On my 4th year with this unit. For about the last year the inside of my burn chamber ( the Walls ) are covered with a black soot that is different than the gray ash that used to be produced after a burn. I have played with the air intake some thinking it needed more air but its further open than it used to be & the black soot has not changed. I'm wondering if something has changed in my pellets. I've run Michigan Hardwood pellets from TSC for 2 yrs now. I did run some Pro Pellets for a while & it seemed to be better. The Problem with those is the bag says net 40# per bag when they actually weigh #36 lbs. So you giving up 10% of your burn time to use those at around the same $ per bag. Any thoughts on the cause of Black soot vs Grey ash on the walls of the burn chamber walls ?