Converging-diverging combustion zones for furnace heat exchanges

A method of manufacturing a heating furnace unit, comprising:

• providing a mounting panel;
• providing a burner assembly having at least one burner located therein;
• providing a combustion tube of a heat exchanger, wherein said combustion tube includes at least a first pass and a second pass, said first and second passes interconnected by a curved pass of said combustion tube;
• mounting said burner assemble to one side of said mounting panel;
• mounting said heat exchanger to an opposite side of said mounting panel;
• and
• coupling one end of said combustion tube to said burner assembly such that said at least one burner can emit a flame into an opening in said one end, wherein:
• said combustion tube has at least three zones,
• a converging first surface of said first zone
• said first surface extends an entire length of the first zone and converges towards a direction of a flame emitted by the one burner during an operation of the heating furnace and forming a first non-straight angle with a diverging second surface of said second zone,
• said second surface extends an entire length of the second zone and diverges away from said direction of said flame,
• said second surface has a second non-straight angle with a third surface of said third zone,
• said first zone is nearest of said three zones to said burner assembly and extends to said first opening and said second zone is in-between said first zone and said third zone,
• said first, second and third zones are within a flame length configured to be emitted from said burner,
• said first non-straight angle is located on said opposite side of said mounting panel, and
• said first non-straight angle and said second non-straight angle are within said first pass and are nearer to said first opening upstream end than said curved pass.