A furnace unit that comprises a burner assembly having at least one burner located therein and a heat exchanger having at least one continuous combustion tube. One end of the combustion tube has a first opening that is coupled to the burner assembly. The combustion tube has at least three zones. A first surface of the first zone has a first non-straight angle with a second surface of the second zone. The second surface has a second non-straight angle with a third surface of the third zone. The first zone is nearest of the three zones to the burner assembly and the second zone is in-between the first zone and the third zone. The first, second and third zones are within a flame length configured to be emitted from the burner.