My system had the same problem. The air handler drain backed up during the summer, overflowed onto the drywall ceiling and through the plenum into the ducts. I replaced the ducts and s/r, had the a/h and plenum cleaned with a bleach solution. Which insurance company paid for the burn and rebuild? I have contactor friend who had mold throughout his house. He replaced everything from the studs out including his furniture. He was told that the spores cannot be removed from porus surfaces.