A flue damper is a device which is used to regulate the flow of air through a flue, a pipe which vents hot air and gases from a wood stove, boiler, or similar piece of equipment. It is important to confirm that the flue damper is in operational condition before starting the piece of equipment it is attached to and the damper may need to be adjusted before use as well. In addition to flue dampers, many devices have vents which are used to control the intake of air.
Flue dampers serve a number of functions. They regulate the combustion rate by adjusting the amount of air which can flow through, and they also keep insects, rain, and the elements out of the inside of the equipment by blocking the flue.
When something like a wood stove is being used, the damper needs to be open. If the damper is not open, exhaust gases can build up and push out into the room. In addition to filling the room with smoke, this can be a health hazard, because byproducts of combustion are not healthy to inhale. In an airtight stove, a situation in which gases build up inside the stove can occur, and when the stove is opened, they can literally explode out the stove door, which is extremely undesirable.
Having the flue damper wide open when a fire is started allows lots of oxygen to move through the stove. Once the fire is established, the damper can be adjusted to slow down the rate of combustion, as may be done when a fire is damped down for the night. The intake vents can also be adjusted, depending on the design of the equipment. When no fire is lit, the flue damper should be closed to keep the elements out, but it's important to remember to open it back up again before a fire is lit.
Sometimes a flue damper becomes frozen in place, especially when the stove has not been used in a long time. Lubricants may need to be used to loosen it so that it can be moved. It is also important to be aware that while many dampers are adjustable from the outside, sometimes the controls for the damper are inside the stove, which means that the flue damper needs to be adjusted before lighting a fire because otherwise it will be too hot to move. People should also be careful about handling the damper handle, because it can get extremely hot thanks to the hot air and gases rushing up the flue.