A rapid warm-up heat exchanger for heating a fluid using exhaust gas includes multiple plate pairs that are joined by braze joints to form a stack. A fluid inlet manifold and a fluid manifold extend through the stack, and each one of the plate pairs defines a tortuous flow path for the fluid that extends between the fluid inlet and fluid outlet manifolds. A housing surrounds the stack, and together the housing and the stack define an exhaust flow path in spaces provided between adjacent plate pairs. A valve element can be provided within the housing in order to selectively direct exhaust flow through the exhaust flow path.