Chimneys: Questions and Metal vs Clay

[ohthetrees]

This is a repost of a question from my build thread because I feel a chimney discussion would be useful on it's own, and I hope it gets a wider audience here, than buried in my build thread.


I'm paralyzed by indecision about what to do about my chimney. In no particular order, here are some thoughts and priorities...

1. I'm going igloo style to comport with my "compact" design priority. I think it is the smallest form for an oven.
2. I don't like the look of a big silver pipe sticking out of my beautiful brick oven.
3. The price of duravent sticks in my craw
4. Clay flue liner is cheap
5. I can only find 12" sections of flue liner around here!
6. Because of the compactness design priority, I'd like to avoid bulding a brick chimney to surround the flue liner

Now I have some questions...

1. Is there a good metal option that is substantially cheaper than duravent?
2. Is single wall OK? Steel instead of stainless?
3. Can I give the exterior of a metal pipe the same treatment I give the outside of the dome? By that I mean insulation, wire-lathe, some sort of mud, and then some sort of waterproof stucco-like materal.
4. If I find longer clay flue liner sections, how long a section can jut unsupported from the brick structure?
5. If I can't find longer flue liner sections, can the sections be joined or supported somehow, without building a brick chimney surrounding it?
6. How do I prevent water from running straight down the outside of the pipe/liner and into the oven entry?
7. What is the shortest chimney you would contemplate?
8. Is 8x8 square flue liner sufficiently large?

It boils down to this..... How can I build a chimney that doesn't look like metal, costs lest that duravent, and doesn't require building a brick chimney surround? Is there anything realistic left?

[dmun]

Re: Chimneys: Questions and Metal vs Clay

Is there a good metal option that is substantially cheaper than duravent?

Not that I know of, but I don't know everything.

Is single wall OK?

No. It'll get blisteringly hot on the outside.

Steel instead of stainless?

No. Flue gasses are corrosive, and they will rot out the pipe in three years.

Can I give the exterior of a metal pipe the same treatment I give the outside of the dome?

The heat of the pipe will need some sort of insulation between it and the outer finish.

If I find longer clay flue liner sections, how long a section can jut unsupported from the brick structure?

I know people do this, but I think the hot interior/cold exterior difference is what causes flue tile cracking.

can the sections be joined

I joined my flue tile sections with a thin layer of heatstop. Any refractory mortar should do.

How do I prevent water from running straight down the outside of the pipe/liner and into the oven entry?

Ah. Here's the main problem. I don't think any chimney shortcuts are going to keep your oven dry. I'm increasingly skeptical of igloo form ovens in wet climates, unless they are under roofs of some kind. What you need is some form of isolation between your flue and insulation, and the exterior.

In a masonry chimney, the exterior is separated from the flue liner by a half inch air space, and the masonry exterior is separately supported from the firebox. The alignment of the flue tile and the masonry chimney is maintained by the chimney cap, which also directs rainwater away from the exterior of the chimney.

What is the shortest chimney you would contemplate?

Two feet is the shortest suggested chimney. More is better. Plan on some smoke staining the exterior of the arch with a two foot flue.

Is 8x8 square flue liner sufficiently large?

The 8x8 square flue liner is exactly right.

It boils down to this..... How can I build a chimney that doesn't look like metal, costs lest that duravent, and doesn't require building a brick chimney surround? Is there anything realistic left?

It looks like you're aiming at a flue tile with a stucco exterior. As long as the stucco is at least half an inch away from the flue tile, and thick enough to support whatever you're doing as a chimney cap it should work. Be sure to avoid any recess between the dome and the flue where water/snow can gather.

[brickie in oz]

Re: Chimneys: Questions and Metal vs Clay

What about a metal flue surrounded with ceramic blanket to allow for expansion, then chook wire then render?

The flue need to be SS or it will just rust away.

You could always cast one with refractory concrete.

[fxpose]

Re: Chimneys: Questions and Metal vs Clay

What about a metal flue surrounded with ceramic blanket to allow for expansion, then chook wire then render?

The flue need to be SS or it will just rust away.

That's basically what I did, except I used cheap stove pipe, not SS.
I expect the stove pipe to eventually rust through over time. Who knows, 3 years, 5 years, 10 years under our dry SoCal climate?
Since it's only partial blanket, vermicrete, and stucco surrounding the stove pipe, I don't expect redoing the entire chimney assembly will be a major project should the time come. And by that time I could probably use a design change anyway....

[ohthetrees]

Re: Chimneys: Questions and Metal vs Clay

Thanks for the tips, particularly the detailed info from dmun. I feel plenty of trepidation about going against such sage advice, but I'm really married to the igloo form, so I'll just do my best to make it waterproof. I'll try to shape the exterior to shed water, and I'll look into fancy coatings. As for the chimney, I'm going to start out with 3' of flue liner, I'll notch them so they interlock, then I'll give them the same insulation/render/coating treatment as the rest of the dome.

fxpose, how did you deal with the top of the chimney. In other words, if I run the insulation/wire/render all the way up the chimney, it has to end somewhere, how did you "tie it off" in a way that was weatherproof(ish)?

thanks for the all the advice!

[fxpose]

Re: Chimneys: Questions and Metal vs Clay

fxpose, how did you deal with the top of the chimney. In other words, if I run the insulation/wire/render all the way up the chimney, it has to end somewhere, how did you "tie it off" in a way that was weatherproof(ish)?

I used a trim collar. It's a round, flat doughnut shaped flashing which slips over the pipe at the top. You seal this collar with high temp chimney caulk.

[AtTheLake]

Re: Chimneys: Questions and Metal vs Clay

I am building a 42" Pompeii in an outdoor kitchen in northern MN.
My chimney is going to need to be 10-12 feet tall.
I am currently planning on using 8" Duravent.
My main concern is whether or not on cold days the flue gases will stay hot enough to keep going up. Would I be better off using a 6" flue?
I have a 10" chimney on my cabin fireplace and on cold days it can be difficult to get the smoke moving the right direction.

Mark

[david s]

Re: Chimneys: Questions and Metal vs Clay

6" is way too small for a 42" oven. I'd go for 10" you can't really oversize the flue with these ovens because it is independent of the oven so you don't lose heat out of it, but a large flue will prevent smoke coming out the front, more so if the pipe is taller too.

[dmun]

Re: Chimneys: Questions and Metal vs Clay

Stick with the eight. A six inch flue, even a long one, will let smoke pour out of your door on start up. Fireplaces need bigger flues because they have bigger openings on the front.

[Mike D]

Re: Chimneys: Questions and Metal vs Clay

What if you put a ceramic drain pipe over/ around a ceramic flew pipe? They look kind of cool. You could put fire blanket between them or just an air gap.

-Anyone?

Mike