Trend Log Analysis for Commissioning
Trend logs provide a time-based record of BAS point values that reveals system behavior impossible to observe in real-time. Effective trend analysis is essential for commissioning verification, troubleshooting, and ongoing performance monitoring.
Why Trend Analysis Matters
Benefits for Commissioning
- Verify sequences over time: Confirms operation during unobserved hours
- Identify intermittent issues: Catches problems that don't occur during tests
- Document performance: Provides evidence of proper operation
- Establish baselines: Creates reference for future comparison
- Support troubleshooting: Shows exactly what happened and when
What Trends Reveal
- Control loop stability and tuning
- Sequence timing and transitions
- Setpoint maintenance accuracy
- Equipment cycling behavior
- Occupancy pattern response
- Energy consumption patterns
Setting Up Trend Logs
Key Points to Trend
For each major system, trend:
Air Handling Units
- Supply air temperature (setpoint and actual)
- Discharge air temperature
- Mixed air temperature
- Return air temperature
- Outside air temperature
- Supply duct static pressure
- Cooling valve position
- Heating valve position
- Economizer damper position
- Fan speed/status
VAV Boxes
- Zone temperature (setpoint and actual)
- Airflow (setpoint and actual)
- Damper position
- Reheat valve/stage
- Occupancy status
Plant Equipment
- Chilled/hot water supply temperature
- Return water temperature
- Differential pressure
- Pump speed
- Equipment staging/status
Trend Configuration Parameters
Interval (Sample Rate)
| Application | Recommended Interval |
|---|
| Control loops | 1-5 minutes |
| Setpoints | 5-15 minutes |
| Status points | Change of value (COV) |
| Energy data | 15 minutes |
| Commissioning | 1 minute |
Duration
- Commissioning verification: 1-2 weeks minimum
- Seasonal testing: Full heating/cooling season
- Ongoing monitoring: Continuous with archiving
Change of Value (COV)
For binary and slow-changing points:
- Status points: Any change
- Setpoints: Any change
- Analog with COV: Define appropriate increment
Analysis Techniques
Visual Pattern Recognition
Stable Control
Temperature follows setpoint smoothly
Small oscillations (±0.5°F) are normal
Quick recovery from disturbances
Hunting/Oscillation
Temperature swings above and below setpoint
Regular sinusoidal pattern indicates tuning issue
Large swings (>2°F) need attention
Saturation
Output stuck at 0% or 100%
Setpoint not being maintained
Indicates undersized equipment or wrong setpoint
Correlation Analysis
Compare related points to verify proper relationships:
Example: AHU Economizer
- When OAT < lockout: OA damper should modulate
- When OAT > lockout: OA damper at minimum
- MAT should track setpoint when economizing
Example: VAV Cooling
- When zone temp > setpoint: Damper opens
- When damper at max: Cooling coil activates
- Zone temp should return to setpoint
Timing Analysis
Verify sequence timing matches design:
- Startup delays between equipment
- Staging delays for multiple units
- Proof-of-operation timeouts
- Alarm delay timers
Common Issues Found Through Trends
Control Loop Problems
Symptom: Oscillating Temperature
- Trend shows: Regular cycling above/below setpoint
- Cause: Poor PID tuning
- Solution: Adjust gains (usually reduce proportional)
Symptom: Slow Response
- Trend shows: Long time to reach setpoint
- Cause: Integral gain too low
- Solution: Increase integral or reduce integral time
Symptom: Overshoot
- Trend shows: Temperature exceeds setpoint before settling
- Cause: Proportional gain too high
- Solution: Reduce proportional gain
Sequence Problems
Symptom: Equipment Short Cycling
- Trend shows: Rapid on/off transitions (<5 min cycles)
- Cause: Deadband too small, staging delay too short
- Solution: Increase deadband or staging delay
Symptom: Simultaneous Heating/Cooling
- Trend shows: Both heating and cooling active
- Cause: Deadband/mode changeover issue
- Solution: Adjust changeover sequence, add deadband
Symptom: Economizer Not Working
- Trend shows: Minimum OA even when conditions favorable
- Cause: Enable logic or sensor issue
- Solution: Verify enable setpoints and sensor accuracy
Scheduling Problems
Symptom: System Running During Unoccupied
- Trend shows: Equipment operating after hours
- Cause: Schedule error, override stuck on
- Solution: Correct schedule, clear overrides
Symptom: Uncomfortable Conditions at Startup
- Trend shows: Slow warmup/cooldown recovery
- Cause: Optimal start not working, undersized equipment
- Solution: Enable/tune optimal start, verify equipment capacity
Commissioning Applications
Pre-Functional Verification
Before FPT, use trends to verify:
- All points updating appropriately
- Sensors reading reasonable values
- No stuck or erratic signals
- Communication is stable
FPT Documentation
During functional testing:
- Start trends before test begins
- Continue through test completion
- Archive with test documentation
- Annotate key events on trend
Post-Commissioning Baseline
After successful commissioning:
- Establish baseline trends for normal operation
- Document expected patterns
- Use for comparison during troubleshooting
Tools and Techniques
BAS Native Trending
Most BAS platforms include trending:
- Configure trends in engineering software
- Set intervals and duration
- Export to CSV for analysis
- Limited by controller memory
External Trend Tools
For more detailed analysis:
- Historian software (PI, Wonderware)
- Spreadsheet analysis (Excel, Google Sheets)
- Building analytics platforms
- Custom scripts (Python, R)
Analysis Tips
- Normalize time axes: Align data from different sources
- Filter noise: Use moving averages for noisy signals
- Compare to setpoints: Always plot actual vs. setpoint
- Include outside conditions: OAT helps explain behavior
- Note events: Mark schedule changes, overrides, etc.
Reporting and Documentation
Trend Report Contents
- System/equipment identification
- Time period covered
- Points included
- Key observations
- Issues identified
- Recommendations
Visual Presentation
- Line charts for continuous data
- Stacked charts for related points
- Annotations for key events
- Color coding for easy reading
Archiving
- Export raw data (CSV format)
- Save chart images
- Document trend configurations
- Include in commissioning report
Best Practices
During Commissioning
- Start trending early—before you need the data
- Trend more points than you think necessary
- Use consistent intervals across related points
- Include outside air conditions always
- Document any manual overrides during trending
For Ongoing Monitoring
- Establish baseline patterns
- Set up automated fault detection
- Review trends periodically
- Archive data for future reference
- Update trending as systems change
References
- ASHRAE Guideline 0: The Commissioning Process
- ASHRAE Guideline 36: High-Performance Sequences (includes monitoring)
- Building Analytics Best Practices
- Continuous Commissioning Guidebook (FEMP)
Trends don't lie—they show exactly what the system did. Learn to read them and they'll tell you more than any instantaneous reading ever could.