A top-fired reformer box is provided. The top-fired reformer box includes a burner row, a tube row, a reformer tunnel including a closed end, an open end, and a plurality of tunnel ports formed along a side wall of the reformer tunnel, the plurality of tunnel ports including a one or more tunnel port located along the side of the tunnel, and a flow resistor positioned at least one tunnel port applying a flow resistance for flue gas entering the reformer tunnel via the at least one tunnel port such that uniform flow is achieved within the reformer tunnel.