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 . <EOS>