I'm noticing a lot of black soot on the inside of my stove so far this year. I cleaned the exhaust thoroughly before the start of the season and replaced the door gasket for another issue that was happening. I'm burning Northern Warmth Doug Fir Supreme. I figured I'd have the cleanest burning year yet with these pellets, but so far not true. Lots of clinkers too. This Quad Castile insert stove is a never ending battle.