An environmental control system includes a cross-flow heat exchanger, a first valve, and a second valve. The heat exchanger includes three cores each with their own respective inlet, outlet, and cold air passage. The first inlet is fluidly connected with a hot air source. The second outlet is fluidly connected to the first outlet. The third inlet is fluidly connected to the second inlet and to a hot air destination. The first valve is fluidly connected with the hot air source, the second inlet, and the third inlet. The first valve is positioned in-line between the hot air source and the second and third inlets. The second valve is fluidly connected to the hot air destination, to the first outlet, and to the second outlet. The second valve is positioned in-line between the hot air destination and the first and second outlets.