Abstract: An improved heat exchanger apparatus that serves alternatively as an evaporator and as a condenser, the heat exchanger apparatus comprising a frame and a heat exchange coil supported by the frame, the coil having a first coil end portion, a coil medial portion and a second coil end portion. The coil passes sinuously between ends of the frame such that the first coil end portion and the second coil end portion are disposed in the first coil row that is first contacted by air blown through the exchanger, and the coil medial portion is disposed to pass sinuously in subsequent coil rows so that approximately one half of the coil medial portion passes in co-flow heat exchange relationship to the air and approximately one half of the coil medial portion passes in counter-flow heat exchange relationship to the air.