A heat exchanger for a gas boiler for producing hot water is provided with a casing extending along a first axis and through which combustion fumes flow; a tube along which water flows, and which is housed inside casing, and coils about the first axis to form a helix having a succession of turns; and deflecting means for directing the fumes between successive turns of a first helix portion in a first direction and between successive turns of a second helix portion in a second direction opposite to first direction; the tube forming the turns of the first helix portion has a first cross section and the tube forming the turns of the second helix portion has a second cross section different from the first cross section.