Sealing up second flue.

[Ctwoodtick]

I have a double flue chimney. The main fireplace flue are use for the wood insert on main floor with stainless liner. The other flu which connects to the basement wall is not connected to anything. A while back, I had stuffed some fiberglass insulation (no paper ) into the thimble in the basement to avoid some heat loss. I get a smoke smell down there from time to time when using the insert. I’d like to do is secure a thin piece of sheet metal on the top of the flu on the roof with some silicone to prevent smoke coming down. While I’m at it, I’d like to seal off completely the Thimble in the basement. Almost positive that this is completely fine to do, but I just wanted to double check that there’s no issues with essentially making this a dead air space within that part of the chimney itself. Thanks in advance.

 

[bholler]

I have a double flue chimney. The main fireplace flue are use for the wood insert on main floor with stainless liner. The other flu which connects to the basement wall is not connected to anything. A while back, I had stuffed some fiberglass insulation (no paper ) into the thimble in the basement to avoid some heat loss. I get a smoke smell down there from time to time when using the insert. I’d like to do is secure a thin piece of sheet metal on the top of the flu on the roof with some silicone to prevent smoke coming down. While I’m at it, I’d like to seal off completely the Thimble in the basement. Almost positive that this is completely fine to do, but I just wanted to double check that there’s no issues with essentially making this a dead air space within that part of the chimney itself. Thanks in advance.

You will most likely have issues with condensation inside the flue which will destroy the clay liners

 

[Ctwoodtick]

You will most likely have issues with condensation inside the flue which will destroy the clay liners

Oh ok, good to know. Is there another way to block that 2nd flue?

 

[Ctwoodtick]

Just wanted to bump this thread. Looking for best way to prevent smoke coming down 2nd flue(unused) , preferably without raising height of other flue. Thank you

 

[moresnow]

No offense @bholler but I'd cap it for a few test burns to see what happens. Harmless for a short term test I'd have to think? Sucking smoke back in the house is no good!

 

[bholler]

Just wanted to bump this thread. Looking for best way to prevent smoke coming down 2nd flue(unused) , preferably without raising height of other flue. Thank you

I usually put a plate over the top with nuts under it to leave a tiny gap. And a small hole in the plate at the bottom

 

[Ctwoodtick]

Ok thanks for the input guys.