A heat exchanger for an aircraft comprising: a hot fluid inlet; a hot fluid outlet; a cold fluid inlet; a cold fluid outlet; and a header connected to the hot fluid outlet, the header comprising: a housing defining a header volume; a baffle separating the header volume into a first volume and a second volume, wherein the first volume and the second volume are in fluid communication with each other; and a bypass outlet in fluid communication with the first volume; wherein the housing includes first and second end walls (186; 188) defining a housing length, and first and second side walls (190, 192), the baffle extending from the first end wall toward the second end wall; characterised in that: the header further comprises a heat exchanger outlet located in the housing opposite the bypass outlet; the heat exchanger further comprises a bypass valve configured to control fluid flow through the bypass outlet such that: when the bypass valve is closed, no air can flow through the bypass outlet, and all air in the header volume will flow through the heat exchanger outlet; and when the bypass valve is open, air from the first volume can flow into and through the bypass outlet, and air within the second volume will flow through the heat exchanger outlet; wherein a length of the baffle is less than the housing length; and wherein the bypass valve and baffle are arranged such that when the bypass valve is open a volume of air comprising 0% to 60% of a total air within the header volume will flow through the bypass outlet. a hot fluid inlet; a hot fluid outlet; a cold fluid inlet; a cold fluid outlet; and a header connected to the hot fluid outlet, the header comprising: a housing defining a header volume; a baffle separating the header volume into a first volume and a second volume, wherein the first volume and the second volume are in fluid communication with each other; and a bypass outlet in fluid communication with the first volume; wherein the housing includes first and second end walls (186; 188) defining a housing length, and first and second side walls (190, 192), the baffle extending from the first end wall toward the second end wall; a housing defining a header volume; a baffle separating the header volume into a first volume and a second volume, wherein the first volume and the second volume are in fluid communication with each other; and a bypass outlet in fluid communication with the first volume; wherein the housing includes first and second end walls (186; 188) defining a housing length, and first and second side walls (190, 192), the baffle extending from the first end wall toward the second end wall; characterised in that: the header further comprises a heat exchanger outlet located in the housing opposite the bypass outlet; the heat exchanger further comprises a bypass valve configured to control fluid flow through the bypass outlet such that: when the bypass valve is closed, no air can flow through the bypass outlet, and all air in the header volume will flow through the heat exchanger outlet; and when the bypass valve is open, air from the first volume can flow into and through the bypass outlet, and air within the second volume will flow through the heat exchanger outlet; wherein a length of the baffle is less than the housing length; and wherein the bypass valve and baffle are arranged such that when the bypass valve is open a volume of air comprising 0% to 60% of a total air within the header volume will flow through the bypass outlet. the header further comprises a heat exchanger outlet located in the housing opposite the bypass outlet; the heat exchanger further comprises a bypass valve configured to control fluid flow through the bypass outlet such that: when the bypass valve is closed, no air can flow through the bypass outlet, and all air in the header volume will flow through the heat exchanger outlet; and when the bypass valve is open, air from the first volume can flow into and through the bypass outlet, and air within the second volume will flow through the heat exchanger outlet; when the bypass valve is closed, no air can flow through the bypass outlet, and all air in the header volume will flow through the heat exchanger outlet; and when the bypass valve is open, air from the first volume can flow into and through the bypass outlet, and air within the second volume will flow through the heat exchanger outlet; wherein a length of the baffle is less than the housing length; and wherein the bypass valve and baffle are arranged such that when the bypass valve is open a volume of air comprising 0% to 60% of a total air within the header volume will flow through the bypass outlet.