“Wilde Lake Middle School initially used a combination of occupancy sensors and the Aircuity CO2 readings to control the ventilation in our building,” said Tim Heinrich, HVAC project manager, Howard County Public School System. “We ran into a series of issues with the occupancy sensors. To remedy that, 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.”

Discovery of the Undiscovered

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 CO2 levels within a classroom space dropping below ambient conditions while rising back up as students and staff returned on Monday morning (Fig 1).

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 (Fig 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.

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 levels 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 14, 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, April 13, 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.

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 dedicated outdoor air system (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.

The Wilde Lake Middle School is the latest example of HCPSS’ commitment to sustainability, as recognized in Policy 6080. The school system is dedicated to performing as an environmentally, economically, and socially sustainable organization, following practices that create a healthy environment, engage staff and students in developing environmental literacy, and strengthen operations. From an architectural design perspective, HCPSS is the only school system in the state to pursue all new school constructions and full systemic renovations to be LEED certified or better.

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

Howard County Public School System’s Sustainability Commitment 

The Wilde Lake Middle School offers a green learning environment with natural daylight, enhanced classroom acoustics, improved IAQ, thermal comfort, and opportunities to integrate green features into the school’s curriculum. From a construction perspective, as a green school, the building conserves energy, water, and materials, thus reducing negative impacts on human health and the environment. This kind of capital investment can also defray operating expenses. For more information, visit http://wlms.hcpss.org.