Help with Temp Variations

[tfrank778]

Hello All!

I've recently created/finished my offset smoker which is pictured below. Last week I did my first brisket on there and noticed that the temps are no where near as consistent as they were from side to side.

Here are the graphs from the last two sessions... Before my temp from side to side was within 5-10 degrees now it's closer to 50 from side to side

How do I correct this issue? It appears the meat is affecting the air flow in the pit pretty significantly. I have great draw due to the oversized stack normally.

[sandro]

Slow your fire down, it looks a bit too lively; and use a stack damper

[Dirtytires]

First off, you are always going to have a larger difference on a traditional offset vs a reverse flow. Eliminate the first 12-18 inches of rack space from your calculations as that is typically a dead-zone for placing meats anyway.

Secondly, it appears you are using the door as the air intake which makes controlling the fire unpredictable and non repeatable as the heat can always just run out the door instead of thru the smoker. In addition to needing air vents for the intake, your exhaust is oversized which lets the far side of the pit release the heat too quickly.

[tfrank778]

First off, you are always going to have a larger difference on a traditional offset vs a reverse flow. Eliminate the first 12-18 inches of rack space from your calculations as that is typically a dead-zone for placing meats anyway.

Secondly, it appears you are using the door as the air intake which makes controlling the fire unpredictable and non repeatable as the heat can always just run out the door instead of thru the smoker. In addition to needing air vents for the intake, your exhaust is oversized which lets the far side of the pit release the heat too quickly.

Any advice on how big to make the vents?

Also I've been toying around with making an exhaust damper, but I've never had an issue with heat like that before. Will choking off the flow through the pit affect performance as well?

[ThinBlue]

With the knowledge that you were able to achieve closer temp ranges on a previous cook I'm inclined to say that your pit is more than capable of performing the way that you expect because, well, it has already done so in the past. Before cutting anything up or making any modifications I would encourage you to take a closer look at your fire management. As Sandro stated, perhaps your fire was a little more lively this last time around. Try keeping the flames a little lower during the cook to reduce some of that radiant heat coming directly from the flames. The key is to start with a good coal bed.

I agree with DirtyTires that using the firebox door is likely going to give you more inconsistent results than a dedicated inlet BUT I will say lots of people do it that way and it works well enough for them. Just a learning curve thing and something you'll have to be willing to accept and be mindful of throughout a cook.

That is how I would proceed if I were in your shoes for whatever that's worth.

Also be sure to rule out any thermometer probe placement inconsistencies/issues. In general just try to be as consistent as possible so that you know you are collecting good data.

Sent from my GM1917 using Tapatalk

[Brewmaster]

Hello All!

I've recently created/finished my offset smoker which is pictured below. Last week I did my first brisket on there and noticed that the temps are no where near as consistent as they were from side to side.

How do I correct this issue? It appears the meat is affecting the air flow in the pit pretty significantly. I have great draw due to the oversized stack normally.

Hi there, what are you comparing it with? I mean, what's changed from before when temperatures were more consistent?

I built the same sized offset and it's the most even side to side of any of our offsets. What do you have installed for a baffle on the throat? Mine works best with no baffle at all and runs really evenly, it's a convection machine Your stack is way too large diameter, a damper will help to slow it down, but "stack effect" will be non-existent. It'll probably work decent enough with a damper in the stack though, give it a whirl! It'd work better to replace the stack with about a 3.5" i.d. diameter stack that's in the range of 38" tall from top of the collector. And remove the baffle if you have one - but let's see some pics of the baffle in there? I'll add some pics of mine. Good luck and feel free to ask questions about my build if you have any!

Actually, I'm going to link you to my google photo album for the build, have a look!

https://photos.app.goo.gl/Nca8PDYaWiGWa8sd8

[sandro]

Mine works best with no baffle at all and runs really evenly

That is one gorgeous "Millscale style deflector". I've been removing mine as well for the past couple of cooks, seems to move the hot spot further to the right although that part of the grate is still pretty much unusable. Have you noticed differences with and without?

[Dirtytires]

Any advice on how big to make the vents?

Also I've been toying around with making an exhaust damper, but I've never had an issue with heat like that before. Will choking off the flow through the pit affect performance as well?

I would run your tank size thru the pit calculator and see what it tells you. You will have better operation of your pit if you mount the intake below the level of the fire basket so that air is fed into the fire from below and then the rising heat is only able to go thru the pit. I like to put an intake on each side of the pit so any wind can be shut out of one side without needing to rotate the whole pit.

