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Taming an out-of-control offset firebox?

 
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kylemorley
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Joined: 06 Dec 2011
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PostPosted: Sat Jul 09 16 4:07 am    Post subject: Taming an out-of-control offset firebox? Reply with quote

Any thoughts on what might make a firebox run completely out of control? I just finished up a quick and dirty wood burning smoker made from a horizontal 55 gallon drum for the CC, and an offset 18 gallon drum below it for a firebox The firebox has an 8' pinwheel air intake on the front (that is to say, what used to be the top) and a door for loading on the top (the curved part). All sizes and openings were calculated using the Feldon calculator. Problem is, it is very volitile, with the temperature jumping all over, to the point where I am always messing with it, and don't dare leave it for more than 15 minutes for fear of coming back and finding the CC up to 400 F and the brisket incinerated, or the the CC down to 150 F and and the brisket barely cooking.

The oil drum was not a good choice, but I am stuck with cause there is not time to build another. It leaks around the door quite a bit and it hard to seal since it warps a bit every time it is used. Could this cause the erraaticity? The 3" diameter, 40" high smokestack also makes me wonder - it looks to skinny and tall to me, but it's what the calculator calls for. I'm using red oak, except for a small amount of lump charcoal I use for starting the fire, and pepping it up when it gets too low.
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SoEzzy
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PostPosted: Sat Jul 09 16 5:21 am    Post subject: Reply with quote

How are you controlling the intake air flow?

Can you post a picture or two?
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Smokin Mike
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PostPosted: Sat Jul 09 16 5:46 am    Post subject: Reply with quote

Sounds like your splits are flaming up and spiking the temps. Probably due to uncontrollable air input. You got any air leaks around the firebox door for example?
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kylemorley
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PostPosted: Sat Jul 09 16 9:55 pm    Post subject: Reply with quote

Pouring rain outside, I'll try to get a picture later.

Airflow is controlled by an 8" pinwheel aka pie slice damper which has a total opening of about 19 sq. in. Door leaks a bit but not excessively - shutting the pinwheel down produces a rapid reduction of flames and temperature.

There's no box or grate for the fire. I wonder if this may be aggravating things by letting coals accumulate excessively.

Odd thing is the firebox, figured by the calculator, seems huge but it is hard to get the temperature in the CC over 200. And once I do, it tends to run away - so I am always fiddling with the pinwheel, no set the pinwheel and go away
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necron 99
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PostPosted: Sun Jul 10 16 11:29 am    Post subject: Reply with quote

Why no fire / coal grate?

To me this is fundamental element - whether a grill or a smoker - to allow air flow THROUGH the fuel and ash accumulation BELOW the fuel.

Seems like you went to a lot of other effort, why did you exclude this element?
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k.a.m.
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PostPosted: Sun Jul 10 16 7:36 pm    Post subject: Reply with quote

What are the dimensions of the drums I would like to re-run your numbers.

Is the cooker a true offset with the firebox on the end or is the firebox mounted under the cooking chamber?

What size splits are you using?

When you start up are you using charcoal or lump to create a coal base or are you relying on the splits to create a coal base?

The height and size of your exhaust could be part of the problem but there are a few other things as well like no coal grate to support your coals and allow air to flow under them.
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castrovillecowboy
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PostPosted: Mon Jul 11 16 12:19 am    Post subject: Reply with quote

I had similar problems with my bigger cooker. I was trying to control mine with the fire door. when, during a cook, I got enough of exactly what you are describing and cut out an intake door - that helped tremendously. I had added a grate for the fire to sit on. Since then I have added a coal basket and now need even a smaller fire. I would definitely give numbers to KAM and let him re-run your numbers. Measure your intake total hole space too.

Also, not usre if you covered this as I type, dont try to use your exhaust to control the heat - just the intake - once you find that happy medium without messing with the exhaust your life as far as fire is concerned will be easier.

Hope this helps, if nothing else - even those of us that have BBQed for years have the same issues on new pits, Very Happy Shocked
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kylemorley
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PostPosted: Mon Jul 11 16 5:52 am    Post subject: Reply with quote

Thank you for your help!

CC is a standard 55 gallon drum

Firebox is similar drum construction, 13" diameter by 21" long

Port connecting them is 6"x11" or so oval, properly located at very top of FB and bottom of CC, at end of the CC opposite the stack

Stack is 3" x 30".

