Creosote, the black or brown residue of combustion that collects over the internal surfaces of the chimney flue liner, is very flammable. If permitted to build up, it might capture fire, triggering cracks in "fireproof" brick, stone or clay flue liners and permit heat to reach nearby wood framing and other https://chimneysweepnearme55654.jts-blog.com/27581844/propane-fireplace-insert-no-further-a-mystery