A heat exchanger assembly for an aircraft with an environmental control system, the heat exchanger assembly comprising: a first heat exchanger with a first hot air circuit passing therethrough, wherein the first heat exchanger is configured to be in fluid communication with a source of bleed air from the aircraft; a second heat exchanger with a second hot air circuit passing therethrough, wherein the second heat exchanger is disposed adjacent to and in fluid communication with the first heat exchanger; a third heat exchanger with a third hot air circuit passing therethrough, wherein the third heat exchanger is disposed adjacent to and in fluid communication with the second heat exchanger, wherein the first, second, and third heat exchangers (18, 16, 14) are fluidly connected in series; and a cold air circuit passing through each of the first, second, and third heat exchangers (18, 16, 14), wherein the first, second, and third heat exchangers (18, 16, 14) are in cross-flow communication with the cold air circuit; a first closure bar disposed between the first and second hot air circuits, wherein the first closure bar prevents fluid communication across the first closure bar between the first and second hot air circuits; a second closure bar disposed between the second and third hot air circuits, wherein the second closure bar prevents fluid communication across the second closure bar between the second and third hot air circuits; a first top Y-septum welded to a top end of the first closure bar; a second top Y-septum welded to a top end of the second closure bar; a first bottom Y-septum welded to a bottom end of the first closure bar; and a second bottom Y-septum welded to a bottom end of the second closure bar. a first heat exchanger with a first hot air circuit passing therethrough, wherein the first heat exchanger is configured to be in fluid communication with a source of bleed air from the aircraft; a second heat exchanger with a second hot air circuit passing therethrough, wherein the second heat exchanger is disposed adjacent to and in fluid communication with the first heat exchanger; a third heat exchanger with a third hot air circuit passing therethrough, wherein the third heat exchanger is disposed adjacent to and in fluid communication with the second heat exchanger, wherein the first, second, and third heat exchangers (18, 16, 14) are fluidly connected in series; and a cold air circuit passing through each of the first, second, and third heat exchangers (18, 16, 14), wherein the first, second, and third heat exchangers (18, 16, 14) are in cross-flow communication with the cold air circuit; a first closure bar disposed between the first and second hot air circuits, wherein the first closure bar prevents fluid communication across the first closure bar between the first and second hot air circuits; a second closure bar disposed between the second and third hot air circuits, wherein the second closure bar prevents fluid communication across the second closure bar between the second and third hot air circuits; a first top Y-septum welded to a top end of the first closure bar; a second top Y-septum welded to a top end of the second closure bar; a first bottom Y-septum welded to a bottom end of the first closure bar; and a second bottom Y-septum welded to a bottom end of the second closure bar.