Hello I have a df 520 I like to think I know it pretty well. I run rice coal typically at 3/4 teeth and 4 air. Aquastat at 160 low 200 high 10 diff and 2 min every 1/2 hour on the timer. My ? Is with no change in coal or settings this winter yesterday during my daily check my coal feed pipe was so hot I can't hold it, I have a good ash ring and dark center and still a hot feed pipe. I have tried all different settings evening going up to 6 teeth and it stayed hot. My draft is 2 in back and 4 over fire. At help would be great.