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Adjusting the top vs. the bottom vent? What's the reason for adjusting one over the other?


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Still a big newbie here but this forum has really helped me out with getting started. I've been pretty happy so far with my meals on the Akorn since I got it a couple of months ago. By far my favorite meal to make is grilled pizza which I am now doing almost weekly. I also made some smoked salmon last weekend that was amazing, After Christmas, I can't wait to try a smaller brisket and see what happens.

 

Anyway, my question is around the difference in temperature control that comes with adjusting the top vent vs. the bottom. Is there any guidance as to why I might choose to adjust one over the other when making temp. adjustments? Is it a fair statement to say that the bottom vent is for large temperature adjustments while the top is for more fine control or is that not the case?

 

 I know to not "chase" temps but I've been a little uncertain as to why I would want to adjust one vs. the other other than my thoughts above.

 

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The bottom vent is for supplying air to your fire.

 

 

The top vent controls the air flow amount through the grill.

 

 

 

The vents need to be set in such a way that they correspond with each other.  That ratio is specific to your particular grill.  We can offer suggested ratios....but you'll get it figured out in quick time.

 

 

 

 

For my Akorn.....wide open throttle was both vents fully open.

 

250 was the bottom vent open the width of a pencil and the upper vent open only enough to put the pencil point through the round part at the bottom of each slit.

 

400 was 2 finger widths open on the bottom vent and a full pencil width for the top.

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This is mostly my personal opinion however some of it is based in science. 

Top vent controls volume out  <-----
Bottom vent controls volume in  <---    this is the science part.

 

^^^^^together they control the air velocity through the cooker and over the fire.

 

Top vent open a bunch and bottom vent open a little creates a cleaner fire

Bottom vent open a bunch and top vent open a little creates a more smoky fire

I personally work them together, I have a setting on top for each setting on the bottom.  That is just what works for me.  There are obviously many different ways to look at it and apparently folks are having success using different methods.

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I do almost all of my low and slow using mainly the top vent for temp control and after not so very long the smoke is so thin and blue I can't see anything but heat waves unless I line up with a dark backdrop or have sunlight directly opposite the cooker from me.

My experience with different cookers, and I assume a kamado is the same, is that if I leave the top vent wide open and try to control strictly with the bottom the temp tends to be harder to stabilize. I have a theory on this that I will state here and I am open for correction.

My theory is that, without an exhaust damper, or with the exhaust wide open, there is obviously nothing to restrict airflow out of a vertical smoker which, since hot air rises, creates a slight vacuum at the bottom of the cooker near the intakes. Small adjustments are amplified because, for example, if the smoker temp is too low and you slightly open the intake to allow more oxygen to the fire, the resulting rise in temperature causes the heat inside the smoker to rise even faster, creating even more of an atmospheric vacuum at the intake, which in turn draws even more air through the vent, which in turn causes the temp to rise, which.... you get the picture. I have experimented with this some, and it seems sometimes the temp can be somewhat stable but too low and a very, very slight adjustment of the intake starts a gradual rise in temp that just keeps going and going (within reason of course).

The top vent counteracts this tendency. By restricting the exhaust, according to my theory, you control the atmospheric pressure inside the cooker and hopefully keep it equal with the outside, so that you have a nice controlled flow of air through the cooker that controls the temp. I also theorize that this "snowball" effect when not using a top damper would be more noticeable at higher elevations / thinner atmospheres.

All this is a fancy way of saying that you generally will have the best luck using both vents to control temps.

Anyone here is more than welcome to bust my theory with no boo-hooing from me but for now I stand by it. I close the bottom vent down just wider than it would need to be for the temp I am trying to hit and dial it in with the top.

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I do almost all of my low and slow using mainly the top vent for temp control and after not so very long the smoke is so thin and blue I can't see anything but heat waves unless I line up with a dark backdrop or have sunlight directly opposite the cooker from me.

My experience with different cookers, and I assume a kamado is the same, is that if I leave the top vent wide open and try to control strictly with the bottom the temp tends to be harder to stabilize. I have a theory on this that I will state here and I am open for correction.

