Abstract: The heat exchanger according to the invention comprises extremely flat oval-shaped tubes (3) extending in a mutually parallel and spaced-apart relationship and are connected on their narrow side to manifold pipes (1,2) for forward and return flow. To ensure a sufficiently stable mechanical connection between the pipes and a satisfactory supply and discharge of the heating medium from the manifolds to the flat tubes as well as in the reverse direction, a trough (5) is formed by material removal in the manifold pipes (1a, 2a) thus creating a connection aperture (6) in the wall of the pipe. The flat tube (3a) is fitted into this trough (5) with its rim on the narrow side, to ensure a solid stable welded joint between the pipes. The flat tube (3a) is provided on its narrow side with a bore serving as a connection aperture (7), which comes to lie centrally against the connection aperture (6) in the manifold (1a, 2a).