Heat exchanger and methods of producing the same

1) A heat exchanger for a gas boiler for producing hot water;

• the heat exchanger (4) comprising a casing (10) extending along a first axis (A1) and through which combustion fumes flow;
• a tube (11) along which water flows, and which is housed inside said casing (10), and coils about the first axis (A1) to form a helix (17) comprising a succession of turns (16);
• and deflecting means (12) for directing the fumes between successive turns (16) of a first helix portion (30) in a first direction (D1) and between successive turns (16) of a second helix portion (31) in a second direction (D2) opposite to the first direction (D1);
• said heat exchanger (4) being characterised in that the tube (11) forming the turns (16) of said first helix portion (30) has a first cross section, and the tube (11) forming the turns (16) of the second helix portion (31) has a second cross section different from the first cross section.