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How to Fix a Leaky Gas Flue Roof Vent – Part 4

Bob Jackson
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by Bob Jackson

How to fix a leaky gas flue roof vent by replacing the flue vents, storm collars and rain caps. This repair is continued from Part 3.

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
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