As the performance of many HVAC systems in buildings are still being evaluated, the ASHRAE Epidemic Task Force has updated its reopening guidance for HVAC systems to help mitigate the transmission of SARS-CoV-2. 

"The Building Readiness Guide includes additional information and clarifications to assist designers and commissioning providers in performing pre- or post-occupancy flush calculations to reduce the time and energy to clear spaces of contaminants between occupancy periods," said Wade Conlan, ASHRAE Epidemic Task Force Building Readiness team lead. "New information includes the theory behind the use of equivalent outdoor air supply, method for calculating the performance of filters and air cleaners in series, and filter droplet nuclei efficiency that help evaluate the systems' ability to flush the building."

Major updates to the building readiness guidance include the following:

• Pre- or Post- Flushing Strategy Methodology: The strategy has been updated to include the use of filter droplet nuclei efficiency, which is the overall efficiency of filter based on viable virus particle sizes in the air, to assist in determining the impact of the filter on the recirculated air on the equivalent outdoor air. This allows the filter efficiency as a function of particle sized, using on ASHRAE Standard 52.2 test results, to be estimated based on the expected size distribution of virus-containing particles in the air. This calculation is currently based on Influenza A data and will be updated as peer-reviewed research becomes available for the distribution of particle sizes that contain a viable SARS-CoB-2 virus.  Additionally, a chart added to help determine the time to achieve 90%, 95%, or 99% contaminant reduction if the equivalent outdoor air changes per hour is known.
• Flushing Time Calculator: There is now a link to a view-only Google Sheet that can be downloaded for use, to help determine the available equivalent outdoor air changes and time to perform the flush.  This sheet is based on a typical mixed air AHU with filters, cooling coil, with potential for in AHU air cleaner (UVC is noted in the example), and in-room air cleaning devices. Provided efficiencies of MERV rated filters are based on the performance of over 200 actual filters from MERV 4 through 16, but the tool also allows users to enter custom characteristics for specific filters.
• This sheet also calculates the filter droplet nuclei efficiency based on the cited research but allows a user to adjust the anticipated distribution of virus as desired. It also allows specification of the zone (room) air distribution effectiveness from ASHRAE Standard 62.1 to account for the impact of the HVAC system air delivery method on the degree of mixing. Default calculations assume perfect mixing. Finally, the tool allows for the target air changes to be adjusted if an owner wants to achieve a different percent removal in lieu of the recommended 95%.
• Heating Season Guidance: The guide now includes data to consider for heating of outdoor air and the potential impact on pre-heat coils in systems. 
• Adjustments to Align with Core Recommendations: The Core Recommendations were released in the last month and this guidance document needed to be updated to ensure that the information provided aligned with the intent of those recommendations. This included minimum outdoor air supply and filter efficiency requirements and their role in an equivalent outdoor air supply-based risk mitigation strategy.

The guidance still addresses the tactical commissioning and systems analysis needed to develop a Building Readiness Plan, increased filtration, air cleaning strategies, domestic and plumbing water systems, and overall improvements to a systems ability to mitigate virus transmission.

To view the complete ASHRAE Building Readiness guide and other COVID-19 resources, visit ashrae.org/COVID-19.