A fireplace flue is the part of a fireplace system that connects the actual opening to the outside. It runs through the chimney, and can be made of masonry or metal. Often, one is made of round or square terracotta pipe stacks, or round metal pipe. The purpose of the flue is to vent smoke and gases from the fire to the outdoors.
If it is connected to a freestanding fireplace, a fireplace flue sometimes includes a chimney thimble. This is a sleeve of pipe, usually metal, that connects the piping coming out of the fireplace to the flue opening. It often has a damper mechanism, commonly a piece of metal that fits in or just below it, which can be opened or closed. The purpose of a damper is to prevent heat loss when a fireplace is not in use. Opening a damper when a fire is burning should allow smoke to travel up the chimney.
If smoke begins to fill the room when a fire is lit, instead of going up the fireplace flue, the damper should be checked, to make sure it is open. If it is, there are some other common causes of a fireplace not drafting properly. One that is easy to fix is a blocked flue, often due to a bird, hornet, or other animal's nest. Having a chimney sweep remove the nest alleviates the problem. Sometimes, a chimney cap is helpful in preventing animals from making their nests inside a flue.
Another situation that can create a smoke problem can be that the fireplace flue must be primed, or pre-heated. Sometimes, a cold flue will not draw smoke up. To fix this problem, a rolled up piece of newspaper can be lit and held as far up into the flue, past the damper, as possible. This may need to be done more than once to warm the air adequately. This whole process is known as priming.
Another potential problem in a fireplace flue is the build-up of creosote — a sticky tar-like substance — that can accumulate due to the incomplete burning of wood. It is important to have an often-used fireplace flue cleaned regularly, so that creosote does not collect on the inside of it. If this substance is not removed, it can ignite, causing a dangerous chimney fire.