Abstract: Apparatus for controlling the relative rates of flow of gases through a pair of ducts including a bellows chamber defining a substantially enclosed interior and a partition member movably mounted within the chamber dividing the interior thereof into first and second partial volumes which are fluidly sealed from each other. The first partial volume is in fluid communication with a first duct in which a gas flows while the second partial volume is in fluid communication with a second duct in which a gas flows such that a change in the rate of flow of a gas through one of the ducts changes the pressure in the associated partial volume whereby the partition member will move to accommodate the same. A linkage is associated with the partition member for transmitting the movement thereof to a damper located in one of the ducts such that the gas flow opening provided by the damper in the duct is regulated by the extent of movement of the partition member.