Fibrous glass insulated flexible ducts are used in residential and commercial heating, ventilating, and air conditioning systems operating at static pressures from 1/2 to 10 in. w.g. (125 to 2500 Pa), 6,000 fpm (30.5 m/sec) maximum air velocity, and 250˚F (121˚C) maximum internal air temperature. Flexible ducts are designed for use as run- outs from trunk ducts to room registers, diffusers, and mixing boxes. They may also be used as supply and return air ducts when the product's flexibility is an advantage in routing ducts through spaces with obstructions.