Exhaust Venting Question

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Pappa Roach1
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Exhaust Venting Question

Post by Pappa Roach1 » February 10th, 2015, 1:37 am

I have looked over lots of builds, and I have seen multiple exhaust set ups.

With 8 each 3/4" Vent holes in the lid, you have 3.54 Square Inches of exhaust opening.

With the 2" Bung Open ONLY you have 3.14 Square Inches.

The 1 Inch Bung will add another .786 Square Inches.

Using 2 each 1.5 Inch stacks you will have 3.52 Square Inches.

I have also seen 2 each 2" Stacks used, which is 6.28 Square Inches.

EDIT I looked at a Weber Dome today 4 ea 3/4" Holes = 1.77 Square Inches

I would presume that most builders think around 3.0 thru 3.5 Square Inches of Exhaust Opening is about right. However the Weber Dome lids seem to be working fine with only 1.77 Square Inches of exhaust.

The Dual 2" Stack Builds with 6.28 Square Inches do not seem to be a problem either.

My reading so far indicates that you need to regulate the heat with the Air Intake ONLY, and not shut down the Exhaust at all.

Am I correct so far??

Bob R
Last edited by Pappa Roach1 on February 10th, 2015, 5:37 pm, edited 2 times in total.



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Re: Exhaust Venting Question

Post by Smokeone » February 10th, 2015, 6:50 am

Yes you should control your cooker by the intake. I personally never adjust my exhaust, but some do. As for how many or size of exhaust, I think you are right that exhaust in the 3" range is sufficient. I do use the weber lid, and it's standard exhaust holes come in at just under 3". I'm not the "BTU" guy, so I can't go all technical on this, but my simple mind says if I there is roughly 3" coming in, how can you let 6" out? Clear as mud! :D

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Re: Exhaust Venting Question

Post by The Czar » February 10th, 2015, 9:31 am

I too have seen guys control by exhaust but I personally think controlling by intake is more effective and easy to control....but I also use a weber lid but I would say you theory of sizes are correct...
I feel like trying to control temp by exhaust would muddle your air flow....


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Pappa Roach1
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Re: Exhaust Venting Question

Post by Pappa Roach1 » February 10th, 2015, 6:02 pm

Using the Weber Dome lid, and the 3/4" Pipe Intake set up you will have (either 3 or 4) 3/4" Intake Ports, and 4 each 3/4" Exhaust Ports.
Since this obviously has been working just fine for years, it looks to me like on start up the UDS must be operating fine with the same amount of Intake Port size as Exhaust Port size.
When you throttle down the Intake Air after start up, you will still have MORE than Double the OPEN Exhaust Port size as Intake.
I am thinking SmokeOne is correct as long as you have atleast as much Exhaust Port size as Intake Port size, the UDS should work fine.

A bigger Exhaust Port size must not hurt anything up to a point either, HOWEVER it may not be improving the Smokers performance.

What do you think?

Bob R



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Re: Exhaust Venting Question

Post by Frank_Cox » February 10th, 2015, 8:43 pm

I Promise not to go all BTU on ya.....
:BDHorse:
LOL... that being said... Once draft is established in the drum the intakes take over and control the amount of combustion air to the fire. I have always started out with a 2" or equivalent exhaust size or 3.14 square inches (length in this case does not matter). bigger is not always better in the case of low and slow cooking hence I put Click here : gaskets on my lids. :beer:


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Re: Exhaust Venting Question

Post by Tom_Heath » February 14th, 2015, 8:06 am

Wow, that was a short response.


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Re: Exhaust Venting Question

Post by The Czar » February 15th, 2015, 1:11 am

Short for him


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Re: Exhaust Venting Question

Post by Big Steve » April 20th, 2015, 1:21 am

Here's my .02 cents on this. Inlet vents control temp. I use the exhaust vent to control draft. I look at both vents as a balancing act but once set right it's pretty much hands off.



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Re: Exhaust Venting Question

Post by Georgiaboy98 » May 6th, 2015, 12:49 pm

6in stack 2in pipe I've never had any trouble controling temps!
[img][IMG]http://i1345.photobucket.com/albums/p67 ... 09e3e9.jpg[/img][/img]



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