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Buildings As a CO2 Battery?

We know carbon dioxide (CO2) can be sequestered in materials, but have you ever thought about buildup indoors? Mechanical or natural ventilation brings in fresh air from indoors to dilute the CO2 emissions we exhale, but when our ventilation systems set back during unoccupied hours, CO2 can be stored indoors and linger on long enough to affect the background indoor levels the following day.

When we think about your office, school, place of worship, or residence, we usually don’t think about CO2.  But it’s there. Even in ambient air outdoors, background levels can typically range from 400-500 ppm at the lowest.

As we strive for the best possible Indoor Air Quality in our office building, Morrissey Engineering staff observed this phenomenon firsthand.  Each day employee exhalations added CO2 to the building and ‘background indoor’ levels only dropped to match outdoor background levels over weekends, when the building remained unoccupied for a longer duration. Weekday evenings saw lingering CO2 levels, even after occupants departed.

The figure below depicts our CO2 trends from a typical work week and shows the weekend reset of CO2 and the weekday accumulation, before and after a programming change we made in an effort to lower overall CO2 levels in the building. As shown in the charts below. Background CO2 average during unoccupied weekday hours was hovering around 560 ppm but after controls modifications dropped down to outdoor ambient levels of 432 ppm average (as low as possible).

Co-authored by Sarah Gudeman & Jerry McNerney