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UDS Air Intake Size

cjclemens

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Joined May 20, 2016
This morning, I fired up my UDS for the first time ever, and I'm not too excited about the results. I filled the basket 1/2 full with generic briquettes and lit it with lighter fluid (that's all i had available). Once the coals had that nice greyed look, I put the diffuser plate and cooking grate in it. At that point, I laid a temp probe on the bottom grate, and set the lid on it.

Anyway, long story short, the temperature more or less stabilized around 280 with all intakes wide open. It was still climbing very slowly - I assume due to the rising morning temperatures. My UDS build has 3 - 3/4" intakes. 2 have caps and one has a ball valve on it. With one intake capped, temp dropped to 230. With 2 capped and one wide open, it all but chokes the fire out completely.

I was hoping this build would be capable of running up to 350-375 wide open, and maybe hit that 250 zone with one of the caps on. Is this a simple case of not enough airflow? Will up-sizing the intakes to 1" do the trick or will I need to go bigger? Any suggestions would be greatly appreciated!
 

krj

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Bigger is always better. You can always choke the air down with a cap or valve, but with a smaller sized air inlet you can only flow so much air at once eve full bore.
 

hardcookin

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It seems to me I didn't run a diffuser.
As mentioned above, what's your intake an exhaust situation?
 

cjclemens

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I’m using the lid off an old Weber kettle grill. It has that dial on it with 4 - 3/4” holes in it. I thought this might be the choke point but, if that’s the case, I shouldn’t have seen such a drastic temperature response from capping one intake. Or at least that was my best uneducated guess....
 

fivetricks

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Yeah, your exhaust size should be sufficient. I only run 2.25" of exhaust, I run 3x3/4" ball valve intakes.

That being said, sometimes I have to extend my chimney with a piece of PVC pipe to increase draft on still wind days. Kind of hard to do that with a kettle lid.
 
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pc farmer

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I have 8 (1/2) inches holes for exhaust. 4 ( 1in) intakes. 2 stay 3 stay capped unless I wan to burn hot. I have ball valves on them. Only small adjustments are needed to change the temp. You need to make small intake changes and not cap one off. Fine adjustments. Learn your smoker.
 

fivetricks

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Yep. Once I get semi dialed in I work in 1/16 or so increments on my ball valves.

Edit: that's why I'm ultimately happy I didn't go with caps and spent the $$$ on 3 valves So I can dial in in different wind and weather conditions.
 

daveomak

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I’m using the lid off an old Weber kettle grill. It has that dial on it with 4 - 3/4” holes in it. I thought this might be the choke point but, if that’s the case, I shouldn’t have seen such a drastic temperature response from capping one intake. Or at least that was my best uneducated guess....
Yeah, your exhaust size should be sufficient. I only run 2.25" of exhaust, I run 3x3/4" ball valve intakes.

That being said, sometimes I have to extend my chimney with a piece of PVC pipe to increase draft on still wind days. Kind of hard to do that with a kettle lid.
4 each 3/4" holes = 1.76 square inches of opening....
1 each 2.25" hole = 3.98 square inches of opening....

Drill 4 to 6 more 3/4" holes around the top edge of the drum to get the approx. exhaust and same control.. Then you will still have some exhaust control using the control on the lid...
 

hardcookin

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I run a drum with a kettle lid. It did ok, but I often thought about adding another daisy wheel to the lid. You can order them from Weber.
Once I got up to temp.
I had 3 - 3/4" intakes one with a ball valve.
I would cap 2 intakes and only run my valve half open.
"But I didn't run a diffuser"
 

cjclemens

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Thanks for all the suggestions. I think I'm gonna just hit this from both angles, exhaust and intake. I'll just order one of those daisy wheels from Weber and upsize the intake pipes to 1". Hopefully, that should do the trick!
 

cjclemens

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I did some calculations and, if my figures are correct, increasing air temperature from 70* to 300* will increase volume of that air by approximately 43%. If I increase my intakes pipes to 1" and double my exhaust by adding a second pinwheel vent on the lid, that'll take my intake area up to 2.37 and my exhaust up to 3.52 square inches. That'll make my exhaust about 48% larger than my intake. Is this the right line of thinking, or am I over-analyzing it?
 

daveomak

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If your intakes are 1", you will lose the fine adjustment necessary for temp control... UDS originally had 1/2" full flow ball valves and usually 1 nipple uncapped and 1 valve 1/2 open had the temp ~ 225-250...

1" hole is 4X's bigger than a 1/2" hole..
 

cjclemens

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Joined May 20, 2016
If your intakes are 1", you will lose the fine adjustment necessary for temp control... UDS originally had 1/2" full flow ball valves and usually 1 nipple uncapped and 1 valve 1/2 open had the temp ~ 225-250...

1" hole is 4X's bigger than a 1/2" hole..
My intakes are already 3/4" so going to 1" isn't quite that drastic. That said, I guess I can start with the exhaust and give it another test run before I mess with the intake. If that works, that'll save me some $ on hardware, anyway.
 

daveomak

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When the air enters the smoker it's about 60-80 deg.... when it leaves, it's about 225-300... Air expands and that means the exhaust should be about twice the sq.in. as the inlet...
 

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