Author Topic: How narrow can the chimney be?  (Read 509 times)

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Offline peteH

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How narrow can the chimney be?
« on: March 30, 2014, 12:12:44 PM »
"a friend" has a barrel oven where the chimney is inside the oven and I was thinking about redoing it and moving it to the outside as it should be.  I am limited by the size of the landing and so I was wondering if I made the chimney on the narrow side (~1-2" x 6" inside dimensions) how that would impact performance?  Maybe I would need to go a couple of courses higher to increase the draw?  Let me know what you think, thanks.  Sidebar question, how much does the chimney effect the performance anyway as the smoke comes out the front and either up the chimney or just out into the open in ovens with no chimney (obviously the chimney keeps the smoke out of your face, but what else).


Offline TXCraig1

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Re: How narrow can the chimney be?
« Reply #1 on: March 30, 2014, 12:29:41 PM »
A proper chimney is most important when the oven is in an enclosed area that you don't want to fill with smoke. If you oven is outside, and you don't mind the face of it getting black, the chimney is much less critical.
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Offline shuboyje

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Re: How narrow can the chimney be?
« Reply #2 on: March 30, 2014, 01:15:31 PM »
You can also just leave the chimney in the oven as it is now and fabricate an extension that brings the inlet down into the oven chamber for a very similar result.
-Jeff

Offline Tscarborough

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Re: How narrow can the chimney be?
« Reply #3 on: March 30, 2014, 01:15:54 PM »
That is certainly true (Craig), but when you have a chimney that is drawing well, it will accelerate the combustion in the oven by "scavenging", i.e. removing combusted air that will be refreshed by new oxygen rich air.  Optimal is 15% of the opening, modified by the height of the stack.  The taller the stack, the smaller the cross section of chimney required.  The shape is important as well, with round optimal, square next then rectangular.

To calculate usable area of a chimney draw a circle(s) in the cross section such that the circle diameter is the minimum dimension of the flue.  That is the usable area of the flue, and as stated, it should fall in the 15% of the opening area to be most effective, since stacks on ovens seldom fall into the height needed to modify that down.

In short:  Make it as big as practical and fire it up.
« Last Edit: March 30, 2014, 01:17:33 PM by Tscarborough »

Offline stonecutter

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Re: How narrow can the chimney be?
« Reply #4 on: March 30, 2014, 01:33:30 PM »
"a friend" has a barrel oven where the chimney is inside the oven and I was thinking about redoing it and moving it to the outside as it should be.  I am limited by the size of the landing and so I was wondering if I made the chimney on the narrow side (~1-2" x 6" inside dimensions) how that would impact performance?  Maybe I would need to go a couple of courses higher to increase the draw?  Let me know what you think, thanks.  Sidebar question, how much does the chimney effect the performance anyway as the smoke comes out the front and either up the chimney or just out into the open in ovens with no chimney (obviously the chimney keeps the smoke out of your face, but what else).

The chimney is inside the oven?...like in the oven itself? That would be something to change, yes...you are dumping a lot of usable heat out of the oven.

 Second, whats your oven size?2"x 6" ID isn't bad( its not great), but I highly doubt a 2" throat will contain the volume of exhaust exiting the oven, if that is how you sized it...you are going to have smoke rolling out to the outside.  If you keep that narrow dimension, then open the throat up and transition it to your flue diameter.

You have any pictures?
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Look at a stone cutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred-and-first blow it will split in two, and I know it was not the last blow that did it, but all that had gone before.
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Offline peteH

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Re: How narrow can the chimney be?
« Reply #5 on: March 30, 2014, 01:53:51 PM »
Here is a pic.  The inside oven dimensions are 23.5" wide by 36" deep.  17" dome height with the door height at 8" ( 60% is ~10").  Anyway, it cooks really good NY style pizza, although it takes a little while to heat up and it cools off faster than I would like.  I posted a pic of the original when I first joined the forum last year and used your suggestions to make some easy modifications.  As I continue to use it, I am ready for some more drastic modifications and as I watch all my heat go up the chimney, I thought that one would make sense.

