You are correct about a blocked vent resulting in low heat. This happened to me appx 2 months ago. The problem was not the "cheap" plastic duct. It was the outlet at the wall which had accumulated lent. To suggest this guy hire a chimney sweep to install a duct is akin to hiring a maid to pick up that beer bottle someone threw into the yard. Move the dryer, clean the duct, move the dryer back. What did that take...10 minutes?