How To Install Furnace Vent Pipe Through Roof

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So, you’re in the process of installing a new furnace and currently need to put up the vent pipe, which you’ve selected to install through the roof. Or perhaps you want to replace the existing furnace roof vent.

Let’s begin by clarifying that you can indeed DIY-install the furnace vent pipe through the roof.

Although it requires technical know-how, the process is straightforward if you have some DIY experience. Below is a step-by-step guide to help you through the entire process.

Before Installation

Safety Warnings and Considerations 

First and foremost, you need to know that installing the furnace vent comes with multiple safety challenges, including the following;

  • Risk of personal injury and property damages: Improper installations, adjustments, alternations, servicing, and maintenance of the furnace venting system can cause an explosion, fire, CO poisoning, electrical shock, and other conditions that can cause personal injury (or death) or property damage. 
  • Appliance damage or failure: Failure to follow proper installation protocols can also result in unit component damage.

Codes and Standards

Besides safety considerations, installers must also keep in mind existing national and local building codes relevant to furnace vent installation. For instance, you need to consider the following;

  • NFGC/NFPA guidelines: All gas appliances installations in the US must be per the National Fuel Gas Code (NFGC) and the Installation Standards Warm Air Heating and Air Conditioning Systems (WAHAC) regulations.
  • MHCSS: All manufactured gas installations must align with the Manufactured Homes Construction and Safety Standard (MHCSS) and the Standard for Manufactured Homes Installation (SMHI). 
  • ACCA Manual: All furnace duct installations must meet the requirements of the Air Conditioning Contractors Association (ACCA) Manual D or the American Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) 2005 Fundamentals Handbook. 
  • NEC requirements: All electrical connections must be in line with the National Electrical Code (NEC). 

Choose a Location 

This is only necessary if you’re installing a furnace flue pipe for the first time. Now, you need to choose the best location for the vent pipe to exit the roof.

Although the decision is ultimately up to you, it’s advisable to abide by a few industry best practices.

  1. Ensure to install the roof vent so that electrical components are protected from water, moisture, and potential damage.
  2. Never install the furnace vent system on a combustible material other than wood flooring. This helps prevent the risk of fire.
  3. Always leave ample space for servicing and cleaning in compliance with fire protection codes.  

How to Install a Furnace Vent Pipe Through the Roof

If everything is set, the next step is to remove the old vent pipe (where necessary) and install the new one.

Requirements/accessories 

  • Flue pipe
  • Screwdriver set
  • Utility knife
  • High-temperature silicone
  • Caulking gun
  • ¼ inch self-tapping screw
  • Electric drill
  • Storm collar
  • Termination cap
  • A ladder and safety gear

Most manufacturers will provide all the gas flue pipes and necessary accessories. For instance, you may find the storm collar included as well as the termination cap.

However, you want to make sure you’re getting a suitable flue pipe. Furnaces use a double-walled flue pipe such that the inner liner carries exhaust fumes outside while the outer lining insulates and protect the outer lining.  

Removing the Old Vent Pipe

You’ll often need to remove the old vent pipe before installing a new one, as most houses come with a furnace vent installed. Begin by wearing your safety gear and positioning the ladder. Then get on the roof and proceed as follows;

  1. Unscrew the termination cap: Remove the screws holding the termination cap on the end of the vent pipe. Most termination caps use ¼ inch screws that penetrate the outer lining, squeezing a compression band to hold the cap’s collar tightly against the end of the vent pipe.
  2. Remove the cap: Hold the vent pipe in one hand and the termination cap in the other. Then, holding the vent pipe still, turn the cap anticlockwise until it comes off the end of the vent pipe. Once it’s out, set it aside.
  3. Cut the sealant holding the storm cover: The storm collar slides over the end of the vent up, resting against the top of the roof’s vent flashing to direct rainwater away from the vent thanks to its cone shape. To remove it, you must first cut through the sealant covering the seam between the vent pipe and the storm collar. Use a utility knife for the job.
  4. Remove the storm collar: Gently pull the collar off the vent pipe. If it won’t come off because of excess sealant, scrap the sealant with the utility knife.
  5. Scrap the sealant at the base of the vent: You’ll notice additional sealant between the vent pipe and the roof flashing, which serves as insulation and protection. Cut through it to loosen the vent pipe.
  6. Remove the old vent pipe: Get off the roof and enter your attic. Then locate the vent pipe joint nearest to the roof flashing. You’ll notice a vent pipe joint around that point. Follow the same process we did in removing the termination cap to slide out the pipe vent to separate the two sections of the vent pipe. Then slide the vent pipe out of the roof’s flashing into the attic.

How to Install the New Furnace Vent Pipe 

You’ve successfully removed the old vent pipe. You can now bring the new vent pipe onto the attic to begin the installation.

  1. Push the vent pipe through the roof: Gently push the male end of the new vent pipe up the roof from the attic. How much you need to push it through the roof depends on the type of vent. Fortunately, most vents have marks to guide you.
  2. Connect it to the furnace vent: Before you get off the attic, slide the bottom (female) end of the new vent pipe into the top part (male end) of the vent pipe that connects to the furnace. Then, holding the gas vent still with one hand, twist the new vent pipe gently in an anticlockwise direction until it’s deep inside.
  3. Lock the two sections together: Most vent pipes use compression bands so you can squeeze the two pipes together. Otherwise, you need to drill screws into the connection to hold them tightly. When done, exit the attic and climb onto the roof again.
  4. Secure the connection between the pipe and roof flashing: Use a caulking gun to run a ¼ inch bead of high-temperature silicon around the seam between the vent pipe and the roof flashing. Then wait several seconds for the joint to secure.
  5. Install the storm collar: Slide the collar over the vent pipe with the wide end facing down. You want to push it until it touches the roof’s flashing. Then, using a caulking gun, cover the seam between the collar and vent pipe using hot silicon.
  6. Install the termination cap: Slip the termination cap’s female end over the male end of the vent pipe and tighten. Again, you should check whether it has a compression band. If so, use the right screwdriver to turn the band’s screw clockwise until the termination cap holds tightly against the vent pipe. Otherwise, use an electric drill to run a ¼ inch self-tapping screw through the cap’s collar. You need at least a screw every four inches.
  7.  Test the vent pipe: You’re done. Now, run the furnace for a while to see if the vent pipe works as intended. If so, congratulations!

Summary

The most important thing when installing or replacing the furnace vent pipe is to find the correct part. Different furnaces use different vent flues.

Additionally, the quality of vent pipes varies significantly from one to the next. So, make sure to find the right one. The best idea is to get an OEM part from the manufacturer.