Gas furnace

A gas furnace, comprising:

• a combustion portion in which a fuel gas is burnt to generate a combustion gas;
• a heat exchanger having a gas flow path through which the combustion gas flows;
• a blower configured to blow air around the heat exchanger;
• and
• an inducer configured to discharge the combustion gas from the heat exchanger, wherein the heat exchanger comprises:
• at least one single path in which a single gas flow path is formed, the at least one single path including an inflow single path in which a combustion gas inlet is formed;
• a single-multiple return bend configured to communicate with the at least one single path and convert a flow direction of the combustion gas;
• and
• at least one multiple path having a plurality of paths forming multiple gas flow paths that are separated from each other, wherein the single-multiple return bend extends in a first direction crossing the at least one single path and the at least one multiple path, and is connected to the at least one single path and the at least one multiple path, wherein each of the plurality of paths is configured to communicate with the single-multiple return bend in parallel with the others of the plurality of paths, wherein a diameter of each of the plurality of paths decreases as a distance from the single path increases in the first direction, wherein an end of each of the plurality of paths is positioned deeper inside of the single-multiple return bend as the distance from the single path increases in the first direction, wherein a diameter of a gas flow path formed in the inflow single path gradually increases from the combustion gas inlet to an end opposite the combustion gas inlet, wherein a plurality of concave portions recessed inward is formed in a surface of at least two paths among the plurality of paths provided in the at least one multiple path, wherein a concave portion of the plurality of concave portions formed in a surface of one side of each of the at least two paths is staggered from a concave portion of the plurality of concave portions formed in a surface of the other side opposite to the one side, wherein a concave portion of the plurality of concave portions formed in a surface of a path positioned on an upper side among the at least two paths is staggered from a concave portion of the plurality of concave portions formed in a surface of a path positioned on a lower side, and wherein shapes of the plurality of concave portions formed in the surface of the path positioned on the upper side among the at least two paths and the plurality of concave portions formed in the surface of the path positioned on the lower side are different.