Microbial growth on vents most often occurs when the temperature of the vent drops below the temperature of the surrounding air, causing condensation. This condition can be caused by:
Low evaporator air flow from poor duct design, high-resistance filters, dirty filters or lack of maintenance. If the airflow is reduced, the supply air is colder than per system design and metal vents can easily get below the dewpoint of the room air.
Excess indoor water vapor in the indoor air can also contribute to excessive condensation on vents. Be sure and run bath and kitchen exhaust fans intermittently, and try limiting use of the clothes dryer to the morning and evening hours to keep induced infiltration of water vapor to a minimum.
We recommend having the HVAC system professionally cleaned and performance-tested to be sure the correct quantity of air is in circulation by the fan and that the system is draining condensate properly.