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1/2 inch thick Fire box issue.
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Chef Folett
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Joined: 28 Jan 2010
Posts: 65
Location: Austin TX

PostPosted: Mon Sep 15 14 6:22 am    Post subject: 1/2 inch thick Fire box issue. Reply with quote

Hi all, I have had a lot of help from you all here and I really appreciate that.

The last issue I am faced with is the size of the fire I need in order to avoid an overheated cook chamber.

The problem is that my firebox is 1/2 in thick and it works really well, in fact it works too well. In order to keep the temp down below 250 I have to have a very small fire.

This is what I start with. I have also added a Digiq fan system to help extend the life of the fire. At this point I have to feed the fire every 30 minutes.


This is before i have to add wood and the temp is the cooker is over 230.


This is after adding some wood and the temp got down below 225.


it looks to me like I have two options.
1. Replace the 1/2 inch thick fire box with a 1/4 inch thick fire box.
2. Add another 2 foot section to the smoker. I have a 29 inch section of 1/4 inch thick pipe that is the same diameter as the tank.
Here is what my current smoker looks like.


This is what it will look like if I added the pipe to the tank. I can thank my wife for her creative photoshop skills.



I am leaning towards adding the 2 foot section to the tank because that would give me more cook space.

The question is do you think that adding this section will help me by allowing me to create a bigger fire without overheating the chamber or do you think that this fire box is just too efficient and will continue to do the same thing?


Here is a link to my original post of my build if you want to see the background of this cooker build.
http://www.thesmokering.com/forum/viewtopic.php?t=38008&postdays=0&postorder=asc&highlight=chef+folett&start=0
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Smokin Mike
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PostPosted: Mon Sep 15 14 8:21 am    Post subject: Reply with quote

Have you tried starting out with a charcoal base and then adding small splits?
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Smokeythebearguy



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PostPosted: Mon Sep 15 14 9:03 am    Post subject: Reply with quote

I would add a 16-20" ft flue between your fire box and the cook chamber, with a damper
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SoEzzy
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PostPosted: Mon Sep 15 14 9:11 am    Post subject: Reply with quote

IMO you need to build a fire base of the correct size for your pit, once you work out what size you need, you feed it, wood, charcoal or wood and charcoal on a regular basis.

Most fire footprints are between 1 1/2" & 2" tall, 10" & 12" long and 6" to 8" wide, of coals and that's about what is needed to run your pit at around 250°!

If you want less temperature you need a smaller fire footprint, if you want to run it hotter you need a bigger footprint.

Each pit has it's own sized footprint for a specified temperature, you need to practice and learn the one that works for the temperature you want to cook at.

By building your fire footprint, with charcoal and/ or wood, you get used to the fuel loaded needed to start your pit and get the cooking chamber up to temperature, and then you need to let it cook for a while, then add enough fuel to maintain the fire footprint, that might be one split, or two splits normally, or it might be some more charcoal and one split, or just a split on it's own.

The volume of the fire footprint, in it's tight, small space will give you longer burns with less fuel, once you get in the groove, you will be able to take one look at your fire, and know ho much fuel to add, when, and how long it will keep things going for. The only way to get confident is to practice, practice, practice!
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brutus1964
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PostPosted: Mon Sep 15 14 10:06 am    Post subject: Reply with quote

i would agree with SoEzzy that you may need to start with a different base. Try to start with about 5 lbs of charcoal with the exhaust vent fully opened with the coals ashed over. If the temp gets too high you can back it down by shutting down the intake. Once this levels out it will give you an idea of how much fuel to add or take away. You can use small splits and preheat them on your firebox for faster ignition when you ad them to the firebox. This will help maintain a more even heat. I would try a few more cooks as suggested and just experiment until you find the sweet spot for your cooker.
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Chef Folett
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Joined: 28 Jan 2010
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Location: Austin TX

PostPosted: Mon Sep 15 14 10:57 am    Post subject: Reply with quote

thanks. I have done all of this I still have trouble keeping the temp between 225 and 240. That is where I want the temp to be. if I have a good base of coals in there and add another split about 6 inches long the temp jumps up past 250.

The other issue is that it will only last for 30 minutes. is it unreasonable to get a fire to last at least 45 minutes?
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SoEzzy
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PostPosted: Mon Sep 15 14 11:46 am    Post subject: Reply with quote

Don't fight the pit, if your pit runs at 250° learn to live with it, map out your temperatures on the grates, don't rely on the external thermometers for your real temperatures.

If you use bigger splits, you will get a longer burn, you need about 50% more material than is giving you your 30 minute burn.

Preheat your splits and always put them on the fire, cut side down, you should get almost instantaneous ignition, and not get too much of a spike in temperatures.
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k.a.m.
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PostPosted: Mon Sep 15 14 7:31 pm    Post subject: Reply with quote

You have changed exhaust placement and size since you built the cooker.
How did she run before this change?
How big is the current exhaust? It looks like 8" ID is this correct?
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Chef Folett
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PostPosted: Mon Sep 15 14 9:19 pm    Post subject: Reply with quote

SoEzzy, I do preheat my splits. They catch fire immediately. if I put more fuel in there the temp will jump to over 280 some times. I use two smoker probes inside the cook chamber side by side. One is from the DigiQ and the other is from a Maverick. I also have tuning plates in there so the temp is mostly even throughout the chamber.

