Furnace heat exchanger

This invention relates generally to furnaces and, more particularly, to multipass heat exchangers therefor. 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.