A heat exchanger comprising: a heat exchange element having a spaced-apart inlet and outlet and a plurality of heat exchange passages extending therebetween, characterized by a bypass valve assembly including: a generally flat main body member connected to the heat exchange element and having: a transverse inlet opening defined through the body member and communicating with the heat exchange element inlet; a cylindrical wall portion defining a transverse outlet opening through the body member and orientated coaxially and communicating with the heat exchange element outlet, the cylindrical wall portion having a bypass opening formed therein, and a bypass passage extending between the inlet opening and the bypass opening; a flexible flapper dispensed within the outlet opening; locating means for maintaining the location of the flapper in the outlet opening, wherein the flapper has a free end portion biased for movement from an open position, spaced from the bypass opening, to a closed position, overlying the bypass opening, the free end portion being dimensioned to restrict flow through the bypass opening when disposed at its closed position. a heat exchange element having a spaced-apart inlet and outlet and a plurality of heat exchange passages extending therebetween, characterized by a bypass valve assembly including: a generally flat main body member connected to the heat exchange element and having: a transverse inlet opening defined through the body member and communicating with the heat exchange element inlet; a cylindrical wall portion defining a transverse outlet opening through the body member and orientated coaxially and communicating with the heat exchange element outlet, the cylindrical wall portion having a bypass opening formed therein, and a bypass passage extending between the inlet opening and the bypass opening; a flexible flapper dispensed within the outlet opening; locating means for maintaining the location of the flapper in the outlet opening, wherein the flapper has a free end portion biased for movement from an open position, spaced from the bypass opening, to a closed position, overlying the bypass opening, the free end portion being dimensioned to restrict flow through the bypass opening when disposed at its closed position. a generally flat main body member connected to the heat exchange element and having: a transverse inlet opening defined through the body member and communicating with the heat exchange element inlet; a cylindrical wall portion defining a transverse outlet opening through the body member and orientated coaxially and communicating with the heat exchange element outlet, the cylindrical wall portion having a bypass opening formed therein, and a bypass passage extending between the inlet opening and the bypass opening; a flexible flapper dispensed within the outlet opening; locating means for maintaining the location of the flapper in the outlet opening, wherein the flapper has a free end portion biased for movement from an open position, spaced from the bypass opening, to a closed position, overlying the bypass opening, the free end portion being dimensioned to restrict flow through the bypass opening when disposed at its closed position.