The BACnet Trend Log object lets a device locally store successive values of a Property with timestamps, which the BMS can later retrieve for historical analysis.

Trend Log is on-device historisation. Sampling can be cyclic (every X seconds) or driven by COV, and the data is stored in the device's RAM or flash, typically up to a few hundred or a few thousand samples depending on the model. The BMS retrieves the data via the ReadRange service. Trend logs are essential for HVAC performance analysis, retrospective fault diagnosis and contractual evidence of operation.

Always check the Trend Log buffer size and overwrite policy: a controller that silently overwrites old samples while the BMS is offline produces gaps that no SQL can recover.