Abstract: An improved flue damper for isolating furnace from chimney-flue when burner is not operating includes in preferred embodiment a damper plate pivotally mounted within a flue section on a control rod extending through the flue section to bearing support in a bracket spaced from the flue section by a frame affixed to the flue section exterior; a bimetallic strip spiral having inner end affixed on the control rod and outer end detachably looped over a stud extending from the frame rotates the control rod under thermal actuation provided by an electric heating element adjacent the bimetallic strip spiral; a damper control strip on the end of the control rod adjustably affixes a switch plate rotatable therewith and bearing a first mercury switch controlling the heating element circuit and a second mercury switch controlling the furnace burner circuit; stop structure on the bracket limits rotation of the damper control strip and control rod.