Pit Smoker Air Flow

[DClovesBBQ]

Hello, I am building a smoker and have a question about the location, length and diameter of the flue and chimney. After years of being disappointed with the limitations of store bought devices I have reviewed a lot of documents and forums but still have one question. My question is if heat rises then why would it make since to have a chimney at the top of the hood? Wouldn’t this cause more heat loss and increase the need for more fire? Wouldn’t it make since to draw the air from the bottom of the pit? This would flow through the the flue and out the chimney.

[tbob4]

Ahumadora and Henrik

[Ahumadora]

Hi there. Usually you want to draw air from the center of the pit, about grate height. You could draw from the bottom of the pit, but would need to mount the firebox as low as possible or increase stack length to compensate. Mounting chimney manifold in the center helps to equalize the vertical heat differential inside the pit. Hot air flows much easier than cold. Post some pics and dimensions of everything and I will give you some numbers to use.

Here's what I have been doin today. Finally arrived in NZ after 4.5 month sailing. 22 tons of pits.

[Ahumadora]

tbob4 You really want to use a 9" grinder under the trailer cutting off the steel supports?

[tbob4]

Yep. My work would be substandard but my effort would be off the chart. Heck, give me plans to make a regular flow pit and I will accidentally make it reverse flow.

[Henrik]

I'm with Craig here. Mount it center, it is the ideal spot. I've tried mounting it lower too, but then you need to compensate with a bigger/longer stack. Smack in the middle is the recipe.

[DClovesBBQ]

Thanks for your advice. I must say my project is not as large as you all are used to dealing with so I appreciate your feedback. It might not matter but I feel if I place it in the wrong spot I will struggle with using and losing too much fuel/heat and worry about the having enough vents to get proper air flow.

[tbob4]

Tag the two respondents so they can see your question with a "@" before their name. This puts the question in their notification folder. Ahumadora and Henrik

[Ahumadora]

The pit is so small, it is not really going to matter much where you place the stack. With a tiny pit, go larger on the intake/exhaust than you think is needed. Intake vents, should be low as possible.

[DClovesBBQ]

Thanks for the guidance. @Ahumadora, Henrik and tbob4

[texastweeter]

That Ahumadora always acting like he knows something about making pits...

[Panhead John]

😂😂

[jfmorris]

I'll comment even though I have no expertise making smokers - but in using them. I've got an offset smoker my dad designed back in the early 1980's, and which he used to have his shop fabricate in two sizes as gifts to customers - a 3 foot long patio model and a larger trailer model. A number of them he fabricated in stainless steel, and I have one of those in my backyard, and it will probably outlast me. It's thin walled unfortunately, as they rolled out the drum shape themselves from stainless and welded ends on it, and basically fabricated their own barrel shape, and mounted a firebox on one end, but it did the job for me for many many years, and I'm kinda stuck with it, since Dad made it and passed it off to me in the 90's.

Anyway, where I was going with this was this. That offset has a 24" x 36" body, with a top mounted flue, and an offset firebox at the other end from the flue. The firebox is lower than the cooking chamber, mounted below grate level, with maybe a 6" x 8" opening into the main cooking chamber. I used to see a variation of up to 75 degrees end to end until I rigged a 9x13 pan as a "deflector" right outside the firebox, basically propped at an angle to force airflow toward the front and back of the cooker. That evened things out, and I see barely a 10 degree differential across the grate now.

Anyway, back to your sketch. The sketch seems to show that you will have the fire IN the smoke chamber, and not a separate box. In a cooker that small, I am not sure how that will work out for you, as it will likely be too hot above the fire to use that area for actually smoking anything - all food will have to be offset. Unless you do something like a reverse flow deflector plate, and even then, I don't think you want the fire directly below the food grate, in a box only 16 inches by 32 inches. If I had issues without a deflector with an offset firebox in a 24x36 cooker, I cannot imagine what you would have in a smaller cooker with the fire in the main chamber.

The other thing I want to point out is that I think you need air intake that extends BELOW the fire grate, not an intake that is at or above the level of the fire grate. The best way to get a draft is with intake below the fire, and on mine, since my intake is just above the charcoal/wood grate, I almost invariable find myself cracking the slide out ash tray he built into the firebox, to get air flowing in through the bottom of the firebox.

[DClovesBBQ]

Thanks for the note jfmorris. Yes, all food will be offset. I have actually considered a way to have the intake airflow come up from the bottom of the box. If successful maybe I will be able to limit the temp variations as you did.