Trend logging and using BAS data

A building automation system (BAS) sees almost everything that happens in a building's HVAC systems: temperatures, pressures, valve and damper positions, fan speeds, run status and alarms. Yet in many buildings that information disappears the moment it is displayed on a graphic. Trend logging is what turns a BAS from a remote control panel into a diagnostic and energy management tool.

Good trend data answers questions that are otherwise expensive to settle: why one office is always cold, whether the economizer works, when the chillers really need to start, and whether a retrofit delivered the savings promised. This guide covers how to set trends up properly and how to read them.

What a trend log is

A trend log records the value of a BAS point over time. Each record has a timestamp and a value, and sometimes a status flag. Trends can be stored in the field controller, in a supervisory controller or server, or exported to an external database or analytics platform.

In BACnet systems, the Trend Log object is the standard way to collect this data. It can record values on a fixed interval or on change of value (COV), and the stored records can be read by any BACnet client that supports the object. Older or proprietary systems use their own methods but the principles are the same.

Interval versus change of value

For most analysis work, fixed intervals are easier to use because every point lines up in time. A common approach is to trend analog points at 1 to 15 minute intervals and binary points on COV.

Choosing an interval

• 1 minute or less: control loop tuning, short cycling, hunting valves and dampers.
• 5 minutes: general diagnostics and most fault detection rules.
• 15 minutes: energy analysis and comparison with utility interval data, which in Canada is commonly provided at 15 minute or hourly resolution for larger accounts.
• Hourly or daily: long-term reporting and benchmarking.

When in doubt, collect at a shorter interval. Storage is inexpensive compared with the cost of missing the data needed to solve a problem.

Which points to trend

A sensible minimum for most buildings is listed below. Exact point lists depend on the systems installed and the sequences of operation.

Air handling units

• Supply, return, mixed and outdoor air temperatures
• Supply air temperature setpoint
• Duct static pressure and its setpoint
• Fan speed command and status, and airflow if measured
• Outdoor air, return and relief damper commands
• Heating and cooling valve commands
• Filter differential pressure

Zones and terminals

• Space temperature and active setpoint
• Airflow and airflow setpoint
• Damper and reheat valve commands
• Occupancy mode

Central plant

• Chilled and hot water supply and return temperatures, and setpoints
• Loop differential pressure and setpoint
• Pump speeds and status
• Chiller and boiler status, load and alarms
• Condenser water temperatures and cooling tower fan speed

Trending setpoints as well as measured values is important. Many problems turn out to be reset strategies or overrides that changed a setpoint without anyone noticing.

Storage and data quality

Field controllers usually hold a limited number of records, and when the buffer fills, the oldest data is overwritten. To keep history, trends must be collected regularly by a server or exported to a database.

Check these points when setting up storage:

1. Retention. Keep at least 13 months so that each season can be compared with the same period a year earlier.
2. Time synchronization. All controllers should be synchronized to a common time source. Unsynchronized clocks make cause and effect impossible to sort out.
3. Gaps and duplicates. Network outages and controller restarts create gaps. Monitor trend health so that gaps are noticed and fixed.
4. Units and naming. Consistent point names and engineering units make analysis faster. Semantic tagging using Haystack, Brick or ASHRAE Standard 223P helps when data is shared with analytics tools.
5. Network load. Collecting many trends from field controllers over slow networks such as BACnet MS/TP can affect control performance. Stagger collection and avoid excessive polling.

Turning trend data into answers

Raw trends are only useful if someone looks at them with a question in mind. A practical workflow is:

1. Define the question, such as "Why is the third floor too warm in the afternoon?"
2. Pull related points on one time-aligned chart: space temperature, setpoint, airflow, damper position, supply air temperature and AHU static pressure.
3. Look for the point where the system stops behaving as intended. For example, the box damper reaches 100% but airflow stays below setpoint, which points upstream to the AHU or duct static pressure.
4. Confirm with a site check before making changes.
5. After the fix, trend again to verify the result.

Patterns worth recognizing

• Simultaneous heating and cooling: heating and cooling valves both open on the same AHU, or high reheat in many zones while the AHU supplies very cold air.
• Hunting: valves or dampers oscillating continuously, usually a sign of poor loop tuning. See PID control loops and tuning.
• Flatlined sensors: a reading that never changes is often a failed sensor or a point in override.
• Equipment running unoccupied: fans and pumps running nights and weekends when schedules say they should be off.
• Setpoints stuck at maximum: static pressure or differential pressure reset strategies pinned at the top of their range, often because one rogue zone keeps calling.

These patterns also form the basis of automated fault detection and diagnostics, which runs the same kinds of checks continuously.

Trend data for energy projects

Trend data is the foundation of energy work. In retro-commissioning, it reveals schedule and sequence problems before any field testing. In measurement and verification, trends of equipment run hours, speeds and temperatures can support savings calculations under the IPMVP retrofit isolation options. For utility incentive programs, trend data before and after a project is often required as evidence.

Frequently asked questions

How many points can a BAS trend?

It depends on the controllers, the server and the network. Modern systems can usually trend every analog point in a mid-size building at 5 to 15 minute intervals, provided collection is managed sensibly. Older systems may need prioritization.

Should trend data be sent to the cloud?

It can be very useful for analytics, but it requires a secure network design agreed with IT. One-way data export through a properly configured gateway is a common approach.

Key takeaways

• Trend logs turn a BAS into a diagnostic and energy management tool; without them, most operating problems remain invisible.
• Trend analog points at fixed intervals of 1 to 15 minutes and binary points on change of value.
• Always trend setpoints and commands as well as measured values.
• Store at least 13 months of synchronized, gap-free data outside the field controllers.
• Analyse trends with a specific question, then verify fixes with follow-up trending.
• A periodic control system verification confirms that the points being trended are accurate in the first place.