what causes clunkers,after running my napoleon np40 for 3 bags the burn pot was caked full .came out in one big hunk.stove is new so its not dirty,pellets were screened before going in,and window had very little black on it,wiped off with paper towel.also noticed that outside air tube is covered in condensation on outside badly,almost frost where it comes through wall.took off hose and no condensation inside tube i can find. help!