Abstract: A furnace heat exchanger with multiple parallel flow passages wherein the last flow passage extends downwardly from its plane as it approaches the outlet so as to result in a lower overall height of the heat exchanger. Preferably both the upper wall and the lower wall of the last passage extend downwardly such that the cross-sectional area of the last flow passage is not reduced and is preferably increased as it approaches the outlet. The bottom wall extension is preferably curvilinear and has substantially the same curvature as a return bend at the inlet of the penultimate flow passage.

Abstract: A furnace heat exchanger with multiple parallel flow passages with at least two of the passages being partially formed of a pair of opposed sidewalls having wavy cross-sectional shapes wherein the downstream passage has at least as many and preferably more waves than the upstream pass. The wavy shapes are preferably generally sinusoidal in form with the waves of the two sides being substantially in phase.