Just read that a single chimney fire can reach 2000 degrees Fahrenheit, found it in an old NFPA safety bulletin. | Chimney Sweeps - The Community Guide
Wow, that's a scary number. My buddy learned the hard way when his wood stove pipe got a creosote buildup he missed. He said the heat gun read fine at the opening, but a few feet up it was a whole different story. The fire department showed him the spot where the steel actually started to sag. He checks his entire flue twice a season now, not just the easy parts.