I have a 6 yr old a/c system in a 6 yr old house where mildew is building up on two vents on the 2ond floor. This is Houston, Texas so its typically plenty hot and humid in the summer and this system runs a lot because of the heat. This is a two story house with two systems. One up, one down. I don't see the problem on any of the downstairs vents. The first vent to do this had a 3 foot plastic duct pipe to the plenum. I have been told by an A/C repairman the duct was too short and the supersaturated air was creating the correct conditions for the mildew to thrive. But the 2ond vent I see this happening to has a 30 foot duct attached to it so I wouldn't this supersaturation is the problem. Could a tear in the duct pipe be the problem?