My furnace flue is a double walled pipe, 6-inch (I think) inside 8-inch (I know). It goes up through a chase in the middle of the house to the attic, where it basically comes out of wooden framework and continues on to the roof. The chase is open to the attic. I want to close off the top of the chase where it enters the attic. I've cut a large piece of roof flashing so it fits snugly around the flue and covers the chase, and I need to close the cut that I made in the cover so I could get it around the flue. I assumed that metal tape would be okay for this, but I'm not sure. (For that matter, is it okay to use the cover of aluminum flashing, since it would be touching the flue, which I assume is not aluminum?) I also was told that I should put fireplace caulk around the flue to close the gap where the new metal cover touches it, and I wonder if anyone has any words of wisdom about doing this.
Thanks for any info.

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Originally Posted by dinkins

My furnace flue is a double walled pipe, 6-inch (I think) inside 8-inch (I know). It goes up through a chase in the middle of the house to the attic, where it basically comes out of wooden framework and continues on to the roof. The chase is open to the attic. I want to close off the top of the chase where it enters the attic. I've cut a large piece of roof flashing so it fits snugly around the flue and covers the chase, and I need to close the cut that I made in the cover so I could get it around the flue. I assumed that metal tape would be okay for this, but I'm not sure. (For that matter, is it okay to use the cover of aluminum flashing, since it would be touching the flue, which I assume is not aluminum?) I also was told that I should put fireplace caulk around the flue to close the gap where the new metal cover touches it, and I wonder if anyone has any words of wisdom about doing this.
Thanks for any info.

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Is there a fresh air pipe at the furnace. The builder may have used this chase to allow for the fresh are to the furnace, quite un-orthodox by most codes, but I've seen it done before. Aluminum tape will be fine, but I have found overtime it loses its adhesive strength.
I would suggest a high temp' silicone caulk. Red and usually called RTV silicone.
This will last a good long time. The vent pipe you describe is called B-vent type. National code books require a 1" clearance to all combustibles. If you want to know more about the fresh air issue you can repost the question and I'll try to explain

Thanks, Northern.
The furnace is in the garage and has plenty of air; the garage is definitely not sealed! (And for that matter, the chase is closed at the bottom. I want to close the top as I'm about to add more attic insulation. So I'll also be adding a collar around the flue pipe to keep the insulation away from it.) Thanks for the info on the caulk. What's your opinion about the aluminum flashing touching the flue pipe?

The only foreseeable problem with the flashing touching is, after many years of expansion and contraction it could where into the outer flue jacket. Just leave a small gap and fill it with silicone as this will remain pliable for many years. As for the pipe going through the garage ceiling there should be a thimble. Code says 1" clearance to combustibles ans drywall is a combustible, unless they used mill board or some other high temp solution.

Well, I don't know what a thimble looks like, and I've never seen this before, but it looks fairly sophisticated. I can't see it all, but there is sort of a wooden box (The flue goes up through it) that has a galvanized plate with a 8" galvanized "collar" that comes down and sort of surrounds the 6" flue. Hard to describe. But it looks like they've kept the flue away from combustibles, but like I said, it's impossible to see because it goes up into a bunch of ductwork.

It sounds like all was done well and you should have no problems with your plan.
Good luck

Thanks a lot; appreciate it.