This category includes VAV systems and other HVAC systems with terminal reheat or double duct, and with and without variable flow. These systems are generally designed for medium and large buildings where the system, whatever the type, connects areas of the building between which people have no reason to move. The greatest risk of viral infection remains the direct contact between people who are infected and those who are not infected. If the ownership of the spaces are different, or if the ownership is single but the movement of people is limited, at least between the various floors, the movement of people must be even more confined and the use of common areas must be managed very well, keeping in mind that toilets and elevators are extremely critical points. From the system engineering point of view, it is absolutely necessary to close each air recirculation damper following the indications given in , to avoid propagating the contagion through the HVAC system to places where it would not be carried by the movement of people from one zone to another.