Abstract: A furnace unit comprising a mounting panel, a burner assembly and a heat exchanger. The burner assembly is mounted to one side of said mounting panel, the burner assembly having at least one burner located therein. The heat exchanger is mounted to an opposite side of said mounting panel. The heat exchanger has at least one continuous combustion tube, wherein said combustion tube includes at least a first pass and a second pass. The first and second passes are interconnected by a curved pass of the 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.

Abstract: 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.

Abstract: A clamshell heat exchanger for use in a gas-fired direct combustion furnace. The exchanger comprises a first clamshell half and a second clamshell half that when joined with the first clamshell half forms a passageway having an inlet and an outlet. The passageway includes a U-bend located between the inlet and the outlet, wherein the U-bend includes a re-entrant sectional profile.

Abstract: 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.

Abstract: A clamshell heat exchanger includes a first clamshell half and a second clamshell half. When joined, the first and second clamshell halves form a passageway having an inlet and an outlet. The passageway has a height and a depth. A ratio of the height to the depth is about 0.5 or less. The heat exchanger has an efficiency of at least about 70%.