Originally Posted By: jpeck
This post was automatically imported from our archived forum.
There are several things going on there which are wrong.
First, the dryer duct needs to be attached to a proper dryer exhaust vent roof cap flashing where the dryer duct is slipped onto the sealed exhaust vent (like it is when connected to a dryer vent wall cap. That also needs to be sealed, so all lint is pushed out of the structure to the outdoors. It also needs a damper on the exterior end.
Now, that the dryer has been removed from there, there are only two bathroom exhaust fan ducts in that roof cap flashing.
The bathroom exhaust fan vents need to be sealed to the exhaust vent for several reasons: a) because that leaves a "building opening" around the exhaust duct, and (at least down here) it is all too common to find that "building opening" within 10 feet of a plumbing vent stack terminal (in fact, they are frequently within a couple of feet of each other); b) because all building openings must be sealed or screened, and those are not always screened on the outside end; c) because the exhaust from all exhausts must be discharged in a manner which will not readily allow the exhaust to re-enter the building (and this is likely to be blowing directly back into the attic to a great extent).
There is nothing inherently incorrect about having two or three of the bathroom exhaust fans vent to the exterior through the same exhaust opening, if done correctly.
Then there is the dryer duct material itself. Most codes require the dryer exhaust duct to be smooth on the inside, and those are not. Most codes also allow for "or other approved material" and some of that material has an approval to be used as a dryer duct. Some code inspectors say "as long as it's smooth inside, you can you that stuff', and we all know it is not "smooth on the inside", and so do they. Other code inspectors look at the listing and say 'Well, it is listed for that use, so I guess it's okay."
Down here (where Harvey and I are), that was not approved, was approved, and has fallen back out of favor, as the code inspectors re-address the "smooth on the inside" issue.
Anyway, bathroom ducts together are okay, but must be sealed around (as should a single one too).
The dryer is all wrong, and needs not only its own exhaust vent, but needs one to which it can be properly attached and sealed.