A recirculation heat exchanger for use in an environmental control system of an aircraft is provided including a rectangular core having a plurality of alternately stacked first fluid layers and second fluid layers. The rectangular core has a width of about 4.750 inches (12.065 cm), and a length of about 10.6 inches (26.924 cm). A first air header is arranged adjacent a first surface of the core and a second air header is arranged adjacent a second, opposite surface of the core. The first and second air header form a portion of a flow path for a first fluid. A first fluid header is arranged adjacent a third surface and a second fluid header is arranged adjacent a fourth surface of the core. The first and second fluid header form a flow path for the second fluid having a multi-pass configuration.