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Lars 06-17-2009 11:33 AM

My Flue Design...
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I really liked the idea of building arches and a dome, so I decided to make an arched vent, like in the FB plans.

One great thing about my keystone, was that it provided a level surface to set my flue section upon.

The only problem is, the flared opening of the arch/vent flares 'up', not down.

The opening at the bottom of the archvent is 56 sq. in. If I use just one brick, cut at the proper angle and mortared in, I bring this down to a 44 sq. in. opening at the top.

Above that is the flue section, which , in my case has about a 70 cross sectional area.

Thoughts? Would it be worth it to go to an 8x8" section ( which, depending on the actual size) might step it down to around 40 sq. inches for the top 2 feet.

I am still building up the bricks for the flue section.

I will be lighting a few preliminary curing fires with this in place just to get a feel for how the flow of smoke will actually go.

[ I am editing this post so no one might see and think it acceptible --- after building this, the smoke collection area is too small. I will eventually chisel out the bottom on either side to give the smoke more area to go up... all across the landing area]

KraemerBAC 06-19-2009 01:00 PM

Re: My Flue Design...

Can you take a picture from the inside shooting up??


Lars 06-19-2009 05:17 PM

Re: My Flue Design...
I just put those triangle bricks in today. I am not really sure how this is going to work, and I was hoping I would get some feedback before I built it.

Oh well. Like I said, I just put them in, and I am going to stick the flue section on and light some curing fires to see what all cracks -- and especially to see how the smoke flows!!

What are your thoughts about this (i.e. why a 'picture shooting up?' ). I am thinking it may be too small an opening for my 39" oven. I will draw it again and attach.


KraemerBAC 06-20-2009 12:04 PM

Re: My Flue Design...
I was more curious to see the construction, I like photos the more the better

I am thinking it should work just fine...I think you have provided more area for venting than some other methods.

Let us know how the fire goes.


Lars 06-20-2009 07:42 PM

Re: My Flue Design...
Hey Peter,
Well, this evening, I mixed up several batches of vermiculite and about 1/3 of the way up the dome, I added in some ceramic blanket and packed in the vermiculite over that and under the 'metal lath' used for regular old stucco finish.

I decided to light up a cardboard box and a few tiny sticks. I didn't have the flue in place, and got smoke all over. After I set the flue on there, it started to draw, and I think it will be okay. Sure got lots of smoke on the inside!

I think once the flue is sealed, it might draw better. What do you think?


Lars 07-03-2009 09:23 AM

Re: My Flue Design...
Just an update.
After a couple of larger fires, I do wish I had made the vent opening bigger. I will use an outer door with a vent in the bottom, OR try a 'smoke shelf' to help direct the smoke up the vent, AND/OR chip away the middle of the solid bricks on either side of the vent bottom to create a larger area opening for smoke.

If you are reading this and have NOT made your vent design... make a LARGER opening for smoke. You can ALWAYS make it smaller more easily than making it bigger!!!


dmun 07-03-2009 10:27 AM

Re: My Flue Design...
I think the number one factor in the success of chimneys is height. There's a reason that factories used to have hundred foot high chimneys, and that's to get that combustion air flow.

It doesn't help to have the flue constrained at the bottom, but I still think that taller is better. Try putting up another piece of flue tile, just set it there, and see if you draw isn't remarkably improved. It's certainly easier than carving up what you have already.

Lars 07-03-2009 01:43 PM

Re: My Flue Design...
Hi David,
Well, another factor may be wind. I do think that the three biggest problems that I have had with the oven are 1.) Low arch needing buttressing, 2.) low arch expanding and causing cracks when heated, and 3.) vent area or chimney height insufficient to prevent smoke from coming out the front.

That being said, it may be, that on a calm day, a systematically larger fire might draw just great and never billow out the door. I will know better as I get more experience firing it up to some decent temps.

For general information, the company that sold me the flue (2'x9x13 terra cotta flue) offered to replace it, and also suggested some type of high temperature silicon. I decided that I would just press on with my build, which meant five more courses of bricks on top of the arch that opens up when fired and closes tight when cooled. I set the terra cotta flue, whose crack is so tiny when cool that you can hardly see it, but 1/8" practically when a fairly raging fire is going. And I put some vermiculite/portland around that at the base.

I plan on learning from this oven. Not only the fun arch/dome brick stuff, but the way to make my next one have significantly fewer problems. If a high chimney is the key, an oven in the house should draw like a champ!

If my whole brick face develops a crack when fired, I am renaming my oven from 'Fire Igloo in Nebraska' to the 'Harry Potter Wood Fired Oven'. [ lightning shaped scar on the forehead that seemed to flare up when danger lurked]

As an aside, my whole dome is covered with insulation now, but I did start chipping away at the bottom of my vent. The brittle dry firebricks and the mortar were similar in hardness. I plan to mix up some of my mortar and leave it in the oven after it is allowed to set for a week ( maybe 5 or so test samples) and check them after 10 fires. I am very curious what the condition of the mortar will be after the portland has burned out. I am getting conflicting information about this. Some say the fireclay never 'sets' ( vitrifies, whatever you call it) others say it will set and not be soluable above, say 800 degrees.


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