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The bathroom ventilation doesn't clear moisture fast enough.
How do I clean the 20ft duct and what must I inspect to make
it work better! Mold is growing on the drywall. Thanks for your interest.
Make certain you have enough make-up air. Door to bathroom is usually cut so that it clears the floor by 3/4" or so. No intake air = no exhaust.
If you've used the cheap plastic flex-ducting, lose it and install rigid.
Make sure you're venting to the great out-of-doors and not into the attic space. Nothing good will come from venting inside the house.
Make sure the internal flapper is actually opening.
If you live in a heating zone, you'll also need to insulate the exhaust duct where it passes thru the cold air space or it could eventually freeze full of ice (horizontal pipe runs)...or at the least......water will come dripping back out of the fan unit.
The unit needs to be properly located. Example: If the unit is installed just inside the door to the room and the shower is 6' farther back into the room.....not all of the moisture will get caught up into the exhaust stream of the fan...if the undercut door is the source of make-up air. Directly over the shower/tub is usually the bestest place.