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Commissioning: Trend Logs

Using data trend logs to supplement in-field functional performance testing has always been an option in the commissioning process, particularly for HVAC systems controlled by direct digital control (DDC) systems. As DDC system trending capabilities and computer hardware storage capacities have improved substantially over the past few years, I propose that trend logs be given a more prominent place in the commissioning toolbox.

A well-defined set of graphical trend logs can be quickly analyzed to ascertain if controlled devices are performing as specified and/or if there are any rogue points that need attention. The best trend graphs have points deliberately grouped together to provide the commissioning professional a visual snapshot of a system’s operation and the interaction between related parameters. For example, the discharge air temperature, the heating and cooling control signals, and the discharge air temperature setpoint would be on the same graph.

In an ideal world, trend logs would be run for at least one week as part of the pre-functional check-out process. The trends would be submitted to the commissioning professional for review and comment and any anomalies would be corrected and trends re-run for another week prior to on-site functional performance testing. This would catch many (but not all) system deficiencies before the contractors, owner, and commissioning professional committed time and other resources to the intense functional performance testing process on site.

Why Not?

This detailed pre-test trending program is not currently a common practice for new construction commissioning for a number of reasons. The top reasons include:

  • The pre-functional trending/review process inherently adds time to the commissioning schedule which may or may not be on the project’s critical path. Realistically, the minimum additional time might be two weeks; a week of trending, a few days for processing the trend data and submitting it to the commissioning professional, and a few days for review and comment. If the trends indicate no problems that need to be addressed by the contractor, that’s it. If there are deficiencies brought to light through the trend analysis, a few days will be needed to correct the issues. After that the clock will begin again for another round of trending to demonstrate that the problems were successfully resolved.

  • Field time will be required by the commissioning professional to validate the control system points prior to relying on those points in the trend analysis. This step could be skipped (with control point issues becoming evident during the trend log analysis) at the risk of rendering the two week trending process ineffective.

  • Pre-functional trending will only capture a subset of system performance conditions. For example, the trending will only be for one climatic season, i.e., whenever the construction project is completed. Also, the trends will not be indicative of a normal occupied/unoccupied facility but of an empty facility with little to no people or thermal loads.

Why?

Even with those caveats, I believe that commissioning professionals and owners should consider the use of trending wherever and whenever possible. The positives include:

  • Field functional performance testing will go smoother and take less time if some system deficiencies can be identified through trend logs and corrected before testing begins.

  • Having the trend logs programmed prior to testing will allow for a record of the functional performance testing and all system responses to the various test steps. Analyzing trend logs collected during testing may help troubleshoot problems and expedite their correction.

  • If trend logs are programmed and validated during the commissioning process, they can remain in place and be used for on-going commissioning and analysis throughout the life of the systems. The trend logs can move from the commissioning toolbox to the operations tool box.

Over the next few months, I am going to use this column to demonstrate examples of systems performance information that can be gleaned from trend log analysis. The format will be one of introducing a system or sub-system, showing a trend graph, and then leaving it up to the reader to discover the operational deficiency illustrated in the trend graph. The “answer” will be given at the start of the subsequent month’s column.

 

Commissioning: Getting It Right

Engineered Systems, September, 2011

by
Rebecca Ellis, PE, LEED AP BD+C, CCP, CPMP, CxA
President
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318
rteesmag@QSEng.com