Originally Posted by Commuter Boy
Don't cheap out on the duct size, moving up to 5" or 6" will let you put in a much quieter model and still move the same amount of air.
That is where I might jimmy-rig mine. The existing exhaust wall cap and fan has a 3 inch corrugated duct. If I go to 4 inch (which my new fan has) I need to replace the wall cap, and its in a right bitch of a spot for me to get to. I did buy a new wall cap tonight, I need to borrow an extension ladder and see if I can even reach the cap. If I can't reach it I'll transition from 4 to 3 inch inside the attic. The fan probably won't be able to hit its rated cfm but it might be the best I can do.