From NEPA Crossroads Knowledge Base
A barometric damper is a device which can automatically regulate the draft in a stove / chimney installation. It can prevent excessive draft from coming through the combustion chamber in a stove by admitting a controlled amount of air to the flue/chimney under normal burning conditions. There is an adjustable counter balance weight on the flapper of the barometric damper to allow the barometric damper to be set to different control points.
If high winds or a change in outdoor temperature increases the draft in a stove/chimney set up, the barometric damper allows the excess draft to be pulled through it rather than the combustion chamber of the stove. A manometer could be used to accurately set the counter balance weight on the barometric damper.
The barometric damper is a "T" style flue pipe fitting installed in line in the flue pipe. It can be in a horizontal, vertical or diagonal section of the flue pipe. The flapper side of the barometric damper is adjustable so that it can be made horizontal no matter what the orientation of the install.