A condensing heat exchanger comprising: - at least one helically wound tube made of a thermally conductive material and inside which a fluid to be heated, such as water, can flow, - a casing inside which said tube is mounted, this casing being provided with a gas discharge sleeve, a bottom and a facade, which supports means for delivering and/or producing a hot gas inside the casing, such as a gas or oil burner, - a deflector which comprises a disk made of thermally insulating material, carried by a sheet metal reinforcement, said deflector being positioned relative to the helically wound tube so as to delimit, with said facade, a combustion chamber, said casing comprises a body consisting of a tubular metal ferrule, this tubular ferrule being closed at one of its two ends by said facade and at its other end by said bottom, in that said tubular ferrule and said bottom are separate before assembly, in that said bottom is formed in a composite plastic material, in that one of the two edges of the tubular ferrule, called "rear edge", has on at least part of its circumference, a plurality of cutouts delimiting fastening tabs, in that the bottom has at its periphery a plurality of fastening slots disposed along at least part of its circumference, each fastening slot being dimensioned so as a fastening tab can be inserted therein, each fastening slot being bordered longitudinally by a first longitudinal rib, which protrudes outwardly, and in that said fastening tab is folded twice around said first rib, so that the ferrule is fastened to said bottom and said helical tube winding is axially stressed at its two ends between the bottom and the facade. - at least one helically wound tube made of a thermally conductive material and inside which a fluid to be heated, such as water, can flow, - a casing inside which said tube is mounted, this casing being provided with a gas discharge sleeve, a bottom and a facade, which supports means for delivering and/or producing a hot gas inside the casing, such as a gas or oil burner, - a deflector which comprises a disk made of thermally insulating material, carried by a sheet metal reinforcement, said deflector being positioned relative to the helically wound tube so as to delimit, with said facade, a combustion chamber, said casing comprises a body consisting of a tubular metal ferrule, this tubular ferrule being closed at one of its two ends by said facade and at its other end by said bottom, in that said tubular ferrule and said bottom are separate before assembly, in that said bottom is formed in a composite plastic material, in that one of the two edges of the tubular ferrule, called "rear edge", has on at least part of its circumference, a plurality of cutouts delimiting fastening tabs, in that the bottom has at its periphery a plurality of fastening slots disposed along at least part of its circumference, each fastening slot being dimensioned so as a fastening tab can be inserted therein, each fastening slot being bordered longitudinally by a first longitudinal rib, which protrudes outwardly, and in that said fastening tab is folded twice around said first rib, so that the ferrule is fastened to said bottom and said helical tube winding is axially stressed at its two ends between the bottom and the facade.