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  #1  
Old 12-08-2013, 02:33 PM
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Join Date: Nov 2011
Location: Lincoln
Posts: 1
Default Smoke Escaping

Planning on starting my build in the spring when things warm up in Nebraska. Something I want to avoid is smoke escaping out the front that would discolor brickwork above the opening. I've noticed some photo posts where that has happened. What's the cause and what can I do to make sure this isn't going to be an issue?
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  #2  
Old 12-09-2013, 11:34 AM
SableSprings's Avatar
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Join Date: May 2009
Location: Roseburg, OR USA
Posts: 195
Default Re: Smoke Escaping

Flyboy, welcome to the forum & the world of WFOs!

There are two main causes for smoke spilling out of the oven's front.

1) The vent size is too small for the oven...or the most common cause...

2) The chimney/flue is blocked or initially restricted (can actually be air or a physical object - assume the flue is physically open).

Air in the chimney/flue column must be warmer than ambient temp in order to work (basic thermodynamics, warmer air rises/cold air drops). That's the reason fireplaces inside a house have a damper...so cold air doesn't flow down the chimney and into the house when there's no fire. It's also the reason a roaring fireplace can be counter-productive. (The air being sucked out the chimney will be replaced by colder, outside air pulled in from all those little gaps around our doors & windows. A house can actually be made colder when you light that nice fire on a wintery night.)

Initially when you light a fire in the oven, the smoke (warmer) seeks a path out & up. Normally the flue air in our outside WFOs is (static) near ambient temps and on initial firing, resists enough to cause at least some smoke to spill out the front (effectively the ambient temp flue air is acting as a partial blockage/restriction to the desired flow pattern). As the air column warms, the static air restriction is removed and the flue volume becomes completely dedicated to upward flow (removing most of the fire's by-products). As the chimney/flue heats up, it will become a literal vacuum cleaner for the WFO smoke & fire by-products taking them efficiently out the stack. The hotter the flue and flue gasses get, the better the chimney works as intended.

Assume your vent diameter (volume) is adequate (and you don't have a creosote build-up ). In order to reduce/eliminate the initial smoke out the front of the oven, there are three choices (IMHO).

1) Preheat your chimney's air column (I've taken a pine cone with a pair of tongs, lit it, and then held the flaming cone just up into the stack to preheat and get the air flowing up.)

2) Power vent the chimney. A lot of commercial WFO have an electric fan built into the system to keep a constant, minimum flow moving up & out the stack.

or...

3) Relax & think of the minimal smoke stain as a badge of honor/proof that you USE your oven!

Increasing the stack height can also increase the "pull" of a chimney, but again it must have the initial warm up before you see any benefits.
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  #3  
Old 12-09-2013, 11:35 AM
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Join Date: Mar 2011
Location: Bucks County, PA
Posts: 185
Default Re: Smoke Escaping

The key to managing the smoke is the design of the chimney and the "Vent". The chimney design is pretty straight forward sizing issue of having the appropriate proper flue size (diameter) and height. The vent however is more of a volume issue. The Vent acts as kind of like a "staging area" that allows the smoke to gather before exits through the chimney flue. Depending on the wind conditions, the chimney will sometimes "breathe", exhausting in the smoke in a cyclic manner (sometimes). The pompeii oven plans show a tapered design; Personally, I dont think this taper is nearly as important as the vent volume.
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Old 12-09-2013, 11:38 AM
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Join Date: Mar 2011
Location: Bucks County, PA
Posts: 185
Default Re: Smoke Escaping

Also, smoke leaking out the front means smoke in your face!!!!
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Old 12-21-2013, 06:40 PM
Laborer
 
Join Date: May 2012
Location: Michigan
Posts: 70
Default Re: Smoke Escaping

I hear ya Flyboy. Before my build I was really concerned about this as well. I found that it is pretty rare that smoke comes out the front. It tends to happen 1) at the fire start up or 2) if I let the flame go out and I put some fresh wood on the coals. To combat this I made two reveals in my entry. One, on the oven side of the flue. This one gets a solid door designed to completely seal off the oven and retain heat. The second, is on the entry side of the flue. This door has an opening in the bottom that allows are flow to feed the fire but seals the top of the entry to prevent smoke from coming out of the entry. If any smoke starts to come out I just simply put this door in place until the fire gets going. You can see this firing door sitting on the patio in the bottom right or this picture. Hope that helps.
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Old 12-22-2013, 02:49 AM
cobblerdave's Avatar
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Location: brisbane australia
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Default Re: Smoke Escaping

