Flue Damper

A flue damper is a movable plate or disc installed inside the chimney flue that can be closed after the fire has completely extinguished to prevent warm indoor air from escaping up the chimney. An open flue acts like an open window, allowing heated air to flow out of the building continuously, which significantly increases heating costs during cold months.

The energy savings from properly using a flue damper are substantial. Studies show that an open chimney flue can increase a building's heating energy consumption by 10 to 20 percent. In a Finnish winter, this translates to hundreds of euros in unnecessary heating costs. The damper should be closed as soon as the fire has fully died and all embers have gone cold — never while any combustion is still occurring.

The most critical safety rule for flue dampers is that the damper must always be fully open before lighting a fire and must remain open until combustion is completely finished. Closing the damper while the fire is still burning or smoldering traps carbon monoxide inside the building, which is a lethal hazard. Many modern dampers include safety features such as partial-close positions that maintain minimum ventilation.

Over time, damper mechanisms can seize, warp, or corrode, preventing proper operation. A damper that does not open fully restricts draft and causes smoke to spill into the room. A damper that does not close fully wastes energy. The damper should be checked during the annual chimney sweep and repaired or replaced if it does not operate smoothly and seal properly.