Abstract: A heat exchanger for exchanging heat between a cooler first air flow and a warmer second air flow is disclosed. The heat exchanger comprises a heat exchanger unit having a housing containing a plurality of heat exchanger plates stacked together to form a stack. Each heat exchanger plate has four corners and an aperture located adjacent each comer, the heat exchanger plates being dimensioned and configured such that the apertures of the heat exchanger plates overlap to form a first, second, third and fourth air passageways in the stack.