Offline stonecutter

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Re: How narrow can the chimney be?
« Reply #6 on: March 30, 2014, 05:28:35 PM »
Yeah, you should think about closing that off and build your throat and flue out of the oven chamber...I'm not surprised to hear that heat up is slow and cool down is rapid.
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Look at a stone cutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred-and-first blow it will split in two, and I know it was not the last blow that did it, but all that had gone before.
Jacob August Riis

Offline Tscarborough

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Re: How narrow can the chimney be?
« Reply #7 on: March 30, 2014, 06:00:53 PM »
That is that abortion of a design sold by these guys:

http://www.brickwoodovens.com/

It is not really an oven, more of an ugly fireplace.  He has improved his plans under pressure, adding some insulation, but the bottom line is DO NOT USE HIM OR HIS PLANS.

Offline shuboyje

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Re: How narrow can the chimney be?
« Reply #8 on: March 30, 2014, 06:03:52 PM »
With the new info I would personally leave the flue exactly where it is and extend it down to 10" above the oven floor.  I would also add a flue damper if you want to do any retained heat cooking.  This will give you a nearly identical result with way less work and retrofitting.  I don't know what's inside the brick work, but I'm thinking you pull of the rain cap, and drop in a round stainless flue that has a flange out all around to sit right on top of the brick work.  Then just put the rain cap back on top.
-Jeff

Offline Tscarborough

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Re: How narrow can the chimney be?
« Reply #9 on: March 30, 2014, 06:36:34 PM »
There is no insulation in that oven.

Edit-Actually, it looks like there is some insulation, it may be salvageable.
« Last Edit: March 30, 2014, 06:46:52 PM by Tscarborough »


Offline peteH

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Re: How narrow can the chimney be?
« Reply #10 on: March 30, 2014, 07:04:32 PM »
I understand the criticism of this design but don't hold it against me.  That is why I have posted questions on how to improve my oven or questions related to a new oven design in general.   I have learned quite a bit from this forum and appreciate the feedback from this post and the others I have posted.  Think of this as my "starter home" and it is not the one I will end up with long term.   I am also an avid reader of a windsurfing forum and like this forum, there are many experienced members who help new less experienced members,  this helpful culture about WFO pizza ovens helps enhance its popularity.  Anyway, thanks for the feedback, good or bad.

Offline Tscarborough

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Re: How narrow can the chimney be?
« Reply #11 on: March 30, 2014, 07:11:18 PM »
I do not hold it against you by any means.  I do hold it against him for selling a sub-standard design for quite a bit of money.

It looks to me like the design has been altered to bring the vent to the front, if not outside the oven, is that so?

Offline stonecutter

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Re: How narrow can the chimney be?
« Reply #12 on: March 30, 2014, 07:12:21 PM »
With the new info I would personally leave the flue exactly where it is and extend it down to 10" above the oven floor.  I would also add a flue damper if you want to do any retained heat cooking.  This will give you a nearly identical result with way less work and retrofitting.  I don't know what's inside the brick work, but I'm thinking you pull of the rain cap, and drop in a round stainless flue that has a flange out all around to sit right on top of the brick work.  Then just put the rain cap back on top.

 :-D
http://oldworldstoneandgarden.com/

Look at a stone cutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred-and-first blow it will split in two, and I know it was not the last blow that did it, but all that had gone before.
Jacob August Riis

Offline stonecutter

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Re: How narrow can the chimney be?
« Reply #13 on: March 30, 2014, 07:24:31 PM »
I do not hold it against you by any means.  I do hold it against him for selling a sub-standard design for quite a bit of money.

It looks to me like the design has been altered to bring the vent to the front, if not outside the oven, is that so?

