A chimney flue is a chamber through which smoke, hot gases, and other byproducts of combustion are vented. The chimney is the upright structure that surrounds the flue and provides heat insulation to reduce the risk of fire, and it is possible for a single chimney to have multiple flues, as may be the case when a home has several stoves or fireplaces which are linked to the same chimney. Chimney flues come in a range of styles and sizes, and they require regular maintenance and inspection for safety.
The purpose of a flue is to provide a vent so that the byproducts of combustion do not back up into a room. Ideally, it carries away gases and smoke with minimal heat loss, so that the fire provides warmth without endangering the people indoors. Flues are usually straight, as this provides the best draft, ensuring that there is a steady flow of unwanted material out of the chimney. The amount of draft can be controlled with the use of a damper mounted inside, allowing the user to strike the right balance between ventilation and heat loss.
Flues are usually made from heat resistant materials, because gases can be very hot. The surrounding chimney adds additional fire resistance, and also reduces heat loss, as the bricks in the chimney heat when the flue gases pass by, radiating that heat outwards into the building. Ceramic, tile, and brick are common construction choices for chimney flues, although it is also possible to use various metals.
Because it is difficult to get a fire to burn with total efficiency, over time, a flue will become clogged with creosote and other byproducts of incomplete combustion. In addition to reducing the available draft and making the flue less efficient, these products also pose a fire hazard. If they are ignited by passing sparks, they can cause chimney fires, which may be very dangerous. For this reason, it is critical to inspect flues regularly, and to clean them periodically to remove buildup and prevent fires.
A professional chimney sweep has the tools and experience to clean out a chimney flue, and he or she can also provide inspections to determine whether or not a flue needs cleaning or replacement. Over time, flues can break down, in which case they may need to be repaired or replaced, and replacement may also be recommended if one was built with substandard materials or materials that are no longer legal under regional building codes.