K.a.m., the smoke stack is 6" in diameter, before that change I was having the draft issue but was also running hot. Once I solved the draft issue I have been working on the temp issue. I have done a lot of testing with the fire size and can for the most part keep it down between 230 and 250 but with this size of fire I have to tend it every 30 minutes. Also once it starts getting below 230 that is when the coals are really low and it drops fast after that so my real operating range is between 230 and 250.
I am just trying to get a little more sleep at night when I cook.
as for adding the 2 foot section to extend my cook chamber I really do want some more real estate to work with. do you think that will help bring my temp down with the current firebox.
the current pit is Cook chamber is 68" long 24" diameter, Firebox is 24"x29"
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k.a.m.
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PostPosted: Mon Sep 15 14 9:38 pm    Post subject: Reply with quote

First I want to say that the thickness of your firebox has nothing to do with your problem, you can take that to the bank.
How does the cooker run without the fans as far as temp zones and length between adding a split?
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Chef Folett
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PostPosted: Mon Sep 15 14 9:45 pm    Post subject: Reply with quote

K.a.m. I will have to do some tests without the fans and let you know.
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crb478
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PostPosted: Mon Sep 15 14 10:46 pm    Post subject: Reply with quote

First off good looking smoker. Before I started cutting it apart I would experiment with reducing the opening between the firebox and the smoke chamber starting with one in less and then 1/2 inch increments. Just clamp some scrap steel in place until you know how its working. This should make the smoker slightly less efficient to keep you in the 225 to 240 range. You could also make the exhaust slightly smaller so less air can flow through. Do only one change at a time though. It really sounds like your smoker would be happy running in the 250-300 degree range, but it that's not where you want to be I am sure you can tweak it enough to get it lower. I would not get rid of the firebox.

The only problem is that each change will probably require more changes. modifying the opening will probably throw the tuning plates off etc.,but with enough tweaking you can get there.
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fiddlertx4
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PostPosted: Mon Sep 15 14 10:55 pm    Post subject: Reply with quote

I agree with KAM that the thickness of the firebox steel has nothing to do with the problem. I like to have a bigger fire with bigger splits and then just choke down the air intake to get a slow enough burn to get the right temp - then you can go hours without even opening up the firebox to add wood.
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k.a.m.
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PostPosted: Tue Sep 16 14 12:17 am    Post subject: Reply with quote

Chef Folett wrote:
K.a.m. I will have to do some tests without the fans and let you know.

Going on this statement I will guess you never ran the cooker without the fans with the new larger exhaust.
If this is the case then I will almost guarantee that is most of your problem.
Offsets do not need fans to run if they are built properly, they operate on their own draft. The fans will only increase your sleep time if you want to cook a lot hotter than 250°
Without the fans you should get about 45 min. to 1.5 hours on a split, that is normal.
You have an offset that is not insulated, if you need sleep I suggest you look into a cabinet smoker.
One of my friends has a totally wrapped offset cooker, the cooker itself is .500 wall, the most he can get out of his cooker without adding wood is about 3 hours and he cooks at around 300°.
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Chef Folett
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PostPosted: Tue Sep 16 14 12:42 am    Post subject: Reply with quote

You guys may be right about the fans. I think that I got them originally to help with the draft issue and never really tried it without them. I will do some tests and see what it does.

300 seems a little hot for me. I used to do my cooks around 1.26 hours per pound when I was having draft issues and the temps were lower because of that. now I am finishing closer to 1 and 1.1 hours per pound. I still manage to get o good brisket but the lack of sleep is killing me.

thanks your your help and i will let you know how my tests go.
I still would like to add the 2 foot section to my tank for more real estate. I should be able to leave the firebox alone even with that extra area correct?

with the current setup with a 68" cook chamber my firebox differential from the pit calculator is 128 %. if I increase the size to 93" for the cook chamber the differential will be 93.6 %.
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Chef Folett
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PostPosted: Tue Sep 16 14 7:22 am    Post subject: Reply with quote

I have another thought.
could my tuning plates be too low?
here is a shot of a tuning plate where it meets the damper between the cook chamber and the firebox.

Here is a pic from inside the firebox.

from the firebox view you can see that the tuning plate is about an inch or so below the bottom of the damper which means that heat can go above the tuning plate.

Should I raise them up so that they are above the highest point that the damper can open. it is currently open all the way.
They are currently 5 inches below my cooking grate. if I raise them up to meet the damper they will be a little more then 3 inches below the cooking grate.
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k.a.m.
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PostPosted: Tue Sep 16 14 7:37 am    Post subject: Reply with quote

Chef Folett wrote:


This is a poor design, allowing your heat to enter the chamber above and below the tuning plate basically renders the plates useless.
While I would remove the adjustable damper ( in my opinion they are useless toys) I would seal the gap between the plates and the chamber. This would keep the heat under the plates where it is intended to be.
I would still can the fans they are not needed.
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Chef Folett
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PostPosted: Tue Sep 16 14 7:44 am    Post subject: Reply with quote

Ok so the first tuning plate should be up against the firebox so all the heat travels below it?
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k.a.m.
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PostPosted: Tue Sep 16 14 7:49 am    Post subject: Reply with quote

Chef Folett wrote:
Ok so the first tuning plate should be up against the firebox so all the heat travels below it?

Yes.

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Chef Folett
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PostPosted: Tue Sep 16 14 7:50 am    Post subject: Reply with quote

Ok, thanks I will give that a try.
I really appreciate all your help.
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