I didn't see this addressed in any other posts so please pardon me if it's already been discussed!
We live in a 2-story townhouse that was built in the mid-eighties. The original a/c unit is STILL working (so the warranty company won't replace it just yet) but it sure is a pain to heat up in the winter/cool off in the summer.
We just noticed a spot in an upstairs bedroom: there is a ceiling register about 6 inches out from a window in the room - usually it has condensation dripping from it in the summer (so we can't have any furniture under it) - but a few days ago I noticed some black colored mold or mildew on the corner of the register.
It's not black mold as in the dangerous kind as far as we can tell. It's not grown in size and there's no mold anywhere else, including the area behind the filters (we wiped inside to see).
Thinking of just spraying it with Lysol to see if that kills it - anyone ever seen something like this before?
Gypsum has paper, mold & mildew loves paper type products. I would be getting into the ceiling to find out why there is air leakage up there allowing the vents to condense. Seal the joins and seal any air leaks, along with more insulation up in the attic should help to eliminate most of that problem.
Black mold is a slow growing mold and is probably not the mold you have but I had an AC unit in an attic and it would rain through the floor because HVAC ducts were not insulated. Just like last post make sure all ducts are insulated very well.