Cap off second thimble

[Ragnorok]

I have a terra-cotta chimney flue that turns out has two thimbles, one in the basement and one on the second floor. The one on the second floor is A) a nuisance and B) quite likely illegal. It's a standard 8" sheet metal thimble that's in a hole in the cement brick, and the flue liner is clearly visible when the sheet metal thimble cap is removed. It orignally had brick around it as a heat shield, thankfully. The thimble moves around in the chimney, like it's not even attached, but given the gaps and utter lack of seal on the cover, I'm not thinking that's a significant added source of CO2, it just looks easy to get the thimble itself out.

To reiterate, I have an 8" sheet metal thimble slipped into an appropriately sized hole, that seems as if it will slide right out of the chimney, which then needs to be capped off.

I have read turnkey devices exist that will seal this hole against heat and CO2, perhaps in addition to some high temp sealer, but I have been unable to find anything like that. I have also found "Duralon", a mica product that claims to work to 1000C, but so far no products that use Duralon. Other than some turnkey thing mention in passing on this or that forum, the only other option seems to be fill the hole with firebrick and refractive masonry cement.

Does a turnkey thing exist to safely seal an 8" thimble hole from one side, or is brick really the only option?

Thanks for your time...
- Rags

 

[stoveliker]

Are you sure that it's on the same flue?
Maybe there are two flues in the chimney?

Tie a stone (or something) to a string, and lower it through the second floor thimble, and see if you can see it via the basement one.

Tips for closing off flues I'll leave to @bholler

 

[bholler]

I have a terra-cotta chimney flue that turns out has two thimbles, one in the basement and one on the second floor. The one on the second floor is A) a nuisance and B) quite likely illegal. It's a standard 8" sheet metal thimble that's in a hole in the cement brick, and the flue liner is clearly visible when the sheet metal thimble cap is removed. It orignally had brick around it as a heat shield, thankfully. The thimble moves around in the chimney, like it's not even attached, but given the gaps and utter lack of seal on the cover, I'm not thinking that's a significant added source of CO2, it just looks easy to get the thimble itself out.

To reiterate, I have an 8" sheet metal thimble slipped into an appropriately sized hole, that seems as if it will slide right out of the chimney, which then needs to be capped off.

I have read turnkey devices exist that will seal this hole against heat and CO2, perhaps in addition to some high temp sealer, but I have been unable to find anything like that. I have also found "Duralon", a mica product that claims to work to 1000C, but so far no products that use Duralon. Other than some turnkey thing mention in passing on this or that forum, the only other option seems to be fill the hole with firebrick and refractive masonry cement.

Does a turnkey thing exist to safely seal an 8" thimble hole from one side, or is brick really the only option?

Thanks for your time...
- Rags

Fire brick and refractory cement is the right way to do it. The wall can then be finished over it

 

[Ragnorok]

Thanks!

 

[Dieselhead]

Claude responded: Het antwoord klopt, maar is minimaal.​

Agreed on fire brick and refractory cement, that's the right call. A few practical details to add:

Use refractory mortar specifically, not regular Portland cement. Portland fails at the temperatures a chimney can see, especially during an actual chimney fire.

Set the fire brick flush with the inner face of the flue, not protruding. Anything sticking into the flue will disrupt the draft for your basement Regency.

For an 8 inch hole in a typical clay-tile lined flue, two or three fire bricks cut to fit usually does the job. Refractory cement bonds them and seals the perimeter.

On the turnkey high-temp sealer products you've been looking for: those alone aren't a code-compliant closure for a thimble under NFPA 211. You need the masonry behind them. The sealer can be a finish layer, not the structural seal.

After it's cured (24 to 48 hours), get a Level 2 inspection on the chimney since the basement appliance is still active. Confirms the seal and the rest of the liner is intact above.

Then you can finish the wall over it (drywall or plaster) as Ragnorok mentioned.

You post some nice AI summaries, but AI still has some work to do to look more human like 🤖