In an exhaust gas duct from a furnace a ventilation opening with a ventilation flap is openable in chronological conjunction with the cut-off of the furnace and it is closable in chronological conjunction with the switching on of the furnace. The ventilation opening is formed by a draft-limiter opening located downstream from the furnace and the ventilation flap is formed by a draft-limiter flap. During a pause in furnace operation, the draft-limiter flap is held in an open position independently of the pressure difference prevailing at the draft-limiter flap. One opening and one flap are eliminated. The draft-limiter flap provides the ventilation function as well as the draft-limiter function. If necessary, it can be installed in a small space together with an exhaust gas flap.
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