In my original post, I asked how much coal soot would have to build up in the flue before it would pose a fire hazard. I should have worded it differently because at one time, I had a soot fire from bit coal. The house had an old sears and roebuck octopus in the basement and was converted to burn gas. After a winter of enormous heating bills, I placed the grates back in and was burning bit for about a month. I threw some paper in to fire off some kindling when it happened. The paper had taken off and I closed the firedoor to get kindling. When I opened the door, the paper was out and I heard the roar of a fire increasing in intensity but no flames in the firebox!! I ran out side to see black smoke rolling out of the chimney and I quickly set off one of the chimfex fire extinguishers and closed it all up. After I calmed down, I open things up and found the fluffy ,black soot about an inch deep all over the inside of the what I would call a clean out chamber behind and below the firebox. Smoke and gasses were directed out of the firebox, down into this chamber before exiting a couple inches from the floor, up the connector pipe and into the chimney. You could see where the soot had ignited and burned to a white,greyish ash. I wanted to hear if anyone else, had experiences like this. It caused me to reconsider not using coal or wood heat for a couple of years .