When I was burning my really nasty Bituminous coal last year, I could blow the chimney pipe off the backof the boiler with the 'backfire'.
The first time it happened I had loaded a five gallon bucket of coal on a hot fire, closed the door and was walking back to the house I heard WHOOSH! And looked back to see a huge discharge of soot, smoke and ash come out the chimney, shooting 15-20 feet in the air.
I ran back to the outbuilding, and the room was full of black soot and ash, what a mess!! The elbow at the back of the boiler had blow off, I had only installed one screw to hold it in place thinking friction was enough to do the job, wrong!! All joints have three screws now.
But of course, I didn't have to worry about cleaning the flue pipes or chimney for awhile!! It was a pretty impressive 'backfire'.