A damper is a metal valve or blocking device that regulates the flow of air inside duct piping or a chimney flue, thereby controlling the combustion rate of the fire in the stove below it. Dampers can be automatic or manual, but manual dampers are much more common on wood-burning stoves. A handle or lever on the outside of the duct work operates the damper to either open or close it. The damper must be partially or fully open during operation of the stove so that there is a draw effect, pulling the smoke up and out of the home via the flue and drawing fresh air from the room to feed the combustion of the fire.
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If an thermostatic control has been installed with your wood stove, it will automatically and constantly regulate the airflow into the flue by moving the damper with a small motor to control the combustion of the fire, keeping it operating at an optimal heat output level.
A Secondary Damper
As a safety precaution, a second damper is sometimes installed much higher in the vertical stack of the chimney solely for the purpose of shutting off the air to the stove in the event a fire ignites in the chimney stack itself. Since wood soot tends to build creosote more quickly and burning embers do find their way into the flue fairly regularly, this can be a wise investment. These secondary or chimney dampers can be purchased from fireplace materials suppliers or can be fabricated at your local sheet metal works.