Fuel burning furnace

A furnace comprising a longitudinally extending burner for use in burning fuel to form hot combustion gases along the length of said burner, a heat exchange enclosure, said heat exchange enclosure including a longitudinally extending exhaust duct portion disposed above said burner for receiving hot combustion gases, and first and second sidewalls at least partially enclosing said burner and extending upwardly from locations adjacent to said burner to said exhaust duct portion and having inner side surfaces which cooperate to define a combustion gas flow path from said burner to said exhaust duct portion, said combustion gas flow path having a width in a direction parallel to the central axis of said burner which is substantially the same as the length of said burner, each of said sidewalls having an undulating cross sectional configuration as viewed in a plane extending perpendicular to the longitudinal axes of said burner and exhaust duct portion, a first series of inwardly projecting undulations formed across the width of said first sidewall and projecting inwardly toward said second sidewall, a second series of inwardly projecting undulations formed across the width of said second sidewall and projecting inwardly toward said first sidewall, said first and second series of inwardly projecting undulations being aligned with each other to form a plurality of spaced apart restricted areas in the combustion gas flow path, a first series of outwardly projecting undulations formed across the width of said first sidewall in an interspersed relationship with said first series of inwardly projecting undulations and projecting outwardly away from said second sidewall, a second series of outwardly projecting undulations formed across the width of said second sidewall in an interspersed relationship with said second series of inwardly projecting undulations and projecting outwardly away from said first sidewall, said first and second series of outwardly projecting undulations being aligned with each other to form a plurality of spaced apart enlarged areas in the combustion gas flow path at locations between the restricted areas in the combustion gas flow path to provide a combustion gas flow path which extends from said burner to said exhaust duct portion and is alternately restricted and expanded to promote a restricted and turbulent flow of combustion gas between said burner and exhaust duct portion, a first plurality of said restricted areas in said combustion gas flow path being offset in a first direction relative to a plane containing the longitudinal central axes of said burner and exhaust duct portion, a second plurality of said restricted areas being offset in a second direction relative to the plane containing the longitudinal central axes of said burner and exhaust duct portion, said first plurality of restricted areas being interspersed with said second plurality of restricted areas so that the combustion gas flow path moves sidewardly back and forth relative to the plane containing the central axes of said burner and exhaust duct portions and the combustion gas flows into and out of each of the enlarged areas of the combustion gas flow path at locations which are sidewardly offset, and means for providing a flow of air to be heated across undulations in outer side surfaces of said sidewalls to enable heat to be transmitted from the hot combustion gas disposed between said sidewalls to the air disposed outside of said sidewalls.