musty vents

[Italics Mine]

When the heat (in winter) or AC (in summer) is not running, there's often a musty smell coming from the floor vents. Any idea what causes this, and how to correct the problem? Many thanks.

[DCDiva]

I know older MH vents are made of a heavy cardboard/foiled combo, not sure when this changed or has changed to regular metal duct work---in the past year we have removed 2 sets of duct work from older 1973 MH and in both cases both ends of the duct work had opened up--they were just stapled with like 2 big staples and the heat/ac would have flowed under the mh and the smells from under the MH and any cridders would have come up from the vents,plus you could have a water leak/water running off roof getting or got the duct work wet--so the only way to find out the sorce is to crawl under the mh and explore--a great time to check everything under there and you might add some 6 ply plastic under the mh at the same time
Melissa