This project is continued from How to Fix a Leaky Gas Flue Roof Vent – Part 2.

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 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 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:

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

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

If Adaseal isn’t available at your home improvement store Rutland 76r 10.3 Oz Red 600°f Rtv Silicone Sealant has a higher temperature rating and is suitable for outdoor use on metal vent pipes and flashing.

This repair is continued in How to Fix a Leaky Gas Flue Roof Vent – Part 4.

Thanks for reading,

Bob Jackson

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