Owners should begin running the system as if people were present several days to a week prior to re-occupying the space. If the facility has small unitary equipment, you may want to run only a few of the units at first, while utilizing fans and open doors to circulate air. The goal of this method is to slowly bring all the surfaces in the space to a normal occupied temperature, and more importantly, humidity level.
There is research appearing to show that viruses are less likely to thrive with relative humidity ranges between 40% and 60%. Furthermore, mold is less likely to propagate when air is maintained below 60% relative humidity. During a period of stagnation, some building materials, such as drywall, carpeting, wood and acoustical ceilings act as sponges, absorbing moisture from the air in the building. This condition progresses over time, so it will also take time to reverse the process and return the environment to equilibrium.
Large buildings with partial occupancy face similar issues as smaller buildings that have had little to no occupancy. These larger buildings have more complex HVAC systems and generally have variable airflow systems that have been operating at a minimum rate for the same time-period. When people return en masse, systems suddenly increase airflow rates, stirring up particulate that has settled during the low usage period. Strategies to bring the system back online include:
- Force air-handling units into economizer mode for several hours to flush the spaces
.When outdoor air conditions are suitable. - Set system to “occupied mode” for several days before anticipated occupancy.
- Turn on local exhaust systems periodically.
- Open interior doors to allow effective transfer air between rooms where practical or allowed by usage.
For more in-depth information on this topic from HVAC industry professionals, please see the following articles:
Operation and Maintenance Considerations Before Re-Occupancy – Barton Associates
Pandemic COVID-19 and Airborne Transmission – ASHRAE
HVAC Systems Should Be Checked Before Buildings Reopen, Due to COVID-19 – ACHR News
Contributor: Stephen Osken, PE, LEED AP, Director of Mechanical Engineering at Barton Associates.
