Abstract: This invention consists of a demountable heat recovery device, which can turn any tube into an efficient heat recovery, for applications in fire tube liquid heaters and steam boilers; the present heat recovery device is comprised by a tube, which may be cylindrical or square, inside which a gas diffuser is placed having a transversal cylindrical, pentagonal, hexagonal, octagonal or square shape, and the most important is that the cavity formed between the fire tube and the gas diffuser shall be narrow, of at least 0.001 m, and that the combustion gases shall flow at high speed, of at least 6 m/sec through said narrow cavity; since only in this way a great amount of heat is transferred to the inner wall of the fire tube; according to this invention, high thermal efficiency heaters and boilers can be built and with a less number of tubes, wherein a mechanical blower has to be installed for them to operate.