After about 4 cords through my Blaze king I decided to check my chimney. I pulled the cap and looked down with a spotlight. It is still clean. I was quite surprised. I guess I can put off buying the 8" brush for a while longer. I imagine the -40 to -60 temps have helped as I have kept this thing cooking 24 hrs a day. I have still only built a fire 2 times. Once this past fall and one time after we left town for 3 days. I still use the boiler for some heat and domestic hot water. I have used 150 gal so far this winter.