I have a harmon SF-260 coal/wood boiler. I get about a half of a quart of black fuzz every year when i sweep. The chimney is a triple wall stainless liner in a older gas flue. I also poured insulation around the pipe to keep as much heat in as possible. I get black flake build up on the screen on the top but nothing towards the boiler from there. I burn 375-450F a night with seasoned wood 18-23%. Most days i turn off the heat on the house when i go to work and then fire the boiler up when i get home and burn 50-125lbs of wood a night to bring the temp back up. I do this so the boiler will get as much wide open burn time as possible. I don't have room in the boiler room for storage(i only wish i did) is there anything else that i am missing to keep the flue pipe hotter other then burn the boiler hotter. I think when i do go to bed that by 2:00-3:00am the boiler is back to cycling and this might be where i am getting my build up from. the little spot where there is no insulation on the lower exhaust part is where i take the temps from with a Fluke IR.