How do I Install a Twin Walled Flue Pipe?


The double-walled structure of a type B gas vent, also called twin-walled flue pipe, provides an additional measure of safety when venting gas appliances such as furnaces, boilers and water heaters by decreasing the minimum clearance required from combustibles to 1 inch and by providing a degree of insulation that prevents the flue pipe from forming condensation and thereby rusting. The drawback to using a type B gas vent, however, is the degree of planning and expertise needed for its installation.

Things You'll Need

  • Level
  • Pad
  • Pen
  • Tape measure
  • #7 sheet-metal screws
  • Locking band
  • Metals straps
  • Pipe hangers
  • Review the venting charts provided by the appliance manufacturer. These will be instrumental when planning the layout of your vent pipe. Also, you will find any special instructions here that are specific to your model appliance along with specifications on the inside diameter of the pipe needed and allowable lengths of horizontal vent pipe runs.

  • Measure from the vent connector on your appliance to the vent section that penetrates your roof. Follow the actual route that the pipe will travel when taking the measurements and take separate measurements between each change of direction. Keep in mind that most combustion gas venting requires a minimum of 1/4 inch per foot rise and there are usually limitations on the length of horizontal runs compared to vertical runs. Check the specifications provided by the appliance manufacturer.

  • Draw a diagram of the vent pipe route using your measurements while at the same time considering standard section lengths of type B vent. Available section lengths of type B vent are limited to a few standard lengths, and the pipe cannot be cut in the field for fitting. Typically, the standard lengths are 18 inches, 2 feet, 3 feet and 5 feet.

  • Use your diagram as a guide to fit your sections of pipe together. Most type B vent is directional, meaning that one end can be slid into another pipe, while the other end of the same piece can have a pipe slid into it. Start with the receiving end of the first section. Attach it to your appliance using #7 sheet-metal screws.

  • Lock the vent pipe together. Type B vent pipe typically uses either a locking band that slides down once two pieces are slid together and seated, or it uses a twist-lock system where the pipe is simply twisted after sliding together.

  • Support your pipe using rigid metal straps spaced no less than every 4 feet. Use your level to check the rise on the horizontal runs as you install the hangers.

Tips & Warnings

  • Gas appliances should only be installed and vented by licensed and qualified installers. In most areas of the U.S., it is illegal to install HVAC equipment without having the proper license and obtaining a building permit.
  • Improperly vented gas appliances can cause fire and/or cause carbon monoxide to be released into your home or structure. Only licensed professionals should attempt to install vent pipe for gas appliances.

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  • Photo Credit gas flue image by Alison Bowden from
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