Any yes, exhaust is just as important as the intake. With too large of a exhaust your heat just runs out and doesn't drop its heat which leaves cold spots. Your problem is made worse with all the cold air that is getting sucked in from the open firebox. Fix that first and see if that gives acceptable results.

[tfrank778]

Hello All!

I've recently created/finished my offset smoker which is pictured below. Last week I did my first brisket on there and noticed that the temps are no where near as consistent as they were from side to side.

How do I correct this issue? It appears the meat is affecting the air flow in the pit pretty significantly. I have great draw due to the oversized stack normally.

Hi there, what are you comparing it with? I mean, what's changed from before when temperatures were more consistent?

I built the same sized offset and it's the most even side to side of any of our offsets. What do you have installed for a baffle on the throat? Mine works best with no baffle at all and runs really evenly, it's a convection machine Your stack is way too large diameter, a damper will help to slow it down, but "stack effect" will be non-existent. It'll probably work decent enough with a damper in the stack though, give it a whirl! It'd work better to replace the stack with about a 3.5" i.d. diameter stack that's in the range of 38" tall from top of the collector. And remove the baffle if you have one - but let's see some pics of the baffle in there? I'll add some pics of mine. Good luck and feel free to ask questions about my build if you have any!

Actually, I'm going to link you to my google photo album for the build, have a look!

https://photos.app.goo.gl/Nca8PDYaWiGWa8sd8

PXL_20220406_153500347.jpg

The difference between then and now is the amount of meat on the pit.. This is the first time doing a large piece of meat (briskets) compared to the other items (ribs, turkey breast, chicken etc)

What size pit did you end up building? The pictures are deceiving on my pit... The tank is actually 18" diameter x 54" length. It's roughly a 75 gallon tank.

Firebox is 20" x 20" x 20"

Smokestack is roughly 7 1/2" I.D.

I think the more and more I look my "throat" is too small especially considering the calc shows 64in^2 required and mine is roughly 16" x 3"

[Brewmaster]

The difference between then and now is the amount of meat on the pit.. This is the first time doing a large piece of meat (briskets) compared to the other items (ribs, turkey breast, chicken etc)

What size pit did you end up building? The pictures are deceiving on my pit... The tank is actually 18" diameter x 54" length. It's roughly a 75 gallon tank.

Firebox is 20" x 20" x 20"

Smokestack is roughly 7 1/2" I.D.

I think the more and more I look my "throat" is too small especially considering the calc shows 64in^2 required and mine is roughly 16" x 3"

Understood! Wowsers yup your stack makes the smoker look way smaller than it is! haha Mine is a 100 lb propane tank, 14.5" x 46" ish.

If you cut the top "shelf" portion out of your throat baffle (similar to mine there, cut that entire top out, allowing the heat to rise, utilizing that round bell-end of the tank to aid in starting smooth convection flow, like a springboard) This should help a lot, couple that with a damper installed in the stack (run it throttled to compensate for being so oversized) and you'll be in the money zone I bet. Definitely way too big of stack on that sucka! Quite a few reputable builders don't put drafts in the firebox door, and while I think a door draft offers better control without a doubt, any that I've used without door draft run just fine. Cheers

[Brewmaster]

Mine works best with no baffle at all and runs really evenly

That is one gorgeous "Millscale style deflector". I've been removing mine as well for the past couple of cooks, seems to move the hot spot further to the right although that part of the grate is still pretty much unusable. Have you noticed differences with and without?

Heyo Sandro, thank you! Yup it runs better without that baffle. But the baffle increases convection/velocity and definitely drives it to the stack more, so I like it more it on smaller/shorter cooks, like ribs. Moves the hotspot over as well, without the baffle the hotspot is pretty small though which is nice. Most of the convection is driven upwards there so I can situate food to within a couple inches of the hot dry spot without the need to juggle food around.

[Dirtytires]

I ran the numbers just to help you confirm but it sounds like you already ran them. Guess you need to decide if you want to tear it apart and fix it properly or just try to get by with it. Honestly depends on how much you use it to determine if it's worth the time and expense to re-work it all.

Given an 18x54 tank...

Stack at 7.5" ID and 9" long (length measured from top of the tank, not exit point.

Firebox of about 5000 sq in--- you could but a plate on bottom of yours blocking it to 20x20x12.5 to achieve but I might suggest a rebuild at 18x16.75x16.75 inches. I like the length on mine to be 18" so i can get a log in.

