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Commissioning: Un-Sampled Systems

Over the past two months this column has addressed the issue of sampling systems for field functional performance testing. In the January 2014 column I noted some of the calculated risks the building owner assumes when making the decision to not test 100% of identical new or modified systems. This month I will present two options to help mitigate those risks for less cost than additional field testing.

For example, if the commissioning plan includes field testing 25% of the variable air volume (VAV) terminals in a project, what can be done with the other 75% to confirm some rudimentary compliance with project requirements? With today’s sophisticated and web-accessed building automation systems (BAS), the commissioning professional can observe system operation from his/her office. There is valuable information to be gained from both live graphics displays and historic trend logs.

BAS Graphics

The commissioning professional can log into the BAS remotely and observe each un-sampled system’s graphic display at a single point in time. This may need to be done 2-3 times to capture different modes of operation (e.g., occupied/unoccupied, economizer/non-economizer, etc.). However, the commissioning professional should know when they can expect a specific operational mode and schedule their BAS graphics review accordingly.

With an in-depth understanding of the sequences of operation, the commissioning professional can quickly observe the BAS sensor inputs and controlled device outputs and determine if the systems are performing as required at the particular time. For example, with a VAV terminal unit, if the damper is fully open and not achieving its set point airflow, there is a problem. If the reheat valve is open and there is no air temperature rise across the VAV, there is a problem. If the reheat valve is closed and there is an air temperature rise across the VAV, there is a problem. Any issues identified in this way can be documented and tracked through the commissioning action list.

In 4-season climates, it is unreasonable to expect the commissioning professional to observe all modes of operation in the relatively short time allotted for systems acceptance at the end of construction. However, BAS graphics review will help identify and resolve issues associated with the modes in play at the end of construction, thus reducing, but not necessarily eliminating, the number of potential failures left to be found by the owner’s operations staff.

Trend Logs

Once a system has “passed” the BAS graphics review process, it can move on to the trend log evaluation process. The difference between the two is a single point of time snapshot of system status versus a view of system operation over an extended period of time, respectively.

If specified in the bid documents, the controls contractor can be required to set up, download, and graph trend logs for 100% of the terminal units, regardless of how many will be field tested. This will allow the commissioning professional to review standard trend graphs for the units not subjected to the full rigor of field testing. The graphs of meaningful control points (inputs, outputs, and set points) can be expeditiously evaluated against performance expectations and any anomalies documented and tracked through the commissioning action list.

The specified trend periods should include at least two “cycles” of expected operation. For a commercial office building or school this will probably be two weeks, including weekends. For a hospital, it may only be two days. For an industrial production facility, it may be however long two production cycles take; it could be a few hours or a few days.

Conclusion

Neither of these methods of remote “testing” will necessarily capture unusual, emergency, or safety modes of operation. However, most building owners would not choose to sample field test systems with those types of requirements anyway.

In addition to the end-of-construction remote systems evaluation, opposite season graphics review and trend analysis can be a worthwhile warranty phase activity in order to economically “test” the systems in all operating modes. This should be relatively straightforward assuming the contractor leaves the trend logs programmed and collecting data. Any problems discovered during the opposite season or otherwise deferred testing can be addressed under the terms of the warranties.

 

Commissioning: Getting It Right

Engineered Systems, March  2014

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