I have had several contractors mention the problem of water forming in the airhandler/duct over the winter while the A/C system is idle over the winter. One case was so severe the complete duct system and airhandler/coil had to be replaced because of mold.
I believe the reason for this is the migration of the warm, humid air (cooking, showers, humidifier, etc) from the radiant heated space to the cooler surfaces of the duct and airhandler/coil. There, the moisture condenses on the cooler surfaces and eventually drips from the cieling registers.
My question is what is the best way to prevent this? I know some HOs try to seal the supplies and returns and some try leaving the blower on "continuous".