By Anthony Capkun
February 4, 2021 – 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 task force team lead Wade Conlan.
“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,” he added.
Major updates to 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 ASHRAE 52.2 test results) to be estimated based on the expected size distribution of virus-containing particles in the air.
Additionally, a chart has been added to help determine the time to achieve 90%, 95% or 99% contaminant reduction when 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 (link not provided – Ed.), 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 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 when an owner wants to achieve a different percentage removal in lieu of the recommended 95%.
Heating season guidance
The guide now includes data to consider for heating outdoor air, and the potential impact on pre-heat coils in systems.
Adjustments to align with core recommendations
The core recommendations were released last month, and the guidance document needed to be updated to ensure 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.
ASHRAE says 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.