This is my setup, 1986 built town home. I just purchased a dryer and before connecting it, I decided to inspect the 4" tube going from the dryer, up behind the wall to the crawl space / attic and out of a hole cut from the roof.
This is what I found:
That's just a 12" segment, I kept, it was like this for 12 feet. While I assume, total blockage is bad, I presume that so is even this amount of lint stuck to wall of the tube. (Is that right or did I remove it for nothing?)
Anyhow, they had a aluminum flex duct tube tube going through the wall, but I'm wondering if perhaps it would not be best to replace it with a rigid smooth (without the flex ducting) aluminum tube for the straight section that goes from the bottom the wall behind the dryer into the crawl space. Then at both ends, replace it with new, clean flex duct tubes.
I'm also thinking of adding a secondary lint filter so this doesn't happen again.
Any advice, or what to use is appreciated, I want the safest solution.