I cleaned my insert's chimney from below yesterday. I used 9 ProFlex poly rods and a 6" poly brush. I put a 2.5" shop-vac hose up the flue as I was pushing the brush. Not possible to see how much dust I got since it went into a bag. Did not see any dust coming out of the hole so it seemed to have worked. It was hard pushing the 6" brush through the bottom transition piece, which I measured at 5.5". I also don't think I made it out the top (although, it was close from measuring) because I could not see it from the ground through binoculars. Perhaps there was another 5.5" piece at the top. I didn't want to push any more - I pushed pretty hard as it was. I saw somewhat of a concern. The stove and liner were professionally installed. The picture below shows what appears to be a gap between the transition piece and the stove top. the picture could be somewhat deceiving because the piece is at an angle and I believe it does actually contact the stove. I am thinking about filling the gap by stuffing in some refractory wool or high-temp RTV. Opinion?