Need some help before demolition/construction project

[Mike S.]

While looking into upgrading our old woodstove, we learned that we need to make some changes to our current setup to meet codes and for safety reasons. We have a chimney through an interior wall. The chimney consists of 16" square concrete chimney blocks with a 6" terra cotta flue running through it. Our problem is this: our wood studs and drywall are too close to the chimney/terra cotta thimble. So, we need to tear down what we have and rebuild the wall to different specs. That's fine but I can't seem to find exactly what those specs are. We are also considering using a Saf T Thimble in the new construction to help reduce our clearances behind the stove.
My questions:
1) How far must our studs (wood) be from our concrete chimnney blocks and is that distance needed on all four sides of the chimney blocks (front, sides and back)? Is it 18 inches from the nearest edge of the unexposed flue on all four sides of the chimney or 18 inches from the chimney block itself? Or is it something completely different?
2) What about metal studs? Are the clearances the same as with wood studs?
3) What is the best non-combustible wall covering product to use in place of drywall?
4) What about the Saf T Thimble? Do they work as well as advertised?
We understand the clearances from the stove to combustible materials, so that's not a problem. Our problem is with understanding the requirements for the framing around the chimney and the material on the wall, especially around the thimble. Our stove pipe goes into the wall behind the stove, not the ceiling.
If anyone knows of a diagram on the Internet for interior wall chimney framing, I would really appreciate the link.
Thanks!
Mike

 

[jtp10181]

I am by far not an expert on this subject but from what I can remember in the code book all framing should be 2" from the chimney structure. Its too bad your clay flue tiles are so small, if it was big enough to take a 6" round liner + insulation you could just do that. What is the inner size of the flue tiles? It is possible I might know a product you can run through there and solve your problems.

As for where the pipe goes through the wall, each pipe and stove will list their own clearances. Just follow those for the horizontal pipe run. Double wall pipe is going to have reduced clearances, and many single wall pipe wont list anything in which case you default to the unlisted pipe clearances in the code book.

 

[webbie]

The pipe that goes through the wall (pass through) is the critical part - and as you are finding out, most chimneys are not built to code. In fact, the code has changed over the years, AND many inspectors used to not be familiar with it (in terms of masonry chimneys), so many of them were passed even through wrong.

Yes, the safety thimbles or even a piece of insulated chimney (with the required "wall patch" I think it is 9" all around) will do the thimble job see:
https://www.hearth.com/what/passing.html for some info.

Also, see the online NFPA 211 guide.

As far as the rest of the chimney, it may come down to "harm reduction" if the entire thing has wood against it.....this is where a little conference with your building official and installer might help. For instance, you can get 5.5" ID flex liner which MIGHT fit down there. Even without insulation, this would provide an additional margin of safety.

 

[Mike S.]

Thanks for the info!
Our story sounds exactly like the one in the link above. We've been using our stove for over 20 years, only to find it has been unsafe all this time. Our problem is the wooden studs around the concrete chimney block may be closer than 2 inches to it and may actually be against it in the back (inside a closet). It has been so long since we built this wall that I don't remember. Also, the drywall in the front is right up against the chimney block and the terra cotta thimble. The wall and thimble don't seem to get hot enough to create a problem so we never gave it a thought. This will all be changed before we burn another piece of wood! I found another thread with some great photos of a new hearth installation and that will be a help.
So, I will assume that 2 inches is a good distance for studs to be away from the chimney block and we know now that drywall is not the material to use for the sheathing. Even at 2 inches, a couple of the studs may be too close for comfort when it comes to the stove pipe, so they may be located a little further away. We will probably go with metal studs in the new wall and in the hearth under the stove.
I'm sure I'll be back with more questions before and during this project.
Thanks for the help!
Mike

 

[jtp10181]

Just to be clear, the 2" was just clearance for the masonry structure. Where the pipe actually goes into the chimney, according to what Web linked, it should be 9" clearance. That is to a crock or a Class A pipe also, no single wall or double wall stove pipe should be in that pass through area according to the article.

 

[Mike S.]

Got it. Our terra cotta thimble (crock?) extends out of our cement chimney block about 8 inches, as it was never cut off. For a couple years, we've had a single wall stove pipe section going into that thimble to the vertical flue opening in the chimney. We plan to remove the wall and reframe it using metal studs and the proper non-combustible sheathing. We will also be changing the hearth under the stove. So, I don't know if we'll be burning wood this winter! We might start the project Christmas week, since I don't work that week. I just want to formulate a good plan first.
Thanks.
Mike