“Wilde Lake Middle School initially used a combination of occupancy sensors and the Aircuity CO₂ readings to control the ventilation in our building. We soon ran into a series of issues with the occupancy sensors,” said Tim Heinrich, HVAC project manager, Howard County Public School System. “Meanwhile Aircuity validated the sub-ambient indoor CO₂ levels that were measured in the classrooms. We have since disabled the occupancy sensors and put our full trust in the accuracy and reliability of our Aircuity system.”

Identifying the Cause of Abnormal Carbon Dioxide

After the building opened in 2017, Howard County personnel noticed something unique happening — after school ended on a Friday evening and through the duration of the unoccupied weekend, Aircuity's data showed carbon dioxide (CO₂) levels within a classroom space dropping below ambient conditions while rising back up as students and staff returned on Monday morning (Figure 1).

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FIGURE 1: Initial discovery of abnormal indoor CO₂ behavior, Feb 2017

MEG went to work trying to figure out the cause. Initially they thought it was a faulty CO₂ sensor, and the sensor was swapped out. The CO₂ readings were reviewed to see if the issue was resolved; however, the indoor readings were still falling below ambient. In addition, data from other classrooms were checked and found to be exhibiting the same behavior, with CO₂ levels dropping below the outside air conditions (Figure 2). Next, they wondered if it was due to a kink in the tubing. However, with multiple rooms falling to nearly the same CO₂ offset below ambient, and Aircuity being a centralized sensing technology, kinked tubing was deemed highly unlikely.

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FIGURE 2: Checking initial solutions and occurrence in multiple rooms, March 2017

Aircuity’s centralized sensing architecture and reliable sensor technology ruled out a number of traditional causes of CO₂ reading errors, such as sensor drift. This provided MEG the confidence to suggest the possibility that CO₂ in the classrooms were indeed dropping below ambient. A third party CO₂ datalogger was installed in one of the classrooms beside the Aircuity sample port and data was gathered over an unoccupied weekend. The datalogger test results validated the Aircuity system’s readings (Figure 3). Note that while the time scale shows the same noon Friday to noon Monday time period as the previous graphs, the initial peak in the classroom CO₂ levels for both the datalogger and Aircuity values begins much lower, at roughly ambient levels. Checking the calendar for Friday, April 14th, 2017, revealed that this was Good Friday and that Wilde Lake Middle School was closed for the holiday. As a result, the classroom had begun its unoccupied CO₂ level decline at least a day earlier on Thursday and from a reduced occupancy level, as the students were off that week for Spring Break and only staff and faculty would have been present on Thursday.

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FIGURE 3: Datalogger test results, April 2017.

After seeing the results of the datalogger test in addition to watching the data and occupancy closely, Howard County's confidence in the accuracy of the data provided by Aircuity was restored.

Aircuity and MEG searched for potential explanations for this counterintuitive indoor air condition. The simplest option, high indoor vegetation density, was quickly ruled out as few indoor plants were present in the building. Further investigation yielded one of the most likely explanations, indicating that the unusual conditions could have been caused by the carbonization of concrete. According to academic research*, “As CO₂ comes in contact with a concrete surface, the calcium hydroxide in the concrete reacts with carbon dioxide in the air to form calcium carbonate.” With Wilde Lake being an NZE building, it had a very tight building envelope, and the installed DOAS unit was turned off during unoccupied times.

Additionally, being a K-12 project meant that the building would have long, unoccupied periods when school was empty during the weekends. In this type of environment, small effects on indoor air quality (IAQ) can build up and appear magnified compared to what is traditionally thought to be possible.

The probable culprit was identified, and as the cement continues to cure over time, CO₂ levels should start to level out closer to ambient. The case of Wilde Lake Middle School identifies new IAQ conditions that need to be considered in future construction projects, especially where tighter building envelopes are being used to meet energy goals or stricter code requirements. Aircuity can help identify and address the undiscovered IAQ conditions that may develop inside future buildings.

 “The accuracy, reliability, and local support offered by Aircuity was an important design consideration, especially towards sustaining net zero energy throughout the operating life of the school,” said Mike Sherren, P.E., vice president, James Posey Associates.

By Sarah Callahan, Aircuity. *Source: Lopez, Ashley Lane. Carbon Dioxide Absorption by Various Types of Pavement Binder Materials

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