superheat and low subcooling; restriction gives high superheat with abnormal subcooling; overcharge gives elevated subcooling.
Worked example
A 3-ton unit cannot hold setpoint. Supply-to-return temperature difference is 10F when 16 to 22F is expected, and external static pressure is 0.9 in. w.c. against a 0.5 in. w.c. rating. On the blower curve this corresponds to roughly 900 CFM instead of 1,200. The fault is airflow, not charge: restricted return, clogged filter or undersized duct. Correct the airflow, then recheck superheat and subcooling at steady state before adding refrigerant.
Traps that cost marks
- Adding refrigerant before verifying airflow and cleanliness.
- Checking charge with a dirty coil or before steady state.
- Skipping combustion analysis, missing carbon monoxide from backdrafting or a cracked heat exchanger.
- Completing pre-functional checklists without functional performance tests.
- Closing out without recording deficiencies in an issues log.