Air damper is a 8" round pinwheel, area about 19 sq in.

No coal grate as I I was running out of time and wanted to see how the darn thing would fire and add one if it seemed necessary.

The CC has serious leakage problems that I hope I can improve on but doubt I can completely eliminate, so I am hoping the problem is in the FB!

Splits vary but are typically 3" to 4" x 14" or half that long. I am proud owner of the worlds most dangerous log splitter so I can make any size necessary, if that were the problem.

I usually use just wood finely split to start but have been using a bit of charcoal to speed up the start with this smoker.


Last edited by kylemorley on Mon Jul 11 16 9:44 am; edited 1 time in total
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Smokin Mike
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PostPosted: Mon Jul 11 16 6:14 am    Post subject: Reply with quote

I'll let k.a.m. and some of the others work their magic but I don't know how in the heck you can get any exhaust out of a 2" pipe.... I don't care how long or short it is.

I put a 3-3/4" x 18" stack (It's 4" tubing) on my 30 gallon New Braunfels cooker and it runs like a champ.
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kylemorley
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PostPosted: Mon Jul 11 16 9:42 am    Post subject: Reply with quote

Whoops, I meant to say 3" stack.
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k.a.m.
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PostPosted: Mon Jul 11 16 7:06 pm    Post subject: Reply with quote

Here is a link to your numbers as specified.
Link to BBQ Pit Calculator
The exhaust if it is a true 3" ID only need be 20" tall.
Your 6" x 11" opening into the firebox yields roughly 60 Sq. in.
You only need 22 Sq. in.
Things I would do to help calm down the cooker.
Reduce the exhaust height to 20".
Close off the bottom of the oval hole by about 1/3 to get closer to 22 Sq. in.
Place a small coal grate in the firebox even if it only allows 1" air clearance.

Starting the cooker: Use 1 chimney of lit charcoal when white hot put in firebox if its a top load then add 1 split. Allow the cooker to heat up and peak then make a note of the temp. Monitor the drop watching the coal base every 15 minutes. What you are looking for is how low the cooker gets when the coal base drops to about 1/2 of what you started with wood included. Once you reach this add a split and monitor what it does. Some adjustments here will probably be needed but those are up to you I am just suggesting a method I use.

What we are doing here is learning the cooker and fire management once you get a handle on this you can run the cooker and let it work for you.

It will take an unusually small fire to run this cooker because of its size and replenishing the coal base using lit charcoal no wood during a cycle on long cooks is common because you cannot maintain a small wood fire only in a cooker this size without having temps everywhere and very hot to do so.
I hope this helps. Very Happy
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kylemorley
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PostPosted: Thu Jul 14 16 3:44 am    Post subject: Reply with quote

I shortened the stack, blocked off part of the opening to pretty close to 22 sq in, and made a coal grate. The difference is quite stunning - the smoker is much better behaved and burns less fuel.

k.a.m. wrote:
Here is a link to your numbers as specified.
Link to BBQ Pit Calculator
The exhaust if it is a true 3" ID only need be 20" tall.
Your 6" x 11" opening into the firebox yields roughly 60 Sq. in.
You only need 22 Sq. in.
Things I would do to help calm down the cooker.
Reduce the exhaust height to 20".
Close off the bottom of the oval hole by about 1/3 to get closer to 22 Sq. in.
Place a small coal grate in the firebox even if it only allows 1" air clearance.

Starting the cooker: Use 1 chimney of lit charcoal when white hot put in firebox if its a top load then add 1 split. Allow the cooker to heat up and peak then make a note of the temp. Monitor the drop watching the coal base every 15 minutes. What you are looking for is how low the cooker gets when the coal base drops to about 1/2 of what you started with wood included. Once you reach this add a split and monitor what it does. Some adjustments here will probably be needed but those are up to you I am just suggesting a method I use.

What we are doing here is learning the cooker and fire management once you get a handle on this you can run the cooker and let it work for you.

It will take an unusually small fire to run this cooker because of its size and replenishing the coal base using lit charcoal no wood during a cycle on long cooks is common because you cannot maintain a small wood fire only in a cooker this size without having temps everywhere and very hot to do so.
I hope this helps. Very Happy
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k.a.m.
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Joined: 12 Dec 2007
Posts: 26012
Location: Southeast Texas.

PostPosted: Thu Jul 14 16 3:58 am    Post subject: Reply with quote

I am glad your cooker is working better. Very Happy
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