1. Triage safety
Rule out combustion, electrical, refrigerant, and drainage hazards before anything else—that comes ahead of routine troubleshooting, not after.
2. Measure
Actual measurements come next: temperatures, pressures, airflow or water flow, control behavior, and venting—not guesses based on the equipment's age.
3. Define the scope
A written scope should spell out the load assumptions behind it, what's changing in the distribution, what permits apply, and how commissioning will be verified.
4. Start up and verify
The job isn't finished until performance is actually measured, controls are configured, drainage is checked, and someone walks the owner through what was done.