Furnaces come in many shapes, sizes and brands. They also have different materials used to exhaust gases out of the home. Traditional, or conventional low to mid efficiency furnaces produce hot exhaust gases, and need metal vent pipes or chimneys to let these hot gases out. Modern, high efficiency furnaces produce much cooler temperature gases and can use plastic pipe materials such as PVC or ABS for the exhaust vents. Typically there will be a second plastic pipe as well that brings in fresh air.