I have a multi-story building that has one AHU per floor taking OSA from a common vertical shaft. The OSA shaft is ducted, though does not have a fan to push the air in the building, and it is equipped with a gravity backdraft damper at the main intake. Each AHU though is not equipped with individual shut-off dampers at each floor. Thus when one AHU is off, the other AHU's that are operating cause the air to be siphoned backwards into the common OSA shaft.
If my interpretation of the Mandatory Requirement is correct, each AHU should be equipped with a means of shutting the airflow from the common shaft when not operating.
I have verified that reverse flow does happen on an identical building the owner has.
I need to know if my interpretation is correct or does the main shut off damper meet the Mandatory Requirement.

Thanks