Flue Mechanism and Updraft
The flue is the metal duct work, ceramic piping or brick chimney stack that runs from a firebox to the top of a chimney to the outdoors. The flue directs exhaust gases and debris from a fireplace upward to vent outdoors into the atmosphere. The flue is effective because it makes use of the stack effect that happens when there's a significant temperature differential between the indoor and outdoor ends of the flue. This has the effect of drawing the exhaust gases and ash, which are lighter weight than the cold outdoor air, up through the flue and venting them outside. Some heat is lost as well in this process, and modifications can sometimes be made in or above the firebox to minimize this ancillary heat loss.
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Fresh Air Supply
The flue has a secondary purpose of allowing fresh air to enter it, which aids in feeding or oxygenating a fire. At the fireplace end of the flue is a lever-operated damper device that can be opened to allow exhaust gases to escape, and fresh air to enter, when the fireplace is in operation. The lever can be closed to prevent cold air from entering a home when the fireplace is not in use.
Troubleshooting Your Flue
Problems with the flue can make the fireplace operate inefficiently or impede the draw effect and the proper venting of exhaust. If your fireplace smokes heavily and the damper if fully open, the flue could be blocked at some higher point by soot, creosote debris, a trapped animal or even a bird nest. If smoke comes into the room, it's best to hire a professional chimney sweeping service. If the fireplace smokes a little or occasionally, make sure the damper is in the fully open position. It may be jammed in a semi-open position or broken, in which case you'll need to hire a chimney service to repair or replace it. Another issue could be the draw, when the top of the flue is not exposed to enough clear air flow. Whether a tree or bush has grown up around the flue vent or a building addition has changed the movement of air around the flue, even subtle limitations of air flow can change flue function dramatically. This can often be solved by increasing the length of the flue with a stretcher piece or a chimney exhaust fan.