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Will Coronavirus Be the End of Demand-Based Ventilation Control?

By Bob MacCamy, PE, Senior Mechanical Engineer

Bob MacCamy, PE

The Centers for Disease Control and Prevention (CDC) is currently conducting an extensive study on the relationship between the spread of coronavirus and outdoor air ventilation rates in interior environments. This study may ultimately lead to revisions to the required outdoor air ventilation rate as prescribed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). But changes to requirements will take time to finalize. Instead, in light of the pandemic, HVAC engineers need to ask ourselves now, “Is it appropriate to reduce ventilation rates using demand-based ventilation of measured CO2 concentrations?”

ASHRAE’s Indoor Air Quality procedure allows for a reduction in outdoor air ventilation rate as long as appropriate CO2 levels are maintained in a space. A CO2 level of 700 parts per million, or ppm, is noted in ASHRAE’s “Ventilation for Indoor Air Quality” Publication 62.1, Article 6.1.2, Appendix C. The rationale behind this requirement is that less people in a space will generate less CO2, and thus, require less ventilation air. Typically used for large areas such as conference rooms, lecture halls, and classrooms, demand-based ventilation allows a mechanical system sized for a large space at maximum occupancy to self-adjust, saving a building owner operating costs. This prescribed reduction, also mandated to some extent in the Energy Code, can best be understood as a room’s fresh air exchange.

According to the World Health Organization (WHO), COVID-19 spreads primarily via respiratory droplets expelled into the air by an infectious person coughing, sneezing, talking, or breathing. Medical researchers have concluded that your likelihood to contract the virus when it is present depends both on its concentration in the surrounding air AND the amount of time you are subjected to it. Because these droplets tend to fall to the ground or other surfaces within as little as three feet, social distancing, wearing face masks, handwashing, and disinfecting surfaces remain our most effective defenses against transmission. There is some evidence still being investigated that certain strains of COVID-19 may remain suspended in the air as particles, or aerosols, for much longer, raising the issue of the air exchange rate. However, there is no clear direction to date on how a mechanical system should be designed or modified to handle these unknowns.  

Current code requires approximately three complete fresh air exchanges per hour for a fully-occupied classroom, which is typically about 800 SF. A reduction based on measured CO2 readings is allowed if a room isn’t fully occupied, but ASHRAE leaves this judgment up to the Engineer. The Energy Code, on the other hand, mandates the use of this reduction method for spaces greater than 500 SF, which includes classrooms. The Energy Code has been adopted by most municipalities and is quite often enforced by zealous code inspectors. It is a matter of law.

My career in HVAC, which has spanned six decades, gives me a unique perspective on this subject. I have seen many changes to the prescribed outdoor air ventilation rate over the years, brought on by two main players in this debate. There is ASHRAE, a professional association of architects, engineers, contractors, and others concerned with HVAC specifically; and there is the energy conservation community, who seek to increase buildings’ energy efficiency to decrease their environmental impact generally.

In the 1970s, the recommended ventilation rate for classrooms was approximately three air changes per hour. In response to the energy crisis of the early 1980s, this was reduced by two-thirds to about one air change per hour. This sudden drastic reduction resulted in what was known as “Sick Building Syndrome,” where poor indoor air quality was found to be negatively impacting occupants’ health. In response, ASHRAE increased their recommendation back to about three air changes per hour in 1989. This standard was quickly incorporated into the building codes in the early 1990s. By 2012, the “International Energy Code” mandated the use of demand-based ventilation systems, where the control system automatically reduces ventilation in any space over 500 square feet when CO2 concentrations below 700 ppm are detected. This reduction is significant, and, as a matter of law, effectively takes air change standards back to the 1980s. This jockeying over prescribed outdoor air ventilation rates between ASHRAE and energy conservationists has lasted more than thirty years, with energy-efficient standards regaining control in 2012.

Today there is a third player: the CDC. With new studies and information concerning coronavirus emerging all the time, it will be interesting to see how this plays out. In the meantime, ASHRAE’s guidance errs squarely on the side of caution, which they explain in a May 2020 statement entitled “Does ASHRAE’s guidance agree with guidance from WHO and CDC?” “Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures.” 1

These changes can include adjustments to HVAC systems’ operating hours, enhanced filtration, or controls like a manual override for CO2 detectors in demand-controlled spaces to bring in more outside air. Until there is clear direction on how to design and configure systems moving forward, we should consider any adjustments or modifications in their specific context, and as supplemental measures to social distancing, mask-wearing, and good hygiene. As a ventilation engineer, I will seek the best and safest design solution for every client based on the latest ASHRAE, WHO, and CDC recommendations and guidelines, as they continue to develop in response to this pandemic.


The views and opinions expressed in this article are solely those of the author(s) at the time of publication and do not reflect any official policy or position of Buchart Horn, Inc., its parent, or affiliated companies. This information is intended for general informative purposes only.