With a view to a heat delivery that is as symmetrical as possible, it is suggested according to a preferred development of the present invention that the heating rib frame is provided with two opposite windows in which an electrical insulation layer is respectively exposed, which covers an associated strip conductor. The window(s) is (are) preferably formed by frame legs of the heating rib frame that extend in parallel with the electrical insulation layers. The insulation layer lies preferably on the inside against these frame legs. The insulation layer with its outer surface lies on the frame legs accordingly. The frame legs can surround the insulation layer on the outer circumference. The aforementioned mass is preferably provided between the PTC heating element and inner walls of the recess and is preferably configured to be electrically insulating. With a view to good processability and the filling of free spaces within the heating rib frame, the viscosity should be between 5 and 8 Pa s. This viscosity value and also all of the viscosity values discussed herein are determined at 25° C.