A Gas Furnace Flue Duct Installation Guide


Venting a gas furnace with a flue helps remove combusted gases from your home. These gases contain harmful toxins such as carbon monoxide, and require a direct conduit from your furnace to the outside. Venting methods and installation procedures vary and special considerations in some setups. Important differences exist between the terms duct and pipe -- flue venting traditionally uses the latter term over the former.


  • Technically speaking, a pipe qualifies as a form of duct. The term “duct” broadly refers to any conduit used to transfer a material from one place to another, particularly a pipe or a tube. Despite the fact that duct and pipe are largely synonymous, literature on gas furnace venting uses the term “pipe” to refer to material used in the flue and the term “duct” to refer to material used to transfer heat from the furnace to interior environments. This distinction exists largely as a means of differentiating between the two to avoid confusion. Also, flue pipes always assume a tubular shape, while ducts can assume square or round shapes.

Venting Methods

  • Two primary methods of flue venting a gas furnace exist: connection to a chimney and connection directly to the outdoors. Venting through a chimney consists of installing pipes from the top of your furnace to your chimney so that flue gases vent out of the home through the chimney. Venting directly to the outdoors entails installing a vent that goes through an exterior wall or window, like a dryer vent might. Chimney venting proves favorable if possible because it carries gases into the air and away from the house. Venting through a wall may cause toxic gases to linger around your home near the ground.


  • Gas furnace flue installation proves relatively simple. Your furnace should possess a flue outlet on the top. The process entails purchasing appropriate piping, connecting it to the flue outlet on the top of your furnace on one end, and to your chimney on the other. When you connect your pipes to your chimney, you must create a hole in the chimney. Do this using a rotary saw. Pull bricks out of the chimney, using a hammer and chisel to break them up if necessary. Use a mounting bracket to secure your pipe in the chimney and apply a heat-resistant sealant to prevent gas from escaping around the connection. When your purchase a furnace, it should come with a manual containing installation instructions for everything, including the flue.

Special Considerations

  • Eugene Silberstein, author of the book “Residential Construction Academy: HVAC,” recommends always using a pipe with the same diameter as the flue outlet on your furnace. Use a B-vent, or curved connection, to transition from vertical to horizontal piping when connecting your furnace to a chimney or other outlet. A pipe should rise at least 1/4-inch per foot when placed in a horizontal position, though Silberstein recommends keeping horizontal sections to a minimum to facilitate venting. Support your flue pipe every 4 of 5 feet with a bracket of some kind, such as a suspended bracket lowered from the ceiling. Never drive screws through the actual pipe during installation -- this can lead to leaking gases.

Building Codes

  • Always check your local building codes before installing flue piping. Building codes may require the use of a specific type of piping, such as steel or aluminum, or certain types of brackets or fasteners. Many codes defer to the National Fuel Gas Code regarding issues like furnaces for gas-burning appliances. You can download old versions of this code for free though the most recent version must be purchased. As an alternative to research, contact a local contractor or furnace-installation service for information on local codes.

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