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Question DetailsAsked on 9/13/2017

how to seal off an unused furnace vent stack

I live in FL, a few years ago I replace a natural gas furnace/AC unit with a heat pump system. The original furnace vent stack was not removed. I believe that when a strong wind and rain storm occurs, water enters the vent stack and causes a leak in the garage. I's like to seal off the vent stack to prevent water from leaking into the garage. Is this an easy problem to fix?

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1 Answer


You can use a domed cap similar to this (but galvanized metal - painted if desired, for longer life) or a conventional galvanized domed metal cap.

You want a tight-fitting cap, if intended to be permanent as your case evidently is should be sealed with duct sealer or a long-life silicone caulk as it is installed, and 3-4 sheet metal screws to hold it in place. Should have a domed or raised profile, not flat, so water does not sit on top and rust through over the years.

You can also cap the bottom in the garage, though I like to leave a drain hole (NOT right over the HVAC unit) in the cap so if there is water getting in you know it - you can divert the duct at the bottom with a flex bend piece to get out from over the appliances if necessary. Reason for bottom cap if to keep any vermin out, and also prevent circulation of warm (presumably) garage air into the pipe, which can introduce moisture which can rust it out from the inside - though not a concern if stainless rather than galvanized. Also seals in the slight sulferous/skunky odor that will form from the accumulated natural gas odorant on the flue walls.

If a DIY person, I would rate this (assuming you are willing/able to get on roof) about a 3 on 1-10 difficulty scale, though will need an electric drill with correct insert bit to remove existing cap sheeet metal screws and put new ones in. This assumes sheet metal flue - if solid steel pipe (typically pre-60's) no tougher to do except you have to predrill pilot holes for the screws.

When you next reroof, if you intend to stay with high-efficiency HVAC unit from now on, I would have them cut the flue back to below roof level and reinstall the cap there (to prevent insect/rodent entry if they get into your attic) and then roof over the penetration hole.

Answered 2 years ago by LCD

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