Originally referred to as factory-built fireplaces, zero clearance fireplaces are made up of a firebox enclosed within a steel cabinet, and a steel chimney or flue. They are built in such a way that they can typically be installed just inches away from combustible materials (hence the zero clearance). Most have glass doors, insulated walls, air-cooled pipes, and blowers that can move the heat produced by the fire into the room. Unlike traditional masonry fireplaces, which are built on site, zero clearance fireplaces are designed to allow for installation at a later date, although they are just as often used for new construction.