A header for a heat exchanger includes a first and a second cylindrical fluid manifold extending in parallel. Each of the first and second manifolds have tube slots that extend through an arcuate wall section of the manifold. A thickened wall section of the header having a generally triangular wall section is bounded by the first and second fluid manifolds and by a planar outer surface of the header. An aperture extends through the thickened wall section to provide a fluid communication pathway between the first and second cylindrical fluid manifolds.