My theory is that, without an exhaust damper, or with the exhaust wide open, there is obviously nothing to restrict airflow out of a vertical smoker which, since hot air rises, creates a slight vacuum at the bottom of the cooker near the intakes. Small adjustments are amplified because, for example, if the smoker temp is too low and you slightly open the intake to allow more oxygen to the fire, the resulting rise in temperature causes the heat inside the smoker to rise even faster, creating even more of an atmospheric vacuum at the intake, which in turn draws even more air through the vent, which in turn causes the temp to rise, which.... you get the picture. I have experimented with this some, and it seems sometimes the temp can be somewhat stable but too low and a very, very slight adjustment of the intake starts a gradual rise in temp that just keeps going and going (within reason of course).

The top vent counteracts this tendency. By restricting the exhaust, according to my theory, you control the atmospheric pressure inside the cooker and hopefully keep it equal with the outside, so that you have a nice controlled flow of air through the cooker that controls the temp. I also theorize that this "snowball" effect when not using a top damper would be more noticeable at higher elevations / thinner atmospheres.

All this is a fancy way of saying that you generally will have the best luck using both vents to control temps.

Anyone here is more than welcome to bust my theory with no boo-hooing from me but for now I stand by it. I close the bottom vent down just wider than it would need to be for the temp I am trying to hit and dial it in with the top.

I think I kinda sorta agree, hence my comment about velocity of the air through the cooker.  Velocity is not the same as volume, a wide open top and smaller opening at the bottom increases velocity.  Fanning a fire versus blowing on a fire are different.  I'll bet that most of us have seen the vacuum effect when we cut the lower vent back while still having the top vent wide open.  Volume goes down but velocity goes up.

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I'm with SmoovD: I've found that the bottom vent's setting isn't nearly as important as the top vent's. I can cook with the bottom vent completely open or completely closed. I think it's best to just leave the bottom vent alone, so that you can concentrate on getting the top vent right.

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I've got some more playing around to do with the Akorn.  I've cooked on it several times now--the food comes out just fine but man do I find myself constantly messing with the top/bottom vents to keep the temps in the 230-280 range.  Here was my experience (typical) this past weekend with a pork butt:

 

1) Brought up temp slowly and started shutting vents down at about 175 degrees.  About a 1.5 on top and a pencil width on bottom.

2) Temps continue to climb and creep to 270 with no sign of stopping, so then i close top down to the half moon point--temps eventually start dropping, and continue to drop (will go to 200 and make me fear snuffing) without stabilizing.

3) At that point, it's a game of opening and closing top and bottom vent every 45-90 minutes until meat is done.

4) This scenario has played out each time I have used the cooker, with varying settings on the top and bottom.

4) The meat comes out great!

 

I like the cooker--it is a step up from my smokenator--i don't have to add charcoal/water, etc and I can leave it for up to 90 minutes or so but it does take some babysitting.  Now that I am comfortable using it, I plan to take a day over the holidays and play with the fire and settings one more time to see if I can dial it in better.  I know I should expect temp swings, but when i set it at what I hope to be a stable temp, and it slowly climbs 20+ degrees something isn't right (I think).  Same with bringing it down--half moon on top with pencil width on bottom brings the cooker from 270 down to 200 in 1.5 to 2 hours--will it snuff?  I haven't wanted to find out with meat on the rack.

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What you've gotta learn is that temperature control is not a matter of fiddling with the vents until the temp stays where you want it. If you never stop adjusting the vents, you will never, ever reach a stable temperature!

 

If you want a stable temp, all you really have to do is set your vents in the RIGHT place and then just leave them there. The trick is learning what that 'right' setting is for the temperature you want to achieve.

 

You mention setting the top vent at 1.5. On mine, I would expect temps to stabilize in the mid-300s at that point. If I had the top vent set to the half-moon point, I would expect it to eventually snuff out, even if I have the bottom vent all the way open (unless maybe it's really windy). For low & slow cooks, I actually leave my bottom vent all the way open and set the top vent to 1 or slightly less. With the top vent at 2, I'll get a stabilized temperature in the low to mid 400s. So we're talking about a 150-200 degree difference just from moving the top vent from 1 to 2!

 

Forget the bottom vent. Once the fire is going, just leave it all the way closed and focus on getting the top vent right. If you can keep a stable low temperature with it closed, try opening it up all the way with the same top setting, see how much difference it makes (probably not a whole lot).

