This is the case of small HVAC systems with one or more ducted zone recirculation terminals (serving several rooms). This can be the most difficult case to discuss, because the aerosol containing the virus can propagate in all of the spaces served by the system. The aerosol with virus does not remain in the rooms where the possible asymptomatic infected person stays. This is certainly true, but it is equally true that it is of no use to close these HVAC systems that are serving small areas in a single property. The greatest danger regarding the risk of contagion is instead facilitated by the movement of individuals in the various rooms and in the common use of the toilets, where the spread of the infection is very likely. In these cases, the concentration of elemental viral loads per unit of volume decreases, because it is distributed over the entire volume served by the HVAC system. The risk is lower for a single person who stays in the same room of any infected person, but the risk is now extended to all the people present in the entire area served by the systems. This means that there is less risk for more people. Anyone who cannot work from home, and therefore is inside the area where the infected person is or has been present, is in any case at more risk from the other two other infection pathways (person-to-person, and touching contaminated surfaces). This does not mean that these HVAC systems perform better than those with primary air, because they still do distribute aerosols (including the virus) everywhere in the building. It also means that their closure does not lead to substantial risk reductions, precisely because of the inevitable movement of people from room to room. Once again, containment must be done by controlling frequentation, promoting smart working and accurately checking the health of those who enter the premises.