DuraVent DuraPlus 6 in. D Aluminum/Galvanized Steel Wall ThimbleDuraPlus 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, including one hour at 1400F, plus three ten minute chimney fire tests at 2100 degrees