A double-wall heat exchanger comprising: a plurality of heat transfer plate pairs (12; 14), each heat transfer plate pair (12; 14) forming a double-wall structure comprising two heat transfer plates (12, 12, 12, 14, 14; 36, 38) that are at least partially separated by a leak space, wherein each of the heat transfer plates (12, 12, 12, 14, 14; 36, 38) having a series of undulations to facilitate heat transfer, the leak space positioned between the undulations of the heat transfer plates (12, 12, 12, 14, 14; 36, 38) of each heat transfer plate pair (12; 14); at least one fluid port (A, B, C, D) defined through each plate pair (12; 14) within which a heat exchange fluid is distributed either into or out a fluid channel (28; 30; 32; 34) that is defined between adjacent plate pairs (12; 14), wherein two adjacent plate pairs (12; 14) are mated together along a boundary edge of the at least one fluid port (A, B, C, D), wherein the fluid channels (28, 30, 32, 34) are fluidly isolated from the leak spaces, and wherein adjacent fluid channels (28, 32; 28, 34; 32, 28; 32, 30) are fluidly isolated from each other and alternating fluid channels (28, 30; 32, 34) are in fluid communication with each other; characterized by a port vent groove defined between the two adjacent plate pairs (12; 14) at a location surrounding the boundary edge of the at least one fluid port (A, B, C, D), wherein the port vent groove intersects and is in fluid communication with a leak space of one of the two adjacent plate pairs (12; 14); and at least one weep hole (50, 52) that is disposed through the plurality of heat transfer plate pairs (12; 14) and intersects the leak spaces of the plurality of plate pairs (12; 14) to channel leaking fluid within one of the leak spaces or the port vent groove to a location outside of the heat exchanger. a plurality of heat transfer plate pairs (12; 14), each heat transfer plate pair (12; 14) forming a double-wall structure comprising two heat transfer plates (12, 12, 12, 14, 14; 36, 38) that are at least partially separated by a leak space, wherein each of the heat transfer plates (12, 12, 12, 14, 14; 36, 38) having a series of undulations to facilitate heat transfer, the leak space positioned between the undulations of the heat transfer plates (12, 12, 12, 14, 14; 36, 38) of each heat transfer plate pair (12; 14); at least one fluid port (A, B, C, D) defined through each plate pair (12; 14) within which a heat exchange fluid is distributed either into or out a fluid channel (28; 30; 32; 34) that is defined between adjacent plate pairs (12; 14), wherein two adjacent plate pairs (12; 14) are mated together along a boundary edge of the at least one fluid port (A, B, C, D), wherein the fluid channels (28, 30, 32, 34) are fluidly isolated from the leak spaces, and wherein adjacent fluid channels (28, 32; 28, 34; 32, 28; 32, 30) are fluidly isolated from each other and alternating fluid channels (28, 30; 32, 34) are in fluid communication with each other; characterized by a port vent groove defined between the two adjacent plate pairs (12; 14) at a location surrounding the boundary edge of the at least one fluid port (A, B, C, D), wherein the port vent groove intersects and is in fluid communication with a leak space of one of the two adjacent plate pairs (12; 14); and at least one weep hole (50, 52) that is disposed through the plurality of heat transfer plate pairs (12; 14) and intersects the leak spaces of the plurality of plate pairs (12; 14) to channel leaking fluid within one of the leak spaces or the port vent groove to a location outside of the heat exchanger.