What a header does
A header is the manifold that splits incoming refrigerant into a coil's individual circuits, or collects it again on the outlet side. It looks like a simple piece of tube with several branch stubs brazed on, but its job is to divide flow evenly. If one circuit gets more refrigerant than its neighbours, that circuit will under-perform on heat transfer while another runs starved — and the coil's rated capacity is never actually reached.
Why copper is the natural choice
Headers are brazed assemblies, often with a dozen or more joints in a small space, so the base material has to tolerate repeated heating without losing strength or developing pinholes. Copper's ductility and its compatibility with standard silver-brazing alloys make it far more forgiving here than alternatives — joints stay sound even after the thermal cycling a header goes through during both brazing and the equipment's operating life.
Distribution design details that matter
Even flow distribution comes down to details that are easy to overlook: branch stub spacing and angle, header diameter relative to the total circuit count, and whether the header includes internal baffling or a distributor nozzle for systems using a thermal expansion valve. Manufacturing tolerance matters too — stubs that are even slightly misaligned change the local pressure drop into that circuit.
Testing before it reaches a coil line
Because a header's performance is invisible until the finished coil is tested, header assemblies are typically checked for leak-tightness under pressure and dimensionally verified against drawing before they're released to the coil-forming line. Catching a bad header at this stage is far cheaper than finding uneven cooling on a finished, fin-stacked coil.