Can anyone tell me what kind of chiminey this is? And if so, how I might be able to extend it. I am burning a jotul f400 castine and am not getting the optimal performance. I have tried just about everything to try and get longer and hotter burns. The only thing left to change would be the draft. I am using a 6" double wall stove pipe that is appx 7 ft long and tying into my existing 8" chiminey that runs an additional 6 ft. Total, I have about 13 ft of pipe going straight up from the stove. The manual said that I need at least 14 ft of pipe to optimize performance. Any ideas on how I might be able to extend what I have without having to replace the whole thing would be greatly appreciated.