Hello there. I'm new to this forum and yes, new to pellet burning. I recently purchased a used 1996 Harmon invincible fireplace insert and began the process of breaking down, cleaning, etc. The previous owner had purchased the home they lived in and it was there, ( installed ) when they moved in. They admitted that they had no idea how to use it and tried to burn pellets this past winter which caused the unit to build up massive amounts of creosote to the point of it not working correctly. Needless to say as I've taken the unit apart and started cleaning it, the unit had a lot of "stage 3" creosote build up through out the inside. I have it completely dis-assembled and have cleaned everything but the exhaust flue from the front to the outlet in back. I noticed that the exhaust ports at the bottom left and right enter into square tubing which goes up then back down then runs to the back and into the exhaust chamber at the rear. It is clogged up pretty bad and I have a question. How do I clean the exhaust, front to back, going up and down and around corners ?? Obviously the tubing is welded and can't come apart. I want to be able to completely clean the unit, fire it up outside, before re-installing everything and mounting it in my fireplace. Thanks for your time.