UDS Exhaust - Help

froggyshooter said:

Question and Request:
Built (building) first UDS: second on the way when this one complete. Just cooked pork butt and it came out great, but had to mess with intake quite a bit.
There are 3 3/4" intakes; 2 with caps one with ball valve. The exhaust is a 1 1/2 metal stack. Got it going with minion method and closed all intakes but 100% on ball valve. When I got home 8 hours later internal (Maverick) smoker temp was 175 F and meat was 150F. This has occurred every time I close off with caps and leave ball valve open 100%. Played with intakes (cap on / off) and it is touchy. The unit is air tight when everything is closed and vent stack is closed.
My thought, but let me know if I am wrong; the 1 1/2" exhaust is causing problem.
Love the way it cooks but cannot get it to hover at 225 like you guys.
Thanks

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If its not going as hot as you like then try opening up your intakes more. Logic dictates that your fire is not getting enough oxygen to idle at the temps you want.

Use everyone else's experience as a rough guideline, no two UDS behave exactly the same since not all are created the same nor do they run on the same fuel.

You didnt mention the ambient environment, how was the weather that day? If it was rainy, windy or cold this may have contributed to your low temps. Also, placement, is your UDS by a wall or out in the open?

I have no clue as to what all these calculations are all about. I just cook food that me and my folks like to eat. With each cook, I learn a little more about my UDS - what works, what doesn't work so well. Over the past year, I've learned different heat setting adjustments for different cuts and types of meat, poultry and fish. I can now make adjustments for the weather too, because I cook year round. So, as has often been said before, "YMMV" ... and keep the Thin Blue Smoke comin'!!

If the area of the exhaust doesn't have any effect than why have one at all? Just because a 2" hole came on the drum that was designed to store and transfer fluids does not mean that by coincidence that it is the optimal size to control exhaust on a UDS at any temperature.
The longer pipe only serves to create restriction and restrict air leaving the smoker plain and simple - for all intents and purposes air restriction inside the smoker is the same and any pick up in velocity as compared between 6" and 12" is negligable. By your argument, any restriction on your exhaust will create rancid tasting food no matter what size the opening.
None of the things you mention at such low relative pressures and velocities contributes at all to your explaination of how the thermodynamics of a domed lid vs a flat lid requires different consideration for exhaust sizing.
The results are 4 - 3/4" holes have adequate area to control the exhaust on a UDS. A 2" exhaust will also work although it will require different intake settings as well how much charcoal you initially light up.
To the OP - this is much simpler than is being made out to be. Start out with a 2" hole or 4 - 3/4" holes and see what works for you. Drum lids are cheap.

I concede, I have been beaten by the superior intellect...... I guess it has something to do with being from chitcago, there seems to be alot of them that know everything....


I have built and used 5 different UDS, with flat and domed lids in the last 2 years.... I know how they run and how the react... OP you can choose to believe somebodys opinion or sombodys experience. it's your decision....... Good luck with your changes....

I won't comment on where you're from either...

So!!!! could any of you cerebral guys hazard an educated guess as to what would be the ideal length 2" exhaust pipe with a 45 deg angle cut on the top. I currently am running 12" and think that the temps are a little hotter than I would like. Not looking to start an argument, just looking for knowledge from people who are a little wiser than me.

Check what lengths of threadded pipe are available at your local hardware store and don't be afraid to experiment. If you like to cook at 225, you may find a shorter pipe works better, and if you like to cook hotter maybe a longer pipe. Try a few pre-cut lengths and give it a shot. I run a weber lid on my uds with no pipe and while I typically run the vent wide open there have been times when it was really cold and windy that I had to close them off by about 30% when the temp started to fall from too much drafting. So in short, there is no ideal length that covers every situation - try a few out and see what works best with your uds.

I have been building drum smokers for many years.(I think I am on the 4th page of the massive UDS thread) So I do have some experience. I think I have built around 25 smoker so far for myself and friends.
The best design I have to date is 4 -7/8 inch intake holes and 6 equally spaced 7/8 inch exhaust holes on the top. Just seems to work the best. Why 7/8 inch.....size of a 1/2 inch NPT slug cutter.
I will say that I built 4 IDENTICAL drums three years ago for a chicken wing cookoff and all four of them cooked differently for some reason. Not that much different but I had to adjust each one differently during the cookoff.

