ASHRAE’s HVAC Guidance for COVID-19 Mitigation Has Changed. Has Your Strategy Kept Up?

As we enter a new year and move forward with vaccines to address the SARS-CoV-2 pandemic, it’s a natural time to evaluate how much we have learned in the past 10 months. We’ve come to understand that indoor air poses a far greater transmission risk than surfaces; that the size, composition, and movement of infectious aerosolized viral particles is quite complex; and that these particles can hang in the air for much longer than originally believed. And as we’ve moved from the short-term crisis mentality of the early pandemic to a longer-term mindset, we’ve also learned to question quick fixes and to replace them with more nuanced approaches.

ASHRAE’s Epidemic Task Force (ETF), comprised of an impressive array of experts, is no different – their guidance has evolved as understanding of the virus has increased. It should come as no surprise to any of us to learn that the ETF’s initial recommendations to increase ventilation as much as possible in an effort to dilute contaminated indoor air with “fresh” outside air have evolved into a more nuanced approach that accounts for a more energy-efficient and achievable combination of ventilation, filtration, and standalone air cleaners. This is reflected in ASHRAE’s recently updated “Core Recommendations for Reducing Airborne Infectious Aerosol Exposure.” The ETF has acknowledged that numerous methods exist to render indoor air safe. It recognizes that some strategies carry massive energy penalties that strain operating budgets and impact ESG goals of energy efficiency and decarbonization.

During our October webinar, Prof. William Bahnfleth, Chair of the ETF, explained that initial guidance from ASHRAE was very conservative without consideration for cost, operational, and seasonal weather impacts. Over time with the benefits of new scientific findings, the ETF has evolved its guidance. The new Core Recommendations refine their findings into a streamlined prescription to mitigate the transmission of infectious airborne aerosols in a variety of building types, including offices and schools.

As stated in the ASHRAE press release, the Core Recommendations are “based on the concept that ventilation, filtration and air cleaners can be combined flexibly to achieve exposure reduction goals subject to constraints that may include comfort, energy use and costs.”

The Core Recommendations for Ventilation, Filtration, and Air Cleaning are the following:

• Provide and maintain at least required minimum outdoor airflow rates for ventilation as specified by applicable codes and standards.
• Use combinations of filters and air cleaners that achieve MERV 13 or better levels of performance for air recirculated by HVAC systems.
• Only use air cleaners for which evidence of effectiveness and safety are clear.
• Select control options, including standalone filters and air cleaners, that provide desired exposure reduction while minimizing associated energy penalties.

According to Prof. Bahnfleth, “ASHRAE’s Core Recommendations are based on an equivalent clean air supply approach that allows the effects of filters, air cleaners other removal mechanisms to be added together to achieve an exposure reduction target.”

Experts from Harvard’s Chan School of Public Health recommend 6 air changes per hour (ACH) as the ideal exposure reduction target for schools. Clean air is defined as outside air or recirculated air that has been filtered by high-efficiency filters such as MERV-13 filters and local HEPA filters.

To calculate the effectiveness and costs of these approaches, enVerid recently published the enVerid COVID-19 Energy Estimator, a free, open-source tool to help building owners, mechanical engineers, and facility managers evaluate the risk, costs, and carbon impacts of different ventilation and filtration approaches. The Energy Estimator shows that installing high-efficiency filtration can be as effective and lower cost than increasing ventilation rates to achieve target air exchange rates to reduce bioaerosol exposure risk.

We are already on record urging facility owners and managers to reevaluate their airborne transmission mitigation strategies in light of this updated guidance. However, we continue to be surprised by building managers that have yet to reevaluate their approach.

In addition to the Estimator, enVerid has developed additional resources around this important topic, including our White Paper on filtration and ventilation, a Guide for Improving Classroom Air Quality for COVID-19 Mitigation and a Webinar Series on COVID-19 & IAQ: New Best Practices that features ASHRAE ETF experts.

The availability of vaccines moves us a significant step forward in bringing this pandemic under control. As building owners and managers prepare to safely bring back occupants and map out major HVAC modernization projects, we urge them to reassess their initial HVAC mitigation strategies. It is possible to achieve two — not mutually exclusive — objectives: pandemic-proofing buildings and ensuring energy efficiency to lower costs and carbon emissions. As indoor air quality and energy efficiency professionals, it’s up to us to counsel facility managers with the benefit of the most up-to-date guidance to help them keep occupants safe while saving money and reducing emissions.