More than 200 scientists recently published an open letter to major international health agencies, including the World Health Organization (WHO) about the emerging evidence showing the COVID-19 virus may be transmitted by airborne particles. In this scenario, the HVAC system becomes a hot topic about the role it plays in spreading the virus.
The majority of the HVAC systems in residential dwellings consist of 100% recirculation of conditioned air. This basically means the supply and return air are transported through an air-handling unit within the same enclosed space. The only fresh air blended into the system enters almost entirely by opening or closing doors or windows (not counting the occasional usage of kitchen and bathroom exhaust vents). Most older houses have wall-mounted return air grilles with space to house a 1-inch-thick pleated air filter. They usually have a MERV filter rating of 2-6, which has an overall efficiency of less than 50%. Most of newer houses have built-in filter plenums in the air-handling unit, which houses a 4-inch-thick air filter. The MERV rating is about 10-11, which has an efficiency of 85%.