In a first aspect of the present invention, a header for a heat exchanger is disclosed that has an inflow side, an outflow side, a bypass port therebetween, and a pressure-sensitive flapper valve proximal the bypass port. As pressure increases at the inflow side of the header, the flapper valve opens proportionally, and, as pressure decreases at the inflow side, the flapper valve closes proportionally. In a second aspect of the present invention, a tube-in-tube heat exchanger is disclosed that includes a helical tube-in-tube assembly adapted for flow therethrough of a plurality of fluids for heat transfer therebetween. In a third aspect of the present invention, a heat system is provided to include an embodiment of the header and an embodiment of the tube-in-tube heat exchanger, so as to provide a bypass of the tube-in-tube heat exchanger under a pressure condition.