Abstract: A heat exchanger for an air heating and cooling apparatus includes a plurality of exteriorly-finless hollow refrigerant-carrying stainless steel spirally wound coils being of flattened oval shape in cross-section and a plurality of angularly-spaced criss-cross grid-forming structures for supporting the coils in spaced-apart relation and aligned in columns and rows in a stack thereof for permitting passage of external air flow across and between the coils. The spirally wound coils are supported by the grid structures to have an annular-shaped configuration defining an open interior in the coil stack core.