A furnace flue is the portion of the furnace that vents away harmful gasses. Depending of the type of furnace, this may be a small pipe, a household chimney or even an industrial smokestack. The gasses created during combustion, such as carbon monoxide, sulfur oxide or nitrogen oxide, are harmful to humans. The furnace flue moves these gasses to a ventilated area, allowing them to dissipate into the atmosphere without causing harm.
Most furnace flues operate on a system called buoyancy. All combustion processes create a wide variety of harmful gasses and particulates. These gases are almost always lighter and warmer than the surrounding air. This means that as the furnace pulls in oxygen to continue the combustion process, the warm and light gasses try to move up and away from the furnace. The flue provides an avenue for them to escape.
Some industrial processes don’t use standard combustion methods. If the industrial furnace creates a substance that is heavier than the surrounding air or that combusts so fast that the gasses don’t have time to escape, the system typically uses a blower. The blower pushes the harmful gasses up the flue and out of the system before they can move into inhabited areas or halt the combustion process. Some smaller furnaces incorporate a blower as well, but often that is more of a safety feature than a requirement.
The furnace flue generally vents the harmful gasses outside the human-occupied areas around the furnace. In a residential situation, this is usually right outside the house. Since the vented gasses are so light, they dissipate almost immediately when put into the air.
Industrial systems vent much more material at any given time. As a result, they use much larger and higher venting stacks. The amount of harmful gas they expel can be great enough to harm people at the point of release, which is why they use stacks that are so tall. The taller stacks keep the harmful gas away from people and get it into higher wind streams, where it will dissipate faster.
The gasses that a furnace flue vents are often very warm. Some household systems use that warmth to create a secondary warming zone. Instead of moving the gasses directly out of the house, the furnace flue travels into the wall or a specially-designed area, where it warms bricks. These bricks draw the heat from the gas as it passes, transferring heat through the house. Even if the gas loses a small amount of buoyancy from the heat loss, the gases below it continue to force it upwards.