Abstract: The present invention relates to a fastening system for fastening a through pipe to a hearth wall, the pipe opening into a box. The fastening system comprises external pipe-securing means for securing the pipe to said screen, and an internal metal guide tube fitted over said pipe and having one of its ends interposed between said pipe and the orifice through which said pipe passes, its other end being externally threaded. The guide tube carries a rigid washer which is disposed so as to abut against the inside face of the inside layer, and against which a wall of the box is mounted, which wall is provided with an orifice that is larger than the cross-section of the guide tube, a sealing device being clamped against the wall of the box by a nut screwed onto the threaded end of the guide tube.