Understanding COV (Change of Value) Subscriptions

COV subscriptions provide efficient real-time data updates in BACnet systems, reducing network traffic compared to polling.

What is COV?

Change of Value (COV) is a BACnet service where a device automatically notifies subscribers when a point value changes significantly.

COV vs Polling

Method	How It Works	Network Traffic
Polling	Client requests value periodically	Constant, regardless of changes
COV	Server notifies on change	Only when values change

How COV Works

Subscription Process

1. Client sends SubscribeCOV request
2. Server accepts subscription
3. Server monitors point for changes
4. When value changes > COV_Increment:
   → Server sends COV notification
5. Client acknowledges (if confirmed)
6. Repeat until subscription expires

COV Increment

The COV_Increment property defines the minimum change required to trigger a notification:

Point Type	Typical COV_Increment
Temperature	0.5°F or 0.25°C
Humidity	1-2% RH
Pressure	0.1 in.w.c.
Setpoint	0.5°F
Damper/Valve	1-5%

Example: If COV_Increment = 0.5°F:

• Value changes from 72.0 to 72.3°F → No notification
• Value changes from 72.0 to 72.6°F → Notification sent

Binary Points

Binary points (BI, BO, BV) notify on any state change (no increment needed).

Types of COV Subscriptions

Confirmed COV (SubscribeCOV)

• Client must acknowledge each notification
• Guaranteed delivery (retries if no ACK)
• Higher overhead but reliable

Unconfirmed COV (SubscribeCOVProperty)

• No acknowledgment required
• Lower overhead
• Suitable for non-critical data

Subscription Properties

Lifetime

Setting	Behavior
Lifetime = 0	Subscription never expires (not recommended)
Lifetime = 300	Expires after 5 minutes (must re-subscribe)
Typical	300-600 seconds (5-10 minutes)

Best practice: Use reasonable lifetime and re-subscribe periodically.

Subscription Parameters

When subscribing, specify:

Parameter	Description
Subscriber Process ID	Identifies the subscription
Monitored Object	Which object to watch
Issue Confirmed Notifications	Confirmed vs unconfirmed
Lifetime	How long subscription lasts

Configuration

Setting COV_Increment

1. Access the point properties
2. Find COV_Increment property
3. Set appropriate value for point type
4. Value too small = excessive notifications
5. Value too large = missed changes

Server-Side Considerations

• COV subscriptions consume server resources
• Most devices limit concurrent subscriptions
• Typical limits: 50-200 subscriptions per device

Client-Side Considerations

• Must handle incoming notifications
• Re-subscribe before lifetime expires
• Handle subscription rejections gracefully

Best Practices

When to Use COV

Use Case	Recommendation
Frequently changing values	✅ COV
Critical real-time data	✅ COV (confirmed)
Slow-changing values	⚠️ Polling may be simpler
Very large point counts	⚠️ Consider hybrid approach
Devices with limited COV support	❌ Use polling

COV Increment Guidelines

Too Small	Result
Sensor noise triggers constant notifications	Network congestion
Minor fluctuations reported	Graphics flicker

Too Large	Result
Significant changes not reported	Stale data displayed
Alarms may be delayed	Missed conditions

Subscription Management

• Track active subscriptions
• Re-subscribe proactively (before expiration)
• Handle subscription failures gracefully
• Monitor for notification gaps

Troubleshooting

No COV Notifications Received

1. Verify subscription active - Check subscription list
2. Check COV support - Not all devices/points support COV
3. Check COV_Increment - May be set too high
4. Network issues - Verify connectivity
5. Subscription expired - Re-subscribe

Too Many Notifications

1. Increase COV_Increment - Reduce sensitivity
2. Check for sensor noise - May need filtering
3. Review subscription list - Remove unnecessary

Subscription Rejected

Error	Cause	Solution
no-space-for-object	Too many subscriptions	Reduce subscriptions
cov-subscription-failed	Device doesn't support	Use polling instead
unknown-object	Object doesn't exist	Verify object ID

COV vs Polling Decision Matrix

Scenario	Recommended
10 points, fast updates needed	COV
1000 points, hourly trending	Polling
Critical alarm points	COV (confirmed)
Legacy device, no COV support	Polling
Graphics with real-time feel	COV
Batch data collection	Polling

Implementation Example

Niagara

1. Create BACnet proxy point
2. Enable "Poll" or "COV" subscription type
3. If COV: Set reasonable lifetime
4. Configure COV increment on source device

Metasys

1. Integration objects support COV by default
2. Configure refresh rate as backup
3. Monitor COV status in diagnostics

Reference

• ASHRAE Standard 135-2020, Clause 13.1 - COV Services
• BACnet Testing Laboratories - Interoperability guidelines