Just finished brushing out my heat exchange tubes and I gazed over at my chimney thimble to see a crack along it's length near top dead center. How concerned should I be? The mason left the thimbles out about 8 inches. I thought this was a little long but it never occurred to me they might crack. Must have gotten a little hot at some point when I was loading more wood on and the flames were going over the top and out.
