# Exhaust question



## tsywake (Aug 4, 2009)

I came across a 6ft piece of 6" diameter pipe (free of course) that I am hoping to use as the exhaust for my smoker.  It's a little larger diameter than I would have liked for an exhaust, but I've also got a chimney damper that I plan to put in there to help choke down some of the smoke going out.  Even with the damper, is it too large?


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## roksmith (Aug 4, 2009)

Not knowing the size of the firebox or cooking chamber..hard to give a precise answer, but my immediate response is you will most likely want to trim the length, but it's not likely to be too large of a diameter.

Unless you go crazy big, it's not likely that you would run into a stack that's too big around.

..btw.. choking down the smoke is almost never a good idea.
control your temp by building the right size fire and controling the air coming in.


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## tsywake (Aug 4, 2009)

The firebox is 20" diameter, 36" tall.  The smoker itself is 30" diameter, and is 5'5" long.  I'm going to drop the exhaust down to meat level, I was just worried that having such a large diameter exhaust would dissipate the smoke too much within the chamber.  Dropping the exhaust should help, and I can always cut a smaller diameter hole in the smoking chamber.


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## Dutch (Aug 4, 2009)

You want to have your smoke flow unrestricted from the chamber. Restricting the flow can lead to stale smoke and a bitter taste on the meat.


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## rivet (Aug 5, 2009)

Why don't you Private Message BBQ ENGINEER? He certainly has the know-how to help or point you in the right direction.

From your dimensions, to me it seems a bit too large diameter for your chamber. 4 inches seems max for it, but then I am not an engineer.

Check him out, if not, then PM me back. I think that I can help you out here  with some engineer's I know.


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## rickw (Aug 5, 2009)

Try this  BBQ pit calculator .


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## tsywake (Aug 5, 2009)

Thanks for linking that calculator, and for all the tips above.  Per the calculator, I'll need ~5" diameter chimney leaving the smoker.  I came across some 2" pipe yesterday (also freebie).  I think I'll go with dual 2" exhausts, rather than the large 6".  That'll keep me close to the 5", and its much more workable than the massive 6".  

I've been amazed at all the engineering that goes into making a smoker.  Before coming to this site, I'd have never imagined all the technical calculations needed to produce the best TBS.


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## deskjockey (Aug 5, 2009)

Honestly, IMO, I would not worry about exhaust, think about how BBQ started, over an open pit, IE infinite exhaust.

You are over engineering your smoker and adding variables which might affect your BBQ.

use the single 5" and work from there


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## swine-n-shine (Aug 6, 2009)

I would think for that size pit your going to run into a lot of problems with 2" pipe for an exhaust.  I know people with similar sized pit with two 4" exhaust who have a little trouble.  When figurung out exhaust size you have to look at volume. In doing this two 2" exhausts doesn't add up to one 4", nor do two 4" add up to one six. My pit has a 24" x 30" firebox and the pit itself is 30" by approx. 7'.  I have a 6" I.D. pipe (not sure of the height) and it works wonderfully.  I'd recommend sticking with the free piece.


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## realtorterry (Aug 6, 2009)

Im a little new to smoking, but i must ask why you would want to choke the smoke down in your cahmber? It may turn out cresote? If you want more smoke flavor why don't you just closer your chimney damper ALITTLE more?


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## mgnorcal (Aug 6, 2009)

Interesting pit calculator spreadsheet on that link.

I'd like to know why the author says that what matters in an exhaust is internal volume and not cross-sectional area.
By that theory, a 6" stack 3' long = a 4" stack 6.5' long.

Keep in mind that cross-sectional area (square inches) quadruples when you double the diameter, so the proportions may not be intuitive.


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## roksmith (Aug 6, 2009)

It's your pit, but I believe you'll find if you use 2 2 inch pipes on a cooker that large, you'll wish you had gone larger.
My unit has a similarly sized firebox ( 20 inch pipe, 24 inch long) and roughly the same size cooking area (main box is 32 inch pipe 4 feet long and and attachad rib box 20x20x36 or so) and I use 2 5 inch pipes for exaust.
As Dutch said, you want unrestricted exaust. Restricted exaust = stale choked smoke and bitter flavor. You want easy fast flowing smoke..especially in a wood burner.

My best guess on the volume of the exaust being what matters.. would be draw. In general, the longer the pipe, the better the draw and the faster the smoke can move thru it. And I buy that to some extent...but in the above example, his firebox has a volume of 11304 cubic inches and needs an exaust volume of about 565 cubic inches according to the spreadsheet. I have a hard time believing that a 720 inch long 1 inch diameter exaust pipe will vent as well as a 20 inch long 6 inch diameter exaust. I know which one I'm using..if I'm building a new pit.

btw thanks for the link..tells me that my inlet should be about an inch bigger than I currently have it...


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## tsywake (Aug 6, 2009)

Looking at the pits that Klose makes, he uses at least a 5" or 6" exhaust pipe on pretty much all his smokers.  According to the calculations from the spreadsheet (once again thanks for that link), the 5" stack it calls for would have to be nearly 4ft long, which is a little longer than I would like.  Bringing it up to 6" diameter, it cuts the length down to 28 inches.  In playing around with the pipe last night, I think that might end up being the better option.


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## tsywake (Sep 1, 2009)

Rick, 

Just wanted to thank you again for that link.  It really is an exact science.  I'm now able to keep my temps steady, no matter how much wood I put in the fire box.  The air intakes vs exhaust length keep it nice and regulated without any additional work.


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## rickw (Sep 1, 2009)

Your welcome, I'm glad it worked for ya.

 More so thanks to the author, Alien BBQ. It is to his credit it was posted at all.


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## sqwib (Dec 13, 2009)

I know this is an old thread but thought it would fit here.
When determining the stack size with the calculator, is the length of pipe from the top of the smoke chamber, or is it from the meat rack. For example I need 20" of 5" pipe according to the calculator. 
And my meat rack is 12" from the top of the smoke chamber, does this mean my pipe needs to be 32"?


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## rivet (Dec 13, 2009)

Very interesting stuff! I used to know this, now I gotta go back and research it!


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## got14u (Dec 13, 2009)

I personally would go bigger just for the fact going smaller is a pain if you need to add. Going to big with a damper would give you the ability to dampen it if you can not get the temps up. Baffling and location of drafts would be more important. that effects the flow of smoke across your meat. just my .02


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## randya (Dec 14, 2009)

Go with the 5", you can alway dammper it if you want heavy smoke.  Make sure you show us the pictures of your build too.  We all like viewing them as you build it.


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