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.