#41  
Old 10-06-2009, 09:45 AM
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Default Re: 36" Tall Man's Oven Build

Stack flow can be sensitive to ambient air temps. If the stack is cold, it will resist hot air trying to rise within it. The old fireplace flue trick of blowing out the stack with hot air might help. As you light the oven fire, you could try sticking a newspaper "torch" directly under the stack to initiate and facilitate the flow. This may be even more true with a tall stack.
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  #42  
Old 10-06-2009, 10:13 AM
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Default Re: 36" Tall Man's Oven Build

I may end up with an eighty four inch chimney. That would put me 36" above the top of my planned roof. I've heard 36" above the highest point of the roof is the minimum desired clearance to make sure there's no interference with the draft pull.
Thanks for the tip on heating the chimney to create pull pdiff, I'm going to remember that one.

Darius
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  #43  
Old 10-06-2009, 04:28 PM
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Default Re: 36" Tall Man's Oven Build

maybe i will throw a couple more courses of brick on top of mine before I try anything else and see how that works ?
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  #44  
Old 10-06-2009, 07:10 PM
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Default Re: 36" Tall Man's Oven Build

Natural draft is a fickle thing. The general rule is that taller is better, but there can potentially be a point where you are too high. As the flue gasses cool, they become more dense and thusly less bouyant which means that this "heavier mass" offers some resistance to flow. However, in the relativley low stack heights we are talking about and the relatively high flue temps we are talking about, this is a bit of a mute point. Pdiff's advice on pre-heating the stack when lighting the fire is excellant and will work every time to establish an intitial draft fast.

I think the over-riding consideration here is an ample throat area servicing the chimney and the effects of wind against the door opening and chimney cap. If you were to measure the draft in the chimney, it would be in the magnitude of 0.8 - 1.1 inches of water column. Very slight in comparison to the outside pressure pulsations that will be imposed by natural wind.

blah, blah, blah... I think that the throat opening serving the chimney and a chimney height designed to carry smoke up-and-away from the immediate surroundings high enough to not be a major nusiance are the main considerations with a WFO.

In our experience with our WFO, we see some spillage out of the door during intense burning. This is accompanied by a dark carbon-laiden flue gas which suggests a very low air-to-fuel ratio. The design of the dome and door opening have a lot to do with effective a/f ratio capability of these ovens. If combustion is not optimal, carbon will be present regardless of chimney design, and it does not take very much spillage out of the door to stain the front of the oven. Having said that, I think the subtle pressure differentials that wind makes on the inlet and outlet of the chimney are the primary reasons for spiilage through the doorway.

My $0.02

Ken Morgan

Last edited by vintagemx0; 10-06-2009 at 08:01 PM.
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  #45  
Old 10-06-2009, 07:24 PM
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Default Re: 36" Tall Man's Oven Build

Ken,,,
are you basically saying Im not getting enough air in ? I am at 63% door to dome height ratio??? Once its hot it burn and draafts well,, Or are you saying at start up I am not getting enough air in ??

Appreciate the input

Mark
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  #46  
Old 10-06-2009, 07:51 PM
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Default Re: 36" Tall Man's Oven Build

Hi Mark,

No, what I'm saying is that some of the influences of optimal draft through the chimney are simply out of our control. Because the inlet and outlet of the chimney are both subjected to wind influence, the pressure differentials between the two ends of the conduit can influence the effectivness of the draw through the chimney. This draw through the chimney is soley relient on the bouyancy of the the constituant gasses.

Just as an automotive engine needs fuel, compression, and spark/timing, combustion of wood requires "the three T's"; Time, Temperature, and Turbulence. Temperature is non-issue in a WFO. Time means time for the fuel and oxidizer molecules to mingle and find one-another. Turbulence is the the flow medium that allows the fuel and oxidizer to fold-together and mix into a combustible mixture. In our WFO's, the basic design is so that the flue gasses (which are high in temperature and more bouyant) rise upward and force there way out of the top of the door opening. This exidous is supplanted with incoming air which comes in low and spreads across the floor to feed the fire. This is all well and good, and works very well as a usable system.

However, at extremely high temperatures, the wood in a WFO will at times volatize at a rate wich is much greater than the static system design can optimize a mixing (time and turbulence) for the air and fuel constituants. Wood is a hydro-carbon fuel and hydrogen oxidizes much more easily than carbon. So, when conditions do not promote complete combustion, the hydrogen will play along, but not all of the carbon. The result is soot.

In short, all I am saying is that WFO's are what they are and that the major attributes that influences the spillage of an outdoor WFO is an ample throat area to the chimney and the effects of wind.

I think all WFO's will spill to some degree. It's OK. We just made sure our chimnry was high enough to keep any dense discharges of smoke from people's faces when burning the oven hard. Also, we're just talking about when the oven is at "high-fire".

Ken Morgan

Last edited by vintagemx0; 10-06-2009 at 07:53 PM.
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  #47  
Old 10-06-2009, 08:28 PM
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Default Re: 36" Tall Man's Oven Build

Hey Ken, your profile says you're a wood fire safety testing supervisor. Man you've got a lot of good knowledge in this area! When I was reading your reply I wondered: What's this guy do for work? Then I looked at your profile and voila! Makes sense why you're "up" on this stuff.

You are suggesting that there is a point where my chimney flue is too high. We have a wood stove we heat our house with and it has great draw and we never have a problem with smoke in the house, even when I have the damper almost completely choked and I'm loading wood. Since this length of chimney is working well in the house, I'll use it as a gauge and make sure my wfo chimney isn't much taller. I hope that sounds reasonable.

Darius
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  #48  
Old 10-06-2009, 09:07 PM
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Default Re: 36" Tall Man's Oven Build

Sounds very reasonable Darius. I was just trying to point-out that tall enough is "tall enough". Of course, local building codes will dictate chimney height requirements when within X feet of the house or other structures... Your household fireplace/wood stove is not subject to winds at the inlet of the chimney, and the chimney is probabaly insulated very well. A wood stove is a sealed appliance and does not scarf-up a large quantity of excess air either. Also, if your stove is younger than 20 years, it has emission-control designed into it. I don't think you can equate an indoor stove with an outdoor oven in regards to chimney performance. I think if you run it hard and it smokes hard, with a definatley black smoke, then you are over-firing it. By over-firing, I mean that the rapid burn-rate is being discounted by less than optimal combustion efficiency. If you backed-off on the amount of fuel, even though the burn-rate is lower, the effective energy release could be just as good if the combustion efficiency is improved.

I guess all I'm saying is that these oven designs are elegantly simple and work very well. Each oven will have it's own attributes and the user just needs to get to know the personality of his/her unique oven. Over-firing and creating mass quantities of soot is a sure sign of waste. As for chimney height, if it draws fairly well when hot and carries the smoke away...all is good.

Ken
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  #49  
Old 10-06-2009, 09:21 PM
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Default Re: 36" Tall Man's Oven Build

Well I'm going to be very gentle with my baby, so I won't be building any raging inferno's. Maybe after testing for a while I'll crank up the heat, but if smoke starts pouring out the oven you're saying I've over-fueled it. I think I got it.

Last edited by KINGRIUS; 10-06-2009 at 09:27 PM.
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  #50  
Old 10-06-2009, 09:25 PM
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Default Re: 36" Tall Man's Oven Build

Black smoke - you'll know the difference, but yeah- you got it!
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