What is the primary purpose of a trend log in a DDC system?

• A. To record historical data for analysis, troubleshooting, and performance verification.
• B. To auto-tune the controller.
• C. To generate real-time alarms only.
• D. To maintain a backup of the firmware.

Answer: (A)

Trend logs in a DDC system capture time-stamped measurements from sensors and controller outputs over time to create a historical record. This enables you to analyze how variables behave after events, verify that the system met its targets, and pinpoint when and where performance drift or faults occurred. In practice, you can review trends to understand how temperature, flow, or valve positions changed in response to setpoints, identify slow drifts, and confirm that the system performed as intended.

Auto-tuning the controller is a separate function that adjusts control parameters based on the system’s response. Real-time alarms are managed by the alarm system, and while trend data can aid in diagnosing alarm events, trend logs themselves aren’t meant to generate alarms. Maintaining a firmware backup serves a different purpose entirely, unrelated to collecting historical process data.