The trending feature in the building automation system (BAS) is a tremendous asset to Owners, facility staff, contractors, and commissioning agents, and can save significant time and money for all of the above. If you are familiar with the BAS and trending, then skip ahead to suggestions on trending at the bottom of the page. Otherwise keep reading!
Due to advancements in building automation technology, increases in computing power, and incorporating more components within the facility into the BAS, building Owners potentially have an immensely useful tool at their fingertips, if is set up properly.
While it is true that the BAS is a powerful tool in its own right to be able to see what is happening in the building at THIS point in time, it is often severely underutilized when it comes to its potential to assist with identifying contributing factors leading to wasted energy and equipment failure.
In order to fully exploit the capabilities of the present-day BAS, the trending feature of the BAS should be set up to collect and store as much data as possible relating to key components (see below). With trending properly in place, the operator can see all data that have been generated by the BAS, allowing them to look back through a window of time to see what has been happening. This information is crucial when trying to determine a chain of events that happened prior to a failure, in seeing how a programming change has affected the operation/efficiency of the system, or if the HVAC system was able to keep up during recent temperature extremes.
With trending we are able to see exactly what the building systems and components have done at any point in time, and how that may have changed over time. This trending is vital in the commissioning process: during the functional performance testing and on-going commissioning phases, and/or for general facility maintenance if you don't have on-going commissioning in place.
Unfortunately, the trending feature within the BAS is often underutilized, not adequately specified, or not fully set up. One of the frequent comments we make when doing design-phase reviews is in regard to the automated temperature control specifications. We usually ask the Design Engineer to specify longer data retention periods, more trending points, trending intervals, etc. In the past, there was a valid reason to limit what was trended and for how long as memory was more expensive and systems were not configured in a way to allow for mass storage. This has changed with lower costs for memory and computing power and a move to web-based controls systems. It is now cheaper and easier to increase the number of points which are trended and the retention period before those data are either deleted or over-written. It is important that the Design Engineer give detailed requirements in the contract documents specifying the number of points to be trended, how long to trend them, and trending intervals. If this isn’t clearly specified, then it can result in a costly change order later in the project to get the trending capabilities increased. It is imperative that trending be specified well in the design phase and set up properly in the construction phase.
During functional testing, both on new projects and when retro-commissioning, we often find that trending was never fully set up, or that very few points are being trended. This makes it difficult to verify how systems have operated under their own control, how they responded to weather conditions over a recent period, or if the equipment is operating correctly.
In my opinion, trending should be mainly used after the initial pass of functional testing in the commissioning process where we verify the mapping of points, t-stats, input/output commands, safeties and alarms, sequence testing, and have verified that all components are working properly, both individually and as a system. The use of trending at this point is to watch the system operate under its own control without manual overrides or manipulations. We can ensure that systems react appropriately to changes in weather conditions, maintain set points without excessive drift and that nothing is “hunting” (as hunting is inefficient and hard on equipment).
Trending is also utilized during the monitoring-based-commissioning process and is also capable of obtaining additional LEED credits under the current version 4 as one of the paths in enhanced commissioning.
BAS Trending Suggestions
We would recommend that ALL data points within the BAS be trended; however, if this is not feasible then at a MINIMUM we would recommend that these points are trended (adjust for your system):
Heating loop supply and return water temperatures
Heating loop pump(s) status:
Speed (percent or hz)
Heating loop differential pressure and set point
Chilled water supply and return temperatures
Cooling coil valve position/Dx stages
Chiller/Cooling Tower/Dry Cooler/etc. status:
Cooling loop pump(s) status:
Speed (percent or hz)
Cooling loop differential pressure and set point
Air Handling System
Air Handling Unit(s) information:
All space temperatures, space temperature set points, and equipment data points for the equipment serving the spaces (i.e., VAV box or FTR valve positions, air damper positions, airflow, discharge temps, etc.)
The above-mentioned minimum points to trend should be trended at intervals as small as possible, but not longer than every 15 minutes. Ideally a trend point would be logged with every change-of-value to truly be able to see what your system is doing in any given period. Anything that saves a trend point greater than every 15 minutes is pretty limited in its usefulness, as a lot can happen in 15-30 minutes. Points should be saved in the BAS for at least a year; this allows you to see how the building has operated over the seasons.
The data trends recommended above are a minimum requirement to really allow for a high-level view of how equipment is functioning. These trends will allow for the identification of any area(s) in which efficiency can be improved as well as pinpoint contributing factors should equipment failure occur.
Utilizing trending data is a powerful tool in our commissioning process and with its reduced cost in recent years should be in place in every facility.
Don’t forget to subscribe to get informed whenever new articles are posted!