A convection drying and/or fixing machine for the thermal treatment of a textile fabric web with a treatment gas includes nozzle boxes above and below the fabric web extending across the width of the fabric web, the nozzle boxes having inlet openings with a given size and shape for receiving the treatment gas and outlet openings for directing the treatment gas onto the fabric web, a pressure chamber directly adjacent the inlet openings for directing the treatment gas into the inlet openings of the nozzle boxes, the pressure chamber having bypass openings with substantially the given size and shape, each of the bypass openings being adjacent a respective one of the inlet openings forming a respective pair of openings, a return flow chamber receiving the treatment gas directly from the bypass opening and circulating the treatment gas to the pressure chamber, and common bypass flaps each being disposed at a respective one of the pair of openings for at least partially closing the openings of the respective pair of openings.