What is the BACnet COV (change of value) service ?

Key Takeaways : What is the BACnet COV service

  • Real-world measurements (temperature, humidity, etc.) change constantly, but transmitting every micro-variation would overload BACnet networks.
  • The COV service filters updates by reporting only meaningful changes (e.g., notify only if the temperature changes by at least 0.5°C).
  • A maximum reporting interval ensures regular refreshes, even without changes.
  • ⚠️ The BACnet standard does not define a minimum reporting interval → if thresholds are too fine, the system can be flooded with notifications.
  • COV reduces network load, optimizes system resources, and improves efficiency compared to continuous polling.
  • Best practice: set thresholds adapted to the use case, always define a maximum interval, and fine-tune configurations.
  • As Olivier Hersent puts it: “The COV service is a cornerstone of BACnet efficiency: it ensures that networks carry only what really matters, balancing precision and scalability.

Table of Contents

Introduction of BACnet COV (change of value) service

In modern building automation, efficient data exchange is essential. BACnet, the international communication standard for building management systems, has become a cornerstone of smart buildings and IoT integration. By providing a common language for devices such as HVAC controllers, lighting systems, and security equipment, BACnet makes it possible to coordinate and optimize building operations seamlessly (What is BACnet?).

However, not all data changes are equally important. Sensors continuously measure real-world values such as temperature, humidity, or pressure — and these values may fluctuate slightly every second. Continuously transmitting every micro-change would quickly overload the network, creating unnecessary traffic. This is where the Change of Value (COV) service comes in.

Why do we need COV?

Real-world measurements, such as temperature or humidity, are in constant motion. A thermometer in a room may detect micro-variations every second, even if the perceived environment feels stable. If every one of these small variations had to be reported through the BACnet network, the result would be a flood of messages that would not only overload the system but also waste bandwidth and processing power.

This is why the COV service was introduced: it allows devices to report only the changes that actually matter. Instead of notifying the network every time the value shifts slightly, a threshold can be defined — for example:
“Notify me only when the room temperature changes by at least 0.5°C.”

By filtering out insignificant variations, the COV service ensures that the BACnet network transmits relevant, actionable updates, while avoiding redundant traffic.

How does the COV service work?

The Change of Value (COV) service in BACnet allows devices to report updates only when certain conditions are met. Instead of a continuous stream of data, the network only receives information when it is meaningful.

Two main parameters are used to configure COV reporting:

  • Change threshold – defines the minimum variation required to trigger a notification.
    • Example: a temperature sensor only reports if the value changes by 0.5°C or more.
  • Maximum reporting interval – ensures that even if no change occurs, an update is still sent after a defined period of time. This guarantees a regular refresh rate and prevents outdated values from remaining in the system.

⚠️ However, it is important to note that the BACnet standard does not define a minimum interval between notifications. If the threshold is set too low, devices may send too many updates, potentially flooding the system. Careful tuning is therefore essential.

Parameter Description
Change thresholdMinimum change in value required before a notification is sent
Maximum reporting intervalGuarantees updates at regular intervals even without changes
Benefits of COV

The introduction of the COV service provides a major improvement in how BACnet networks handle data. Instead of being overwhelmed by redundant messages, systems receive only the information that matters. This leads to:

  • Reduced network load – less traffic means smoother communication and fewer risks of congestion.
  • Relevant notifications – operators only see significant changes, which improves decision-making.
  • Optimized performance – devices and controllers save processing resources.
  • Improved energy efficiency – avoiding unnecessary communication reduces overall system stress.
Continuous polling COV service
High network loadOptimized traffic
Repetitive & redundant dataOnly meaningful updates
Waste of bandwidth & resourcesEfficient use of system resources

By focusing on useful updates rather than raw data streams, the COV service helps ensure both scalability and reliability in modern building automation systems.

Best practices and caveats

While the COV service is a powerful tool, it must be used wisely to avoid unexpected issues. Here are some best practices to keep in mind:

  • Choose a relevant threshold: The value should reflect what is meaningful in the specific use case. For example, in temperature monitoring, a threshold of 0.5°C makes sense, but for pressure or humidity, different values may be appropriate.
  • Always define a maximum reporting interval: This ensures that the system is regularly updated, even if no significant change occurs.
  • Avoid thresholds that are too fine: If the change threshold is set too low, the system may generate a large number of notifications, leading to network congestion.
  • Test and adjust configurations: Every building and system is different. Proper testing allows operators to balance accuracy and efficiency.

⚠️ Remember: the BACnet standard does not define a minimum reporting interval. If thresholds are poorly configured, the system could easily be flooded with notifications, undermining the efficiency that COV was designed to deliver.

Expert insight
Olivier Hersent

The COV service is a cornerstone of BACnet efficiency: it ensures that networks carry only what really matters, balancing precision and scalability.

This perspective highlights the key role of COV in making large-scale building automation both scalable and sustainable.

To conclude

The Change of Value (COV) service is a fundamental feature of BACnet that ensures building automation systems remain both responsive and efficient. By reporting only meaningful changes and enforcing maximum reporting intervals, COV prevents network overload while still providing operators with the data they need to make informed decisions.

For a broader understanding of how BACnet enables interoperability across building systems, you can also read How does BACnet work?.

Frequently Asked Questions (FAQ)

COV means Change of Value. It is a BACnet service that reports data only when a property changes significantly, instead of continuously sending all variations.

Because real-world measurements (temperature, humidity, etc.) change constantly. Without COV, reporting every micro-change would overload the network with unnecessary traffic.

COV filters out minor fluctuations and only transmits updates that exceed a defined threshold. This reduces redundant messages and makes the system more efficient.

  • Continuous polling requests data repeatedly, creating high network traffic and redundant updates.
  • COV only sends a notification when the data has changed enough to matter, optimizing network resources.

Yes. You can set a change threshold (e.g., 0.5°C for temperature changes) and a maximum reporting interval to guarantee periodic updates.

No. BACnet does not define a minimum reporting interval. If you set the threshold too low, you may flood the system with notifications.

 

COV is particularly valuable in HVAC systems, energy monitoring, lighting control, and any scenario where real-time but efficient reporting is needed.

By reducing unnecessary data traffic, COV makes networks more scalable, saves energy, and ensures that only meaningful information reaches operators and controllers.

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