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Furnance Roof Vent Replacement Tutorial (Step-by-Step)

Bob Jackson
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by Bob Jackson

This article goes through the step-by-step process of replacing a roof vent. It is continued from How to repair a leaky gas flue roof vent.

The right vent stack in this photo continues to have a slow leak only when it rains. Resealing the storm collar didn’t fix the leak. I learned after the roofer resealed the vent stacks there was severe rusting under the rain cap that couldn’t be seen from the ground. It’s also possible rain water was following the vertical seam down the pipe and getting inside. Given the extent of the corrosion, I decided the best option was to completely replace the vent stacks.

Type B Gas Furnace Roof Vents
Type B Gas Furnace Roof Vents – Still Leaking

The rusted vent pipe beneath the rain cap was unusual for galvanized pipe and could be a letting in rainwater.

Rusted Vent Pipe at the Rain Cap

Bids to Replace the Roof Vent Stacks

I obtained several proposals to replace the vent stacks from highly rated Heating and Air companies on Kudzu.com. The minor issue was finding HVAC contractors that also did roof work. The bids were in the $400 to $600 range to replace the vent pipe starting at the first joint in the attic and all exterior roof components: vent pipe, flashing cone and rain cap. I chose the low bidder, which included a written 100% satisfaction and 1 year guarantee in addition to be highly rated on Kudzu.com. The company manager double checked the 4″ diameter vent pipe was properly sized to ensure condensation wasn’t an issue and a 2-man crew came out the next day.

New Type B Roof Vent Materials

The 2-man crew arrived on time the next afternoon with complete inventory of Hart & Cooley, Inc. Type B gas vent pipe and roof flashings. The double-wall vent pipe is made in various lengths that snaps together.

New Hart & Cooley, Inc. Type B Vent Pipe

Hart & Cooley 4RWH Rain Cap – top view:

Bottom view of the same. The function of rain cap is to vent the combustion gases, help prevent back drafts, keep out rain and discourage birds from nesting in the flue vent. The HVAC technician said the H&C product works well. Were a bird to build a nest, it will block the vent and a safety switch in the furnace will sense the back pressure and shut off the furnace.

Hart & Cooley 4RWH Type B Gas Vent (Rain Cap)

The HVAC contractor used the Adaseal International, Inc. HVAC/R silicone caulk to seal the vent flashings. It’s 100% silicone rubber rated for 450°F.

Adaseal HVAC/R Silicone Caulk

Disconnect the Old Gas Flue Pipe

The HVAC contractor began the work by disconnecting the old vent pipe at the first joint below the roof deck in the attic. To disconnect the pipe, just slide the retaining collar up and wiggle/lift the pipe to separate. The shiny new pipe sections are shown in this photo.

Furnace Vent Pipe in the Attic

Roof Gas Flue Vent Replacement

With one man working in the attic, the other HVAC technician went to the roof top to disassemble the vents. He began by lifting the retaining collar and wiggling off the top section of vent pipe. The top section with the rain cap is laying to the right in the photo below.

The storm collar is removed by cutting the caulk line with a utility knife and wiggling the collar up the pipe. Once clear of the flashing cone, the storm collar was cut in two with tin snips and pulled free of the vent pipe. Cutting the collar was quicker than working it off the length of the pipe.

Removing the Storm Collar from the Old Vent Pipe

After removing the storm collar, the caulk line at the flashing cone is cut and the old pipe is pulled down into the attic. The retaining collar at the bottom of the vent pipe inside the attic will not fit through the flashing cone, so the pipe must be pulled down.

The second vent stack is removed by the same procedure.

New Roof Gas Vent Installation

Flashing Cones

We originally planned to remove the old flashing cones. The HVAC technician advised against it, explaining the cone was in good shape and he was concerned about the shingles being brittle in the cool (mid-40’s) December weather. We agreed to leave the flashing cones in place.

The old caulk is trimmed and peeled away from the neck of the flashing cone with a utility knife. A clean bare metal surface is needed for a water tight caulk line.

Cleaning Up the Flashing Cone Neck

Flashing cone after cleaning the old caulk from the neck.

Roof Vent Flashing Cone Cleaned and Ready for New Pipe

The second man inside the attic pushes up the new section of vent pipe through the flashing cone. The man in the attic then attaches the vent pipe existing lower section inside the attic.

Pushing Up New Type B Vent Pipe

The joint between the vent pipe and flashing cone is caulked with Adaseal HVAC/R 100% RTV silicone high temperature caulk.

Caulking the Vent Pipe at the Flashing Cone

The storm collar is installed by sliding it over the pipe and down to the flashing cone. The storm collar is sealed with a generous bead of silicone and smoothed (radiused) with a finger.

Caulking the Storm Collar

The rain cap simply slides onto the vent pipe. The technician secured the rain cap with a short 1/4″ sheet metal screw to ensure it doesn’t blow off in a very strong wind. The screw is optional and must not penetrate inner section of the double wall pipe.

Installing the Rain Cap on the Roof Vent Pipe

The vertical seam on the flashing cone was caulked with silicone and the vent stacks are spray painted black for appearance. The two vent stacks are also now equal height for a better appearance.

New Gas Vent Stacks

Rusted Flue Vent Stacks

The old vent stacks were saved for inspection to see if there was an obvious source of the rain water leak. The black RTV sealant beneath the rain cap and on the storm collars is only one week old and did not stop the leak. The vents are a bit dented having been dropped off the roof onto the lawn.

Old Type B Gas Vents

The rust hasn’t corroded through the pipe wall, but the rust lines indicate rain water getting inside the pipe.

Rain Cap and Vent Pipe Rust

It’s really hard to say if this was the leak source, or maybe water was getting inside the vertical seam and channel past the storm collar.

Rusted Vent Pipe

No Vent Leaks!

The new Type B roof vents were installed only a day before a new cold front brought steady rain. The vents are water tight and no water trails are to be seen on the shiny new sections of pipe in the attic.

The job took about 2 hours and the final cost was $350 versus the $400 original quote because we decided not install new flashing cones.


Several years after this repair the entire roof was replaced due to hail damage. I had the gas flue vents completely rebuilt, including the base flashing cones. You can read about that project here.

Hope this helps,

Bob Jackson

adaseal hvac/rattic leak around vent pipegas flue vent repairHVACinstall a new type b furnace vent
Bob Jackson
Bob Jackson
Technology product manager by day and a prolific handyman in the evenings and over the weekends. Bob was the founder of the original Handyman How To website and that tradition continues on this site with excellent new handyman content into the future.
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