Vent/flue/chimney of pure firebrick

[kebwi]

There seem to be many ways to build the vent/flue/chimney. The four big categories seem to be firebrick, castable refractory, terracotta, and steel pipe. In all cases, an external facade might subsequently be appended, although in some cases it is more necessary (terracotta) than others (steel).

I'm curious about the all brick option for the opening round of this thread.

Some discussions on FB have suggested that a corbeled vent would not be a problem from the concern that steps on the inside of the vent might impede air flow relative to an angular and smooth internal vent surface. Would anyone care to comment on this?

Above the vent, if the flue and chimney are built out of firebrick, I have a few questions:

Would splits, standing on edge, be thick enough (1.25" thick) -- either thermally or structurally -- for a two to three foot chimney, or would whole-bricks have to be used (may I presume that wholes on-edge (2.5" thick) are certainly sufficient or must they go face down (4.5" thick)?

Would firebrick that is so dramatically exposed need to be covered or facaded against the elements? I realize that some people disregard any concern over exposing firebrick directly to the elements, but I'm seeking corroboration, especially for a super-hot chimney.

Finally, and somewhat mitigating the previous question, my hope is to stucco my entire oven, including the chimney. Could stucco be applied directly (with wire lattice) to a flue of 1.25" firebrick or would that get so hot that the stucco would fall apart? Should an external framework with an airspace be constructed, either out of rebar and chickenwire, or alternatively, as an external brick wall?

I realize I'm asking a lot of questions in this thread. I hope I don't burn everyone out on this one.

Cheers!

[dmun]

Re: Vent/flue/chimney of pure firebrick

There is nothing about a firebrick chimney that is bad practice: It's mostly just a cost issue. Around here, splits are the same price as full size bricks, they are mostly used when there is a specific reason to use them for this reason. So, an eight inch square flue would take four times six firebricks (24) for a two foot section of flue, plus refractory mortar. That's about three times what I paid for flue tile.

Fire brick IS porous, and usually isn't exposed to the elements. A cold damp structure, if radically heated on one side, is going to be more subject to cracking.

Why the airspace between flue liner and chimney exterior? It's a little insulation, but it's mostly expansion difference compensation. Your flue liner will grow when it heats up and if it's rigidly affixed to the masonry surround, something is going to crack.

Stucco? I try to keep my mouth shut on this topic because I have fixed opinions on the appearance of the stuff. I'm also not very good at it. But still, why would you build a beautiful brick structure, and then cover it with gray mud? [/rant]

[kebwi]

Re: Vent/flue/chimney of pure firebrick

I have access to 50 cent splits (for the moment, could end anytime), so they are cost effective if they are useful. I was thinking splits, on edge, would be lighter than wholes, and would thus put the arch under less stress. A secondary concern would be to keep the chimney to as aesthetically a confined space as possible.

Cost is a reasonable concern. I haven't found steel pipe to be very cost effective, unless you believe single-walled pipe is an option (I guess black pipe would have to be encased anyway, stainless could be exposed, but costs a lot more). Double-walled is expensive and I had one guy at a fireplace store basically say he wouldn't sell me anything less than Class A pipe so I wouldn't come back in a year and complain that my pipe rotted out. Class A is, of course, even more expensive than double-walled pipe.

I've never heard of "flue tile". Is that the terracotta I referred to above. I admit, that is reasonably priced, and the option I am leaning toward the most, but I haven't figured out how to do the exterior. If I brick around it, then it's still all brick, so how have I saved any money (you said brick was an expensive option). I guess those can be cheaper bricks of course, but there is the added expense of the terracotta then. It's pretty even, right?

You ask why I would build with brick and then cover it. First, as I said, I wasn't sure if firebrick should be directly exposed, regardless of its appearance. Second, my suggestion of brick wasn't aesthetic so much as a solution the fact that I haven't found any other options. Steel pipe seems insansely expensive although I haven't specifically done a cost calculation on all the various methods and compared them. I've been leaning toward terracotta, but have been unsure how to do the exterior. If I brick around the terracotta then I haven't saved any money.

