Uses a coaxial pipe system (small pipe within a large pipe) and draws combustion air from the outside through the outer pipe. Exhaust is vented out through the inner pipe. Can vent horizontally through an outside wall, or vertically through the roof - no chimney required! Convenient to install and more economical. Suitable for well-insulated homes or homes with no existing chimney. Can be installed directly over combustible flooring. (except for carpeting or linoleum)