Q: Is 100% outside air needed? What are the energy impacts of increasing air flow?
A: 100% outdoor air generally is not necessary. The 800 ppm CO2 recommendation for indoor air quality provides a good balance between improving air quality and energy efficiency. This recommendation results in significantly improved air quality while minimizing utility bill impacts. And though there will be a relatively small increase in energy costs, these costs will be more than offset by improved employee productivity and fewer sick days.
Q: What percentage of outside air is recommended for crowded spaces?
A: Crowded hallways, entrance spaces, and other confined spaces need sufficient air flow to keep carbon dioxide 800 ppm (40 cfm per person). During the pandemic, distancing must be practiced to keep direct-hit infections reduced. A 12' by 60' hallway should keep occupant density to no more than 24 people (~30 sqft per person), with a fresh air flow of 960 cfm. If the ventilation system operates at 2000 cfm, outside air dampers should be set to 50% fresh air. Ideally, CO2 sensors that can modulate fresh air should be used for active fresh air control.
Q: At what level of concentration does CO2 affect cognitive thinking or cause minor discomfort affecting performance?
A: According to a study from the Harvard TH Chan School of Public Health, cognitive performance in multiple areas decreases when CO2 levels exceed 1000 ppm. See this follow up study to explore value of cognition performance versus cost of ventilation. Our recommendation of 800 ppm CO2 is well within recommended guidelines for CO2 levels.
Q: Should I open windows for increased ventilation?
A: When the temperature and humidity is appropriate, natural ventilation is more cost effective than mechanically supplied air, but it's very important to consider how the air moves through the space. Air entering through windows is uncontrolled and variable. Wind on the face of a building will blow into the building while an open window on the opposite side of the building may be drawing air out of the building. This can be potentially problematic if an asymptomatic person is sitting next to a window with air blowing into the room. The air from the window may carry contaminants from that person into the room. The opposite could happen on the other side of the building. Open windows will affect airflow as the pressures outside an open window fluctuate. We recommend using supplemental fans when opening windows to ensure circulation.
Q: What should I do about increasing fresh air if my building doesn't have fresh air intake?
A: Simply providing recirculated air will not dilute contaminants and in fact can help spread them. A high efficiency filter may capture some of the contaminants, but only so much. Overall, concentration of pathogens in the air will increase over time without fresh air dilution. Depending on the layout of your system, you may be able to hire a contractor to add fresh air ducts and damper controls to your system at a reasonable cost. The lowest cost option is to open windows (see above). You can keep a local exhaust fan on to draw out air from the space, and let air in through an open window.
Q: How much of a benefit is additional outside air vs. focusing on improving comfort through retro-commissioning?
A: Both air quality and comfort improvements should be considered together; don't assume improvement of one requires exclusion of the other. Clearly, occupant comfort is an important concern, as both ventilation and comfort impact productivity. In general, increasing fresh air ventilation and improving filtration will make a space more comfortable, in addition to promoting health.
Filtration and air cleaning
Have a question about building modifications during COVID-19? Contact SEDAC at email@example.com or 800.214.7954.
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