Just my $0.02. There is no BEST design....just find what works for you and cook enough to "learn about" your drum. The ones I equipped with a fan and PID controller could control temperature on the dot but what fun is that!

foam2 said:

.... Just because a 2" hole came on the drum that was designed to store and transfer fluids does not mean that by coincidence that it is the optimal size to control exhaust on a UDS at any temperature.
......

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No, but it DID dictate what size most experimenting was done on.

Personally, I just drill 8 3/4 or so sized holes in the perimeter of the lid.

My new drums use a single 2 inch intake, but my old ones had the smaller intakes. My temperature was chiefly dictated by charcoal basket. I had a small basket and never could get above 250. With the bigger basket.... 350.

Lots of variables at play, but I am just a hick from the sticks.

I love how people always want to reinvent the wheel. Why do you mock the ones who have brought you something so simple. Practice makes perfect!

For the record I use a 2" PVC exhaust with a ball valve along with 2, 3/4" trade size intakes both with ball valves. Mine holds temps very well.

I appreciate all the time put in by others who have shared their knowledge.

Napper said:

Just my $0.02. There is no BEST design....just find what works for you and cook enough to "learn about" your drum. The ones I equipped with a fan and PID controller could control temperature on the dot but what fun is that!

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I thought the same thing. I just built my first one this past week. I thought about an electric hot plate and a cast iron bowl full of wood chips, then maybe a PID controller, and after a few cold ones and some tinkerin', it hit me. Half the fun is building it and the other half is keeping it dialed in and tasting the fruits of your labor.

Mine has 4 half inch holes and magnets for intakes 2" off the bottom, and a 18" x 2-1/4" exhaust pipe (90* elbow and 18" stack w/rain cap)) welded on the side of the drum at grate level. Ran it yesterday for the first time to season it, it ran for 16 hrs, before I finally hit the rack. Maintained it between 180-300. 27 hrs later, it still has some hot coals left in it.:thumb: I am going to drill out the intakes to fit 3/4" pipe nipples, I added a lid thermometer about 2 hrs . into the smoke, (mailman was kind enough to bring it), and I have a turkey thermometer 1/2" below the grate protruding to the middle of the drum, just to see how the temp varies from spot to spot. It stayed within 12* from grate to top and back to front. At some point I want to scrounge a Weber lid for it, add some handles and the obligatory bottle opener.


Ol'duke already standing guard.

froggyshooter said:

Question and Request:
Built (building) first UDS: second on the way when this one complete. Just cooked pork butt and it came out great, but had to mess with intake quite a bit.
There are 3 3/4" intakes; 2 with caps one with ball valve. The exhaust is a 1 1/2 metal stack. Got it going with minion method and closed all intakes but 100% on ball valve. When I got home 8 hours later internal (Maverick) smoker temp was 175 F and meat was 150F. This has occurred every time I close off with caps and leave ball valve open 100%. Played with intakes (cap on / off) and it is touchy. The unit is air tight when everything is closed and vent stack is closed.
My thought, but let me know if I am wrong; the 1 1/2" exhaust is causing problem.
Love the way it cooks but cannot get it to hover at 225 like you guys.
Thanks

Click to expand...

Sory that everybody missed the obvious answer...

Drums have a sweet spot temp wise that they like to run. You can run at any temp you'd like but you have to find its sweet spot to make it fiddle proof.
I find with 1 intake fully opened and my ball valve about 25% open my drums will settle in about 238*. Dont force 225 or 250, let it run where she's happy!

I've been using 2" PVC exhausts about 8" long with a 30* angle cut on top since the beginning of (drum) time. It works.

let me try this again, greenhorn at work....


I am a heavy equipment operator by trade so seeing a stack with smoke coming out of it makes me feel warm & fuzzy.

Never seen an exhaust on the side at grate level. May be hard actually getting any smoke on your meat.

I've seen a few on other sites. I had pretty good smoke coming out around the lid when I seasoned it, which is why I put the band on it to seal it up . I'm going to try it out Wednesday, I will let you know how it works.

froggyshooter said:

Why does the UDS stabilize after a series of 'cooks'?

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The gunk build up seals up the leaks