Gudday
At this stage of the build I saw a lot of smoke stained ovens and living in the sub tropics not that many wood fired stoves so lack of choice with steel chimneys. I made my chimney wider than the oven door and 8 ins deep of brick. Smoke goes straight up that puppy, can't get to the front.
Regards Dave
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Old 12-22-2013, 03:11 AM
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Join Date: May 2011
Location: South Australia
Posts: 470
Default Re: Smoke Escaping

Mine drafts pretty well, except when the flaming sou'wester (seems like every afternoon, from the September to April , though it probably isn't every afternoon) blows into my carport at an angle, swirls around and fires turbulent air into my oven entry, which actually faces north.

At that point, I have to prep the pizzas inside if I don't want wind blown grit in them, and use my door with the cut out at the bottom, which lets enough air in for combustion, and closes up the top of the outer arch so the smoke can only go up the chimney.

For the other 4 months of the year when I can count on a few calm afternoons, drafting isn't an issue at all.

By the way, the smoke staining on my arch is pretty minimal, given the PITA wind I have been dealing with. I've been using it once a week on average, for over a year. An oven with bad staining must draw very badly.
Note the staining is mostly to the left. The oven faces north, the sou'wester comes in from the right as it swirls into my carport/barby area which runs east west, the turbulence washes a bit of smoke out the front and the wind takes it to the left.
I imagine if you construct the vent area in a fairly conventional manner in a reasonably sheltered spot, you shouldn't have an issue, though I think a lot hinges on using tinder dry kindling, good quality dry timber, etc.

Discussion on proper sizing of vent here:

http://www.fornobravo.com/forum/f9/c...est-20230.html (Cold Oven Draw Test)
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Last edited by wotavidone; 12-22-2013 at 03:19 AM.
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Old 12-22-2013, 03:52 AM
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Location: brisbane australia
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Default Re: Smoke Escaping

Gudday Mick
I know about the winds where you live . With the cold southern ocean on one side and 1/2 of Australia desert on the other you can tend to get winds.
I live just off Morten Bay. Great in summer time, a nice cool bay breeze in the afternoon. I had a built in gas BBQ which came with the house. Connected to town natural gas. Great. Except it was in the north east corner close to the house. The wind would swirl around the house and blow out the gas flame. One minute would be coming one way and next the other.
When it came to positioning the WFO , I positioned it again in the north east and facing the evening winds but away from the house. No swirling winds. You can deal with that. After all this time I have plenty of vegetation at the rear and sides of the oven. That slows the wind and stops the gusting which helps.
Steady winds and direction you can deal with but gusting and sudden changes in direction is a different matter.
I can certainly see why you added the brickwork at the sides and the " blast" door.
Regards Dave
PS the gas BBQ is long gone!
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Old 12-22-2013, 01:48 PM
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Location: South Australia
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Default Re: Smoke Escaping

Yeah Dave I thought I was being clever, pretty much putting her back to the wind. It seems I might have been better off facing it.
Still, I manage to deal with it.
My mate that I am building an oven for has picked his spot himself. His approach is that it needs to go where it needs to go to fit in with his overall outdoor kitchen plan, and if he has to wait for a calm day to use it, so be it.
As for vegetation to help I should be right soon.
This year I was a bit slack picking the quandongs. The other day the missus called me out to our one quangdong tree to point out the dozens of seedlings that have sprung up from the fruit that fell to the ground. We will have a grove of quangdong trees soon.

Last edited by wotavidone; 12-22-2013 at 01:52 PM.
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Old 12-22-2013, 04:16 PM
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Location: brisbane australia
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Default Re: Smoke Escaping

Gudday Mick
It's pretty hard to control the wind so don't be hard on yourself . What I've learnt the hard way is that you can't stop it with a solid structure you have to an open one to slow it down. Vegetation is a great options but it takes a long time to grow.
If you can get a bit of lattice or a bit of open brushwood fencing and place it at the sides of the oven. The object being to slow the wind down and stop the turbulence on the lee side of a solid structure. Now I'm no expert but I recon it would be worth a try. Might be an idea to not to fix it to start with so you can experiment with placement.
Hope this might help
Regards dave
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