When reviewing your building automation system, your control’s system trend logs are arguably the most useful tool to diagnose equipment issues you are having. Recognizing the difference between change of value logging and timed interval logging is critical for using your trend log information and analyzing the graphs.

Each unique controls point that is set up for trend logging can typically be set to log at certain time intervals or log any change of status seen by the equipment controller. As a rule of thumb, digital inputs and outputs are assigned to log during a change of value in status. As a reminder, digital inputs/outputs control devices that are simply On/Off or Open/Closed. For example, an air handler supply fan status trend log will log as either ON or OFF. This trend log only logs a new point when the status of the fan changes from Off to On or visa-versa. The benefit of this type of log is that you get the exact time to a fraction of a second when the fan unit changes status. This information can be very valuable when troubleshooting an issue.

That same supply fan might also have its fan speed controlled by a variable frequency device (VFD). This analog output control signal will vary and control the fan speed between 0 and 100%. A fan with a VFD is most likely not running at a constant speed therefore it is more effective to log the performance (fan speed) at a pre-determined time interval. As a rule of thumb, analog inputs and outputs are assigned to log at set timed intervals. 5 minute intervals are usually acceptable time increments for logging a point like fan speed.

Current building automation systems allow you as the building operator to set up the trends you would like to log. The value of your trend in the graphs that are populated when you select the trends and time period you want to view. Older systems may require you to export the trend log data in CSV format to Excel where graphs can be created from the imported data.