Abstract: The panel, as constructed of two plates or sheets marginally seam-welded together and subdivided internally into parallel longitudinal flow passages between vertically spaced inlet and outlet fittings, provides for a well distributed, low-volume flow of an appropriate heat transfer liquid therethrough, preferably from bottom to top thereof.In one illustrated embodiment, such equalized flow distribution is insured by restricted central throat or diverter formations lying between the respective fittings, hence the opposed inlet and outlet ends of said longitudinal flow passages. Moreover, the center-most of said flow passages is longer than those at either side thereof, which progressively decrease in length to the side-most thereof, so that said header spaces incline mildly downwardly in reference to the vertical. This contributes to improved drainage of the panel when desired; it also tends to make for an even distribution as to the vertical flow.