I guess my preferred option is terracotta with a rebar-chicken-wire-stucco "tube" around it, I just haven't got that idea all put together yet.

FINALLY, there is still the issue of the vent, from the arch, up 8-12 inches to a platform for the flue, which I am consideredering doing in corbeled firebrick. Again, the reason is that I simply haven't found a better option. Metal seems extremely expensive. Am I wrong about the costs of using metal? I could go castable for the vent too, I dunno, I'm just shopping around for ideas.

[SCChris]

Re: Vent/flue/chimney of pure firebrick

Yep, the SS vent is just awful as far as dollars go. The clay flue that I could find around here was way too big and the idea of cutting and mending seemed like a lot of work and I wasn't sure that the result would be acceptable. I considered having a SS duct built, but decided that the cost would likely be close to the SS vent system. I considered building one of insulating board and wrapping it to unify the creation, again the dollars. I also considered if a cast system made sense, again the dollars and time.. I finally gave up... In the end I spent $500 for 3 pieces and was able to pick them up the next day. I wanted to verify the parts and have the system now so I can get the oven buttoned up before the winter storms start rolling in. Oh yea, and start cooking.

One more factor was that I'm close enough to the ocean where although salt air isn't a factor humidity is. I don't want to fight rust or the potential for the clay flue to slough off into the entry area.

Quality costs..

Sigh!!

Chris

[kebwi]

Re: Vent/flue/chimney of pure firebrick

Well, I guess I'm not the only one. Funny thing is, most of the design has fallen into place. I even made final decisions about insulating the hearth, which is a big unknown for me. I simply could not make up my mind between board or pure vermicrete. I finally decided to order 2" InsBlock 19 (still haven't decided whether I'll put down just a few inches of vermicrete under it, seems unnecessary, but I have a pile of vermiculite). I decided on my dome style, my dome insulation (InsWool, probably with vermicrete over the top to make a smooth shape for the stucco), arch shape, everything is designed.

...but I cannot make up my mind about the vent and the flue. I just can't find a feasible way to do it.

Thanks for the input. Worse comes to worst, I'll corbel-brick the vent, terracotta the flue, and chickenwire-stucco the veneer...if I can ever figure out how all those heterogeneous pieces go together.

[dmun]

Re: Vent/flue/chimney of pure firebrick

I've never heard of "flue tile". Is that the terracotta I referred to above.

Yes.

Modern flue tile is supposed to be made from refractory material, like firebrick, as opposed to the plain baked clay, or terra cotta things like drain tile.



As always, the language is fluid: when I asked at my brickyard for 'refractory flue tile' the counter guy said 'you mean chimney pipe?'

[kebwi]

Re: Vent/flue/chimney of pure firebrick

Okay, thanks for the clarification. I like the clay liners. No possibility of rust or any other kind of metal break-down. They do crack, but so does everything. I need to design a vent from the slot in the arch up to the flue (I've been thinking corbeled brick, but not with any serious dedication), and I need an exterior around the clay flue, either brick (as in your picture above) or a stucco-based chicken-wire and rebar structure. This is by far the vaguest part of my design...of course it's also one of the last to go up.

Thanks again.

Cheers!

[tmarsala]

Re: Vent/flue/chimney of pure firebrick

Hi all-

I was going to build my chomney out of firebrick, but the cost was great AND the weight on my vent openning would have been great as well! At my local brickyeard they produce a pumice precast flue liner that is only $10 per foot-cheaper and lighter than clay. It works pretty well and insulates great. They boast it can handle the temps that are present, but I still have some cracks in the inside of the liner after several huge 1200 degree firings. Has anyone else out there experienced cracks in their liners? And, if so, what do we do about it? Do we leave tham, like we would do inside the dome?

Tom
Menifee, CA
42"dome