What type of exhaust vent material is required, and what is prohibited?

You must use a heavy metal vent with a 4-inch diameter — do not use a plastic vent or a metal foil vent, as failure to follow these instructions can result in death or fire. The dryer must be exhausted outdoors and must not be exhausted into a chimney, wall, ceiling, or concealed space. Using a plastic vent may also void the warranty. (Pages 4, 25)