A heat exchanger comprising an outer jacket (9) with an inner set of vertical pipe elements (1) fastened at their opposing ends in upper and lower sieve walls (2, 3), and having a gas combustion chamber (4) located above the upper sieve wall, as well as partitions (5, 6, 7) mounted crosswise of the pipe elements. The partitions have openings for the pipe elements. The heat exchanger is also provided with liquid, gas and exhaust fumes inlet and outlet stub pipes. The outer jacket is conically shape with its diameter increasing upwards. An upper sieve wall (2), is essentially flat as viewed from the side, while an upper partition (5) situated below the wall, is shaped as a cone with its vertex pointing down. The upper partition (5) has a central opening (8) in its central region and an outer diameter corresponding to a diameter of the outer jacket (9).