Abstract: A scrubber bypass damper comprises a plate pivotally-mounted to a shaft running across the bypass duct transverse to gas flow. The damper plate self-actuates and rotates about the shaft in response to any pressure differential established across it. A scrubber booster fan is disposed in the main flue at a location downstream of the scrubber and upstream of the outlet of the scrubber bypass into the main flue. When the scrubber is in operation, the pressure rise imparted to the flue gas by the scrubber booster fan is adjusted to keep the damper plate disposed transverse to gas flow through the bypass duct thereby preventing gas flow therethrough. When the scrubber is taken out-of-service, the booster fan is shutdown and the damper plate self-actuates, opening in response to pressure forces exerted on it by the induced draft fan, thus preventing over-pressurization of the furnace.