Flue

A flue is a duct, pipe, or channel that carries exhaust gases from a combustion source to the outside atmosphere. In buildings, flues are typically part of a chimney system that vent smoke, heat, and byproducts from fireplaces, stoves, boilers, or furnaces. Flues are designed to withstand heat and corrosion and to minimize condensation and backdraft.

Flues can be made of masonry with clay tile liners, or of metal in single-wall or double-wall configurations. Metal liners and prefabricated flues are common in modern homes, and flexible stainless steel liners provide retrofit options for existing chimneys. The choice of material depends on temperature, moisture, and the type of appliance.

Draft, sizing, and routing influence performance. A properly designed flue should be tall enough and unobstructed to create a steady upward draft. Horizontal sections or excessive turns can hinder draft and promote condensation and creosote buildup. Flues require regular cleaning, inspection, and a cap or spark arrester to keep out rain and animals and to prevent embers from escaping.

Safety and regulation: Construction and installation are governed by local codes and standards. Clearances to combustibles, proper connection to the appliance, and the presence of an approved cap are important to reduce fire risk and moisture intrusion. Creosote buildup is a leading cause of chimney fires; periodic professional cleaning is recommended.

In industry, flue refers to a flue gas duct or stack that conveys exhaust gases from boilers, furnaces, or waste heat boilers to the atmosphere. Such systems often include pollution control equipment to remove particulates and gases before discharge.