I'm under the impression the vent is in the oven chamber...that's what I had asked about earlier.
http://oldworldstoneandgarden.com/

Look at a stone cutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred-and-first blow it will split in two, and I know it was not the last blow that did it, but all that had gone before.
Jacob August Riis

Offline peteH

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Re: How narrow can the chimney be?
« Reply #14 on: March 30, 2014, 07:49:09 PM »
Hi Stonecutter and Tscarborough (and anyone else who would like to comment),

Yes the vent is in the oven chamber.  I would like to move that outside if possible.  Tscarborough - the vent is a little closer to the front because the firebrick size I get up here in CT was a little different then the plans so my oven is a little longer.  I have built a dryfit oven at another location and it bakes excellent pies.  I was thinking of building that dryfit next to my barrel oven while I modify the present barrel oven (so I didn't have to go too long without WFO pizza!).  If by chance, I wanted to do a complete rebuild, what would be the best design with the limitations that I have a 48" square slab to work with?  Again, many thanks for the advice. 

Offline shuboyje

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Re: How narrow can the chimney be?
« Reply #15 on: March 30, 2014, 08:32:37 PM »
Just in case what I'm saying isn't making sense here is a graphical version.

Looking at a section through the oven from front to back.

A is what you currently have, a straight up draft.  The flue gases go right up and out.

B is what the others are preposing.  The flue is moved outside the door, and the door height now controls the flow of flue gases(and heat) out of the oven.  In your case the flue gases must drop bellow 8" to exit the oven chamber.

C is what I am preposing.  The flue is left in the oven chamber, but extended down to 10" above the floor.  The flue gases now have to drop down to this level before exiting the oven chamber.  Thermally this will function nearly identical to option B with MUCH less work on you part to implement it.  The only Caveat is you need a flue damper as well as a door to seal the oven chamber for retained heat cooking.  This isn't unheard of, it is a fairly common design in large commercial baking ovens, and I've used it on my newest oven with some added features with good success. 
-Jeff

Offline stonecutter

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Re: How narrow can the chimney be?
« Reply #16 on: March 30, 2014, 09:17:55 PM »
How is that much better than he has now, using a highly conductive material that is extending inside the chamber...a big heat sucking straw. I doubt that that doing it that way would function even half as well as a rebuilding a correctly vented chimney. 

Tear it down and do it right...or leave it be.  Whatever option,it's your oven after all...and  these are just opinions anyway.

Btw, where are you in CT?
http://oldworldstoneandgarden.com/

Look at a stone cutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred-and-first blow it will split in two, and I know it was not the last blow that did it, but all that had gone before.
Jacob August Riis

Offline stonecutter

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Re: How narrow can the chimney be?
« Reply #17 on: March 30, 2014, 09:24:23 PM »
That is that abortion of a design sold by these guys:

http://www.brickwoodovens.com/

It is not really an oven, more of an ugly fireplace.  He has improved his plans under pressure, adding some insulation, but the bottom line is DO NOT USE HIM OR HIS PLANS.


Wow, those are not much above a 55 gallon drum with a chimney. 
http://oldworldstoneandgarden.com/

Look at a stone cutter hammering away at his rock, perhaps a hundred times without as much as a crack showing in it. Yet at the hundred-and-first blow it will split in two, and I know it was not the last blow that did it, but all that had gone before.
Jacob August Riis

Offline shuboyje

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Re: How narrow can the chimney be?
« Reply #18 on: March 30, 2014, 09:31:48 PM »
The atmosphere is the big heat sucking straw, there is no avoiding it regardless of the design.  What we do when we build an oven is create some form of barrier between the chamber and the atmosphere.  Since we are talking about convection heat which will always rise making that heat drop no matter how you do it leads to the same result.  The gases are hottest right off the fire.  They rise to the top of the chamber and begin giving off heat to the mass.  As they do this they cool and begin to drop.  In a standard masonry black oven they drop until they are lower then the door, at which point they exit the chamber.  You WILL get the same result if they are forced to drop in the same manor to get to a flue inlet in the oven.  You've created the same barrier to the atmosphere which is all that matters.
-Jeff

Offline Tscarborough

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Re: How narrow can the chimney be?
« Reply #19 on: March 30, 2014, 09:44:40 PM »
Jeff, while that is better than nothing, it does not provide the same effect as moving the vent outside the oven.  The extended tube will preserve some heat but it will not assist in scavenging the oven and assisting in a laminar flow of cool outside air moving under the hot escaping flue gases.  It will introduce turbulent air flow which could even block the incoming fresh air.


 

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