Abstract: This invention relates to an automatic damper device for use in exhaust flues, such as flues of combustion devices such as furnaces and water heaters, devices such as air conditioners, and the like, to prevent undesirable loss of hot air or cold air out the flues during periods when no combustion is taking place. The damper has a rotatable damper plate held in normal open position which provides a desired fail safe in the open position assurance. The damper is also adapted to remain in the open position during periods of combustion in the furnace or water heater but otherwise be retained in the closed position to avoid the unnecessary loss of heated air or cooled air.