 

And don't obsess over hitting some specific temperature exactly. Great BBQ can be made pretty much anywhere in the 200 degree range. If you want to easily hit a bulls-eye as a beginner, give yourself a big target!

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  • 1 month later...

I actually had a fairly free Sat/Sun (first one in loooooong time) with decent weather to play around with the Akorn, specifically trying to maintain sub-275 temp for bbq.  I don't have any issues with higher temps.  I had very interesting and happy results.

 

Long story short, I had a fire on Saturday that finally settled in around 260, and a fire on Sunday that settled in at 255.  Both of these held fairly steady at these temps once locked in, and I kept them both going for about 10 hours each.  I used the wireless Maverick to monitor temps at the grate.

 

One highlight was the conversation with my wife--Her:  What are  you cooking?  Me:  Nothing I'm playing around with temperature control.  Her: (silence, blank/confused stare, eye roll).

 

Details:

Saturday--beautiful day, 55-60 degrees, slight chilly breeze.  Had mixed in some briquettes with the lump on the last cook after wondering if that might help with  a more even burn (more on that later).  So about a 50/50 mix of lump and briquettes.  After getting fire going and internal being 180 degrees, I shut down bottom vent to 1 and top vent to half moon plus a very slight sliver (my akorn does not have the markings on the cap).  I have found that the half moon setting eventually snuffs out the fire so I edged it a little past that--just a tiny bit (sorry no pics).

Temp rose to 293 before slowly dropping and settling in at about 260, with various 10 degree fluctuations throughout the day.  Held here pretty good for the remaining 7 of the 10 hours I ran the thing.

 

Sunday--50-55 degrees but very windy--akorn exposed to some swirling winds on the patio but not in direct line.  This time, used 80% briquettes and 20% left over lump.  Shut down vents at 175 degrees with a little less than 1 on the bottom and same half moon plus small sliver on top.  75 minutes later temps peaked at 230 and then started to drop.  At 225 I opened the bottom back to 1 and cracked the sliver on top just a hair more--I guess this would be about a 1 on the top if I had the dial marked.  In two hours the temp had reached 261 so I cut the sliver back on top to its original setting.  After an hour the temp had dropped to 252 so I adjusted back to bigger sliver and did not touch it after that--for the next 6 hours (Hours 5 through 11) the temp ranged from 252 to 261.  It started to fall slightly after that and at Hour 12 when I shut her down she was at 242.

 

I was pleased with this performance and glad I took the time to do both of these practice runs.  I am amazed at the sensitivity of the fire to those minute adjustments at the top.  Plus it was good to have both windy and fairly calm conditions.  It was also nice to see that I could get these results without the use of an external temp controller.

 

I'm new to the Kamado style cooking and have about 4-5 cooks under my belt, and had been perplexed by the temperature fluctuations I experienced in trying to keep things sub 300 degrees.  I have wondered if the lump charcoal and its unevenness played a role in this, which is why on the last cook I threw in some briquettes.  The 50/50 mix on Saturday performed well after the initial spike at 293, and Sunday really surprised me with the 80/20 mix.  It seemed to want to hold at 225-230, which made me a little nervous as I thought it might snuff out as I have never been able to hold it there before, so I didn't let it sit there long.  Once I got it to the 250-260 range it held there very well and steady until I was ready to shut the thing down.  My personal plan will be to go with briquettes in the future, as I know some of you have.  If performance suffers or is uneven, I may try the lump again.  I am not using the thing often enough to have the additional ash from briquettes become a pain.  

 

But all in all, this was enlightening, and has given me more confidence with how the cooker behaves.

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  • 4 years later...

Smoke volume will depend on fire temp., smoke wood, and air flow. Using the vents, whether top, bottum, or both, will limit the air flow. A few cooks in and you should get a handle on how to control the temps and level of smoke desired.

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You should be more concerned about the quality of the smoke vs quantity of smoke, if that makes sense. The thin blue stuff you barely see is what you want, it’s not bitter or overpowering. White smoke on the other hand is bad news... it’s a result of poor/incomplete combustion, grease fire, and the like. You don’t want that... keep your cooker clean and use quality fuel/smoke wood and you’ll get the good stuff. 

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