We just moved into a rental home. It turned out after living there for a few weeks that we discovered the vents were innudated with mold. They were so bad the landlord had to replace all the vents in the home both upstairs and downstairs.
Toward the end of the job, the guy in charge of the job told me that a major cause of the mold was likely a bathroom floor vent close to a shower. Water dripped slowly over time into the vent and pooled in the vents causing mold to flourish. Now there's a couple exacerbating factors. The AC only runs in the summer. So a few drips during the summer were not a big deal. They would dry up as the AC ran. In the winter, though, the home doesn't use the AC vents. Instead radiator heating is used. So water pools from the vent, but no AC comes on to dry up the water.
Additionally, the bathroom has no exhaust fan. So with the door shut, the room gets rather steamy. I imagine that in the winter, the hot steam can easily travel down the vent and condense underneath the home. This might be a bigger cause.
What can we do to deal with this situation? How can we keep our vents dry and still take showers in this bathroom? Keep in mind we're renters, so we have to ask for everything. It seems likely that the mold problem will repeat unless we can find a way to deal with this problem.