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The Cx Files

Chapter 2: The Case of the Stuffy Building

Jan 19, 2022

Michael Flemming PE, LEED AP, BD+C, CxA, CEM

It all started with a sensor, a carbon dioxide (CO2) sensor to be exact. Why would a sensor read an empty room as if it were jam packed with people? Simple you think – an improperly calibrated sensor. Except, replacing the sensor didn’t fix the reading and then additional sensors were discovered to also be reading far higher than expected. No hidden graveyard was discovered in the bowels of the building, and we quickly ruled out ghosts; so the real commissioning began as the team got started on solving the mystery of the stuffy building.

New tenants renovate existing office buildings often, as was the case for this project. Frequently, the existing equipment in the building gets reused so that the new occupant can takeover the space as quickly as possible. But, as we learned here, utilizing existing equipment that hasn’t been properly tested can cause everything from surprise to extreme frustration and headaches.


In modern buildings, engineers use CO2 as a method to determine how many people are in a space and to ensure that the building’s HVAC systems reacts to provide enough fresh air for the people inside. But why would a building that’s empty have CO2 levels equivalent for a conference room filled with people? Why would multiple brand-new sensors throughout the building read so much higher than expected? Could it be that the sensors aren’t really at fault and are instead the first of many clues?

Clue #1: The sensors

The first clue was finding multiple sensors throughout the building with high CO2 readings. One or two sensors out of calibration can sometimes occur, but to have the majority of sensors throughout the building read high likely represents another problem. Short of somebody breathing heavily on all the sensors, the readings in the space definitely didn’t represent a fully functioning system which was easily observable.

Clue #2: The lack of air

The second clue was that throughout testing, the building seemed to struggle circulating the sufficient amount of air. Although the contractors were able to adjust the equipment to meet the design requirements, the fans seemed to run at max speed and use more energy than the energy model expected. While the team assumed that this was simply cause by the existing ductwork, to the untrained eye it did not appear to be part of any bigger issue.

Clue #3: The difficulty of cooling

The final clue came on a mild day as the team tested the cooling capacity of the building and discovered that although the outside temperature was cool, the space inside was not cooling down. Typically, on mild days, a system like this is designed to bring in fresh air to cool and refresh the space and have the mechanical cooling locked out from operation to conserve energy.

On their own, each of these clues could be different minor issues, but together, like any good mystery, they told a bigger story. The first issue told us that the space was not getting sufficient fresh air, while the second clue showed there was likely an obstruction in the airflow, and the final clue, like the first, also led the team to believe that the amount of fresh air the equipment was providing wasn’t doing the job.

The Great Reveal!

In the end, these clues led us to discovering a problem with the existing equipment that had not been previously identified. After extensive testing, we determined that although the existing controls said the ventilation dampers on the rooftop equipment were opening, the controls’ actuators had been disconnected, and no fresh air was entering the building! This lack of fresh air resulted in CO2 levels getting higher and higher in the building although there were very few people inside.

How could this have been avoided? Simple! Existing equipment should not be assumed to be in proper working condition. Tenants can avoid inheriting facility problems by ensuring that the commissioning scope includes at least a cursory review of existing equipment, so that old problems don’t lead to new headaches.

Mystery Solved!

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