Abstract: 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.

Abstract: A draft limiting device for controlling the draft in a flue gas passage, includes a flap which is supported on a swing shaft in a secondary air inlet for swiveling movement between a closed position, at which the flap extends in a substantially vertical plane to block entry of air from a room into the secondary air inlet, and a fully open position at which the flap is displaced a certain angle about the axis of the swing shaft to admit air through the air inlet to the flue gas passage. A solid, semi-cylindrical member is fixed to the flap so that the center of radius of the member is parallel to the swing shaft axis, and the member extends on diametrically opposite sides of the swing shaft axis. The member thereby provides a restoring moment to the flap as the flap is moved in response to a differential pressure from the closed to the open position. The magnitude of the restoring moment is either constant or decreases as the flap moves in the direction from the closed to the open position.

Abstract: A shut-off device for installation in the waste gas line from combustion spaces. The device includes a flow-through housing, a shut-off plate mounted in this flow-through housing and adjustable, and a positioning motor arranged on the outside of the flow-through housing for the shut-off plate. The shut-off plate is biased by a spring in a direction toward a first terminal position defined by an abutment and is capable of being adjusted by the positioning motor in a direction toward a second terminal position defined by a second abutment. At least one limit switch is provided for actuation by a control element in the open position of the shut-off plate and serving to initiate operation of combustion spaces arranged upstream from the shut-off device. The positioning motor is a rapidly rotating electric motor having a motor shaft connected with the transmission input of a step down transmission for converting the rotary speed of the motor into a slower speed.