ASHRAE Updates Guidance to Mitigate Transmission of SARS-CoV-2
ASHRAE issued an updated version of the organization’s “building readiness” document in August that addresses guidance for heating, ventilating, and air conditioning (HVAC) systems. The document is intended to help mitigate the transmission of SARS-CoV-2 as buildings reopen during the COVID-19 pandemic.  One goal of the ASHRAE Epidemic Task Force, which updated the guidance, was to provide “information and clarifications so that owners can avoid operating their HVAC systems 24/7,” according to Wade Conlan of ASHRAE’s Building Readiness Team. “By rolling out this updated guidance, we are hoping to provide a more robust structure for building owners to complete the objectives of their Building Readiness Plan and anticipate the needs of building occupants,” Conlan said. The task force updated recommendations related to the pre- and post-flushing strategy, which is intended to ensure that a building’s ventilation schedule helps remove bioburden before, during, and after occupancy. According to ASHRAE’s guidance, buildings should be “flushed for a duration sufficient to reduce concentration of airborne infectious particles by 95 [percent].” ASHRAE also updated recommendations related to the operation of energy recovery ventilation systems and building readiness modes of operation. The guidance considers several situations for buildings operating in epidemic conditions, including buildings that are occupied at pre-epidemic capacity, occupied at reduced capacity, and temporarily unoccupied. ASHRAE’s guidance also discusses operation during building closure for indefinite periods.
When buildings are operating in epidemic conditions, ASHRAE stresses that each HVAC system must be analyzed for proper engineering controls that can improve the system’s potential to reduce virus transmission. While increasing the system’s ability to use more outside air is desirable, ASHRAE cautions that doing so has financial consequences and other implications that need to be considered. One example of a typical system discussed in the document indicates that increasing the percentage of outside air from twenty to ninety percent would require twice as much chilled water and triple the pressure drop at the system’s coil. Other potential problems that could result from increasing outside air include doors that will not close, excessive noise between adjacent spaces, and a reversal of the pressure required for a space. “Care should be taken when increasing outside air but keeping exhaust and relief air systems as designed,” the document reads.  Unintended consequences could also result from upgrades to filtration. ASHRAE recommends filters rated MERV 13 and higher to help mitigate transmission of infectious aerosols, but these filters require air pressures that may exceed the HVAC system’s capacity. If a MERV 13 filter is installed in a system designed for a lower-rated filter, it may struggle to maintain pressures between spaces and required indoor temperature and humidity conditions. For buildings operating in post-epidemic conditions, the guidance separately identifies concerns that should be addressed prior to occupancy and operational considerations that may arise once buildings are occupied.  Building owners and operators will find guidance related to the development of a building readiness plan, increased filtration, air-cleaning strategies, domestic and plumbing water systems, and system improvements to mitigate virus transmission. The complete building readiness guide is available as a PDF. Other COVID-19 response resources from ASHRAE are collected on the society’s website.
ASHRAE stresses that each HVAC system must be analyzed for proper engineering controls that can improve the system’s potential to reduce virus transmission.