A heating apparatus (1) including a rear wall with an opening (6) provided with an edge (41) spaced apart from the edge (40, 40', 40') of the rear wall (2), and having a standard size predetermined according to the power of the heating apparatus but not according to the apparatus type or the fuel used therein, as well as an intermediate member (7) having a standard shape selected according to the fuel used, and being fitted in said standard opening (6) for sealing same, said member having a through-hole (8) enabling communication between the inside of the heating body and the flue gas outlet, optionally via flue gas discharge means.