DuraPlus is designed to stay cool on the outside, to provide a hot draft on the inside, to boost stove efficiency, and to provide for a fire-safe design that protects both the chimney and the building. Designed for normal, continuous operation at 1000 degrees F flue gas temperatures, DuraPlus is subjected to rigorous and stringent HT requirements of the UL standard. This includes one hour at 1400°F, plus three ten minute chimney fire tests at 2100 degrees F.