figure 1. Sample data log of system during commissioning.

And yet, the worst system trend is ignoring all the trends.

It is amazing how little we know about what really goes on in most buildings. While much of what happens is none of our business, we are concerned about how key building systems are operating. One of the greatest advantages of using a networked BAS is that we should be able to gain valuable insight into system operations. Unfortunately, what we often see is only a glimpse, an observation at a point in time that may not represent what is actually happening the rest of the time. There are several valuable tools we would recommend for gaining better understanding into the depths of system operation.


Almost all BAS installations provide the ability to trend system variables. Trends (or trend logs) are typically configurable, and can be set up to occur at a fixed interval or upon a change of value or state. While some systems feature a limited amount of trending automatically enabled, most require the operator or controls contractor to set up the trends. Defining a trend is a task that should be fairly easily accomplished, and operators should receive instruction on doing this during training. Despite being a very valuable tool, we find many systems where this function is rarely used. Why?

There are several problems with BAS trends. The largest problem is that the process to set them up is generally not user-friendly; users need to set them up in anticipation of problems, systems have limited capacity to store trend data, and the tools available to view and manage trend data are fairly limited. Most systems provide the ability to view trend data, but in-depth analysis typically requires exporting the data and analyzing it with a tool such as Microsoft Excel. The process of data export can be tedious, and properly using spreadsheets requires skill and training. In short, we find that trends are a valuable but often underutilized tool.


Another alternative for collecting building system trend data is the portable datalogger. These devices are inexpensive, robust, and versatile. The models we use cost around $100; feature built-in temperature, humidity, and light sensors; and collect 43,000 data samples. They are easy to set up and deploy and do a reliable job of collecting information.

When we are evaluating buildings that don’t have a current BAS installed, the use of dataloggers is the way to go. Dataloggers are also an excellent means for validating and commissioning BAS. Deploying a logger allows for ready measurement of values, providing verification as well as often being easier to set up and analyze than trend logs. Of course, the task of analyzing the data typically comes back to the use of a spreadsheet. So, again, good Excel skills are invaluable.


The goal of data collection through trends and loggers is to analyze how well the building is working over time. Is temperature control stable? Are systems shutting off at night? Is the economizer really working? The challenge is to use this information to close the gap between a functioning building and one that is optimized. By collecting and analyzing this data over time, we can start to analyze proper operations and move toward high-performance operations. ES