What It Is
Change of Value, usually shortened to COV, is a BACnet subscription mechanism that reports a point when its value changes instead of forcing a client to poll continuously.
That can reduce network traffic and improve responsiveness, but only when the device actually supports the required COV behavior and the subscription settings are appropriate.
COV Versus Polling
| Aspect | COV | Polling |
|---|---|---|
| Network traffic | Only when subscribed values change | Every poll interval |
| Response timing | Often near real time | Limited by polling interval |
| Support model | Optional and device-specific | Universally available through standard reads |
| Commissioning risk | Requires subscription and increment tuning | Easier to start, heavier at scale |
Key Concepts
- COV Increment: the minimum change that triggers an update for a value-oriented point
- Subscription Lifetime: subscriptions expire and must be renewed
- Confirmed Versus Unconfirmed Notifications: affects acknowledgement and delivery behavior
Why It Causes Integration Confusion
Teams often assume BACnet devices that expose points also expose useful COV behavior. That is not guaranteed. A device may support polling well, support COV only for selected objects, or support it in ways a supervisory platform does not use cleanly.
Before designing around COV, verify the device documentation, the expected object types, and the actual behavior seen in tools or packet captures.