Abstract: An economically manufactured heat exchanger is provided and includes a plurality of elongated, straight, flat tubes (16), each formed of two identical halves (78,80) joined to each other in mirror image fashion and each having opposed open ends and all arranged in a stack (18). At least one spacer wall (36) is disposed within each tube and extends generally from end to end thereof to define at least two side-by-side first fluid flow paths within a tube. An elongated housing (10) contains the stack and includes spaced headers (20,22) with each header including a tube slot for the adjacent end of each tube (16) in the stack (18). Spacers (26) in the stack separate adjacent tubes in the stack from one another and the spacers include ribs defining a serpentine flow path for a second fluid. An opening (30) is located in the housing (10) near each end thereof and is in fluid communication with the second fluid flow path (28).