A self-contained, vented appliance complete with grilles or equivalent, designed for incorporation in or permanent attachment to a wall, floor, ceiling, or partition, and furnishing heated air circulated by gravity or by a fan directly into the space to be heated, through openings in the casing. Such appliances shall not be provided with duct extensions beyond the vertical and horizontal limits of the casing proper, except that boots not to exceed 10 inches beyond the horizontal limits of the casing for extension through walls of nominal thickness may be permitted. Where such boots are provided, they shall be supplied by the manufacturer as an integral part of the appliance and tested as such. This definition shall exclude floor furnaces, unit heaters, and central furnaces.