Commercial HVAC troubleshooting should start with the operating symptom, not a parts list. Use this field guide to diagnose the failure boundary before replacing equipment. The articles below connect BAS evidence, mechanical checks, electrical status, operating sequence, and field verification so facility managers and technicians can identify the correct failure boundary.
Start With the Symptom
The unit runs, but the space is not cooling
- RTU Running but Not Cooling: Six Checks Before Calling for Service
- VAV Box Not Cooling: What to Check Before Replacing the Controller
- AHU Supply Air Temperature Will Not Reach Setpoint
- Commercial HVAC Compressor Will Not Start
Airflow, fan, or duct pressure is wrong
- AHU Static Pressure Alarm Keeps Coming Back
- Fan Coil Unit Fan Not Running
- VFD Running but BAS Says Off
- BAS Graphic Says ON but the Equipment Is OFF
A safety, alarm, or operating sequence does not make sense
- AHU Freezestat Tripped but Cooling Is Still On
- Commercial Chiller Keeps Alarming
- Commercial Boiler Short Cycling
- Cooling Tower Fan Problems
A damper, valve, sensor, or economizer is not responding correctly
- 0–10 V vs 2–10 V Actuator Control
- BAS Sensor Reading Wrong
- RTU Economizer Not Working
- HVAC Pump Hunting
Use a Five-Layer Diagnostic Model
| Layer | What to Verify | Typical Evidence |
|---|---|---|
| 1. Operating condition | Schedule, occupancy, demand, mode, setpoint, outdoor condition | BAS status, alarm history, trend timestamps |
| 2. Control command | Effective command, priority, interlocks, safeties, sequence state | Command value, priority array, program logic |
| 3. Field response | Relay, actuator, VFD, contactor, valve, damper, fan, compressor | Voltage, feedback, proof, local display, physical movement |
| 4. Mechanical result | Airflow, pressure, temperature change, water flow, heat transfer | Independent meter readings and equipment measurements |
| 5. Verification | Normal operation after correction under realistic load | Stable trends, alarms cleared for the correct reason, documented test |
Before Replacing a Controller or Mechanical Component
- Confirm the complaint and capture the original operating state before resetting equipment.
- Separate a BAS command from equipment status and physical proof.
- Verify power, safeties, overrides, and local or hand modes.
- Compare the BAS reading with an independent field measurement.
- Follow the actual manufacturer sequence and approved drawings.
- Correct the confirmed cause, then test both normal operation and protective shutdown.
Preventive Maintenance and Seasonal Planning
- AHU Maintenance Checklist
- Commercial HVAC Preventive Maintenance Checklist
- Commercial RTU Preventive Maintenance
- Commercial Chiller Maintenance
- Commercial HVAC Maintenance Frequency Guide
Safety: Commercial HVAC equipment can involve hazardous voltage, rotating equipment, refrigerant, hot water, steam, combustion, and critical building operations. Follow site procedures, lockout/tagout requirements, manufacturer instructions, and qualified-person requirements.