roasting coffee beans creates smoke. the smoke is pushed up the exhaust pipe. the pipe is left with deposits of creosote. creosote is flammable, so buildup becomes a fire hazard. we have 25 feet of pipe, so once a year we get up on the roof, unassemble the exhaust system, and brush it clean.
one peer in the industry told us that we could just light it on fire at the bottom to create a controlled fire all the way up the pipe. but we choose to do it the old fashioned way and not play with fire.