Interrupter unit for a circuit breaker, comprising - two electrically conductive arcing contact pieces (5, 6), which are movable relative to one another along a switching path between a switch-off position, in which the arcing contact pieces (5, 6) are separated from one another by the switching path, and a switch-on position, in which the arcing contact pieces (5, 6) are in electrical contact with one another, - an insulating nozzle at least partially surrounding the switching path and having a nozzle channel, which passes through the insulating nozzle and through which the switching path passes, - a heating volume, which is connected to the nozzle channel, - a separating housing, which splits the heating volume into a cold gas region and a hot gas region and has at least one connecting opening connecting the cold gas region to the hot gas region, - a cold gas channel, which passes through a nozzle channel end section of the nozzle channel and is connected to the cold gas region of the heating volume, and - a hot gas channel, which passes through the nozzle channel end section of the nozzle channel and is connected to the hot gas region of the heating volume, - a compression volume, which is separated from the heating volume by a compression wall, wherein the compression wall is coupled to an arcing contact piece (5, 6), with the result that said compression wall reduces the size of the compression volume in the event of a relative movement of the arcing contact pieces (5, 6) from the switch-on position into the switch-off position, and wherein the compression wall has at least one compression wall opening, which is closed by an overflow valve when a pressure in the heating volume in the region of the overflow valve is higher than a pressure in the compression volume, characterized in that the overflow valve closes at least one connecting opening between the cold gas region and the hot gas region when the pressure in the heating volume is lower than the pressure in the compression volume. - two electrically conductive arcing contact pieces (5, 6), which are movable relative to one another along a switching path between a switch-off position, in which the arcing contact pieces (5, 6) are separated from one another by the switching path, and a switch-on position, in which the arcing contact pieces (5, 6) are in electrical contact with one another, - an insulating nozzle at least partially surrounding the switching path and having a nozzle channel, which passes through the insulating nozzle and through which the switching path passes, - a heating volume, which is connected to the nozzle channel, - a separating housing, which splits the heating volume into a cold gas region and a hot gas region and has at least one connecting opening connecting the cold gas region to the hot gas region, - a cold gas channel, which passes through a nozzle channel end section of the nozzle channel and is connected to the cold gas region of the heating volume, and - a hot gas channel, which passes through the nozzle channel end section of the nozzle channel and is connected to the hot gas region of the heating volume, - a compression volume, which is separated from the heating volume by a compression wall, wherein the compression wall is coupled to an arcing contact piece (5, 6), with the result that said compression wall reduces the size of the compression volume in the event of a relative movement of the arcing contact pieces (5, 6) from the switch-on position into the switch-off position, and wherein the compression wall has at least one compression wall opening, which is closed by an overflow valve when a pressure in the heating volume in the region of the overflow valve is higher than a pressure in the compression volume, characterized in that the overflow valve closes at least one connecting opening between the cold gas region and the hot gas region when the pressure in the heating volume is lower than the pressure in the compression volume.