I had a kozy heat csk-31 rf gas insert installed into an existing woodburning fireplace. It has an intake and exhaust venting, each 3" flexible ducting, similar to a clothes drier vent duct. One of the ducts wasn't long enough, so they had to extend it with another piece of metal pipe. On the off chance that it was the exhaust side they had to extend and if they only connected it with metal or aluminum tape, what is the worst case if the exhaust side comes loose a few feet from the termination cap on the chimney? I'm hoping that it just wouldn't stay lit if that's the case and nothing serious. The ducts are inside a chimney designed for wood-burning that would get much hotter, I just don't remember for sure that they put a fitting between the two metal ducts. But I do remember them taping around the two connected ducts.