How to Vent a Gas Furnace

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  • 4 hours
  • Intermediate
  • 50-150
What You'll Need
Gas furnace ventilation pipe
Tape measure
Elbow joints
Silicone caulk

The vent pipe of your gas furnace plays an important role by carrying all toxic gases created during the combustion within the furnace out of the building. You should know how to vent a gas furnace and get rid of the harmful gases, for the health and safety of anyone living in your building.

By preventing these gases from contacting other combustible items, your vent will also prevent other possible fires.

Identify Your Vent

When venting your gas furnace, you need to know the different kinds of vents that can be installed. A direct vent will directly throw all the air into the atmosphere. Most old gas furnaces use such a mechanism, while modern furnaces use the B-pipes. These pipes are not only environmentally friendly for residential use, but also authorized for use by regulatory bodies and insurance companies.

Take Proper Measurements

Measure the distance from the furnace vent to the chimney. Purchase the correct size metal vent pipe from a home improvement store and make sure you get enough elbow joints to give the vent's path the necessary twists and turns. Use your measurements and plan the path out ahead of time so that you will know exactly how much material you will need.

The diameter of the vent pipe should always be larger than that of the gas furnace flue, ensuring that no gas escapes from the pipe. The vent pipe must overlap at each joint to prevent gas from leaking. Gas furnaces usually come with their own specifications. Adhere to them even if you are building the vent on your own.

Make the Attachments

Starting at the furnace, connect the various pipes by using the elbow joints, working your way towards the chimney. Be sure that each end of the pipe that is crimped faces the chimney. Crimping allows one pipe to connect to the next more easily.

Secure the Vent in Place

Using at least four sheet metal screws, connect the pipes together and screw tightly. If the length of the pipe is more than five feet, support the weight of the pipe with galvanized hanger straps. Simply screw in the ceiling joist and attach the galvanized hangar strap onto it using a screw before looping it under the pipe. Then, take it back again to the ceiling joist where you can screw it in.

Seal the Joints

It is extremely important to ensure there is complete air-tightness at all the joints and sections. Use silicone caulk to seal all joints and seams on the metal pipe, as well as on the gas furnace and chimney.


If any gaps remain and the vent isn't airtight, the toxic gases will leak back into your home. This undermines the entire purpose of venting the furnace in the first place.

Taking care of the vent on your gas furnace is extremely important, but working improperly and damaging your vent can have catastrophic results. If you're not sure you have the necessary expertise to handle the equipment, tools, and materials required to vent a gas furnace, don't hesitate to hire a professional.