Abstract: A parallel plate air-to-air counterflow heat exchanger employs a parallel array of spaced plates of similar elongated hexagonal shape. A pair of inlet and outlet ducts is connected to each lengthwise pointed end of the array. Each duct communicates with alternate airways via rectangular openings staggered on opposite sides of the ends of the array. The ducts extend parallel to a lengthwise midline of the array to enable air flow through the heat exchanger and ducts with minimal change of momentum and turbulence. The plates each have parallel widthwise margins which are oppositely rolled to different, complementary radii and adjacent plates are mutual mirror images. The plate margins nest together to interconnect the plates and enclose one side of each airway. A housing around the array encloses the opposite side of each airway.