A heat exchanger (1, 1') comprising: - a gas-tight casing, provided with a sleeve for discharging burnt gases, - at least one tube, made of a thermally good conductive material and inside which a fluid to be heated, such as water, can circulate, this tube being helically wound so as to form a helical winding with a longitudinal axis (X-X'), this helical winding being disposed inside said casing, - means for supplying and/or producing hot gases inside said casing, such as a gas or oil burner, so as to define a combustion chamber therein, this helical winding being arranged so as to provide an interstice between two adjacent turns, said at least one tube having an front face and a rear face opposite to each other, an intrados side oriented towards the longitudinal axis (X-X') and an opposite extrados side oriented towards the casing, said front face and said rear face of the tube being planar or substantially planar and being located on either side of the interstice, said at least one tube having a shoulder on its front face and/or its rear face, this shoulder extending from the intrados side of the tube in the direction of its extrados side, over part of the height of the straight section of this tube, this heat exchanger being characterized in that this shoulder also extends either over the entire length of said at least one tube located in said combustion chamber, or over the entire length of said at least one tube located in said combustion chamber with the exception of the first or last turn of the helical winding, and in that each interstice between two neighboring turns is calibrated either using a tooth of a comb introduced into the interstice between two turns, on the extrados side of the tube, or using several protruding elements, such as bosses, formed on the front face and/or on the rear face of said at least one tube, each protruding element formed on a turn of the tube bearing respectively against the rear face and/or the front face of the adjacent turn, so that the shoulder makes it possible to recover the soot and the slag entrained in the hot gases that pass through the interstices. - a gas-tight casing, provided with a sleeve for discharging burnt gases, - at least one tube, made of a thermally good conductive material and inside which a fluid to be heated, such as water, can circulate, this tube being helically wound so as to form a helical winding with a longitudinal axis (X-X'), this helical winding being disposed inside said casing, - means for supplying and/or producing hot gases inside said casing, such as a gas or oil burner, so as to define a combustion chamber therein, this helical winding being arranged so as to provide an interstice between two adjacent turns, said at least one tube having an front face and a rear face opposite to each other, an intrados side oriented towards the longitudinal axis (X-X') and an opposite extrados side oriented towards the casing, said front face and said rear face of the tube being planar or substantially planar and being located on either side of the interstice, said at least one tube having a shoulder on its front face and/or its rear face, this shoulder extending from the intrados side of the tube in the direction of its extrados side, over part of the height of the straight section of this tube, this heat exchanger being characterized in that this shoulder also extends either over the entire length of said at least one tube located in said combustion chamber, or over the entire length of said at least one tube located in said combustion chamber with the exception of the first or last turn of the helical winding, and in that each interstice between two neighboring turns is calibrated either using a tooth of a comb introduced into the interstice between two turns, on the extrados side of the tube, or using several protruding elements, such as bosses, formed on the front face and/or on the rear face of said at least one tube, each protruding element formed on a turn of the tube bearing respectively against the rear face and/or the front face of the adjacent turn, so that the shoulder makes it possible to recover the soot and the slag entrained in the hot gases that pass through the interstices.