What is a Gas Flue?

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  • Written By: B. Turner
  • Edited By: W. Everett
  • Last Modified Date: 28 November 2019
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A gas flue is a device used to vent exhaust air from a combustion system. These systems include things such as fireplaces and furnaces that burn natural gas or propane to produce heat. A large-scale gas flue is also used in commercial settings to vent exhaust air from heating systems or industrial equipment.

As a furnace or other system burns gaseous fuels for heat, they also produce by-products in the form of carbon monoxide and other toxic fumes. Without an effective exhaust system, these fumes would pass into the home, where they could pose serious health risks to occupants. A gas flue connects to the fireplace and directs these fumes outdoors to keep occupants safe.

The gas flue can rely on one of two basic types of operation. Some contain integral fans or blowers that direct exhaust up the flue or chimney. Others operate according to the stack effect, which is based on the natural principles of buoyancy. The exhaust gas produced during combustion is still very hot when it enters the flue. Because it is hotter than air in the home, or air outside, it naturally rises up and out of the chimney or flue without the need for a blower.


The exact design of a gas flue can vary based on local building codes, as well as the size and design and the furnace or fireplace it is being used to vent. Engineers size the flue to maximize the amount of exhaust air that exits the home, and to minimize the risk that dangerous fumes will enter the living spaces. The material used for these flues must also be chosen carefully to prevent damage and reduce maintenance requirements over time.

A gas flue that is improperly maintained or poorly designed can pose serious health risks and fire danger. A leaky flue may allow toxic gases to escape into the home, especially when this flue is installed within an interior wall or ceiling. If the flue is not cleaned at regular intervals, gases and combustion by-products can build up and cause blockages, which can interfere with the exhaust process. Some flues contain a special combustion chamber that collects excess heat energy and reuses it to heat the home. Others capture toxic materials so they can be filtered or disposed of safely, which helps to minimize the effects of pollution.

When the furnace or fireplace is not in use, a damper may be used to block air from entering or exiting the home. These dampers improve energy efficiency and help keep the home more comfortable. It is critical that the damper is opened before the fireplace or furnace is used however, as a closed damper would allow fumes to quickly build up within the home.


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