Throat needs to be 64 sq in. --- Probably worth increasing yours as it will allow you to maintain a smaller fire and have less conductive heat from the big ones you now need.

Vents---you need 24 sq in so I would put a 6x2" slider on each side of the firebox at the same level as the log-rack.

[tfrank778]

I ran the numbers just to help you confirm but it sounds like you already ran them. Guess you need to decide if you want to tear it apart and fix it properly or just try to get by with it. Honestly depends on how much you use it to determine if it's worth the time and expense to re-work it all.

Given an 18x54 tank...

Stack at 7.5" ID and 9" long (length measured from top of the tank, not exit point.

Firebox of about 5000 sq in--- you could but a plate on bottom of yours blocking it to 20x20x12.5 to achieve but I might suggest a rebuild at 18x16.75x16.75 inches. I like the length on mine to be 18" so i can get a log in.

Throat needs to be 64 sq in. --- Probably worth increasing yours as it will allow you to maintain a smaller fire and have less conductive heat from the big ones you now need.

Vents---you need 24 sq in so I would put a 6x2" slider on each side of the firebox at the same level as the log-rack.

I plan to and currently use this pit a lot so I'll definitely take the time to make the adjustments.

The throat shouldn't be hard to adjust.. I'll get the torch/cutting wheels out.

The door "rest" actually has notches cut into it to keep placement somewhat consistent. The picture with the door wide open was after a new split was added trying to get it to ignite. I'm really torn on the vents. Not really sure how/where to place them to get the volume required.

Would it be wise to cut the exhaust down or just install a damper?

[tfrank778]

The difference between then and now is the amount of meat on the pit.. This is the first time doing a large piece of meat (briskets) compared to the other items (ribs, turkey breast, chicken etc)

What size pit did you end up building? The pictures are deceiving on my pit... The tank is actually 18" diameter x 54" length. It's roughly a 75 gallon tank.

Firebox is 20" x 20" x 20"

Smokestack is roughly 7 1/2" I.D.

I think the more and more I look my "throat" is too small especially considering the calc shows 64in^2 required and mine is roughly 16" x 3"

Understood! Wowsers yup your stack makes the smoker look way smaller than it is! haha Mine is a 100 lb propane tank, 14.5" x 46" ish.

If you cut the top "shelf" portion out of your throat baffle (similar to mine there, cut that entire top out, allowing the heat to rise, utilizing that round bell-end of the tank to aid in starting smooth convection flow, like a springboard) This should help a lot, couple that with a damper installed in the stack (run it throttled to compensate for being so oversized) and you'll be in the money zone I bet. Definitely way too big of stack on that sucka! Quite a few reputable builders don't put drafts in the firebox door, and while I think a door draft offers better control without a doubt, any that I've used without door draft run just fine. Cheers

Interesting idea on the "throat" dimensions! Looking at your pictures compared to what the calc spits out, is yours at the exact size required? I like how you cut yours back to the inner wall of the tank, but if I were to do that the throat would be way to large

[Dirtytires]

Exhaust is a personal preference. Most people choose a diameter so that it is long enough to put smoke over the users head while using the pit. To achieve this, I imagine you would need a 3.5-4 inch dia pipe. If this is not a concern for you, I would cut it off to proper length. You could use a damper but that is a massive restriction to get it down to where it needs to that I'm not sure what it would do to the airflow. Might be worth trying.

Just for the record, I like intake vents on either side but you can put one on the bottom of the door as well. Works fine but means you either need to move your pit or block any wind directly into the vent.

[Brewmaster]

The difference between then and now is the amount of meat on the pit.. This is the first time doing a large piece of meat (briskets) compared to the other items (ribs, turkey breast, chicken etc)

What size pit did you end up building? The pictures are deceiving on my pit... The tank is actually 18" diameter x 54" length. It's roughly a 75 gallon tank.

Firebox is 20" x 20" x 20"

Smokestack is roughly 7 1/2" I.D.

I think the more and more I look my "throat" is too small especially considering the calc shows 64in^2 required and mine is roughly 16" x 3"

Understood! Wowsers yup your stack makes the smoker look way smaller than it is! haha Mine is a 100 lb propane tank, 14.5" x 46" ish.

If you cut the top "shelf" portion out of your throat baffle (similar to mine there, cut that entire top out, allowing the heat to rise, utilizing that round bell-end of the tank to aid in starting smooth convection flow, like a springboard) This should help a lot, couple that with a damper installed in the stack (run it throttled to compensate for being so oversized) and you'll be in the money zone I bet. Definitely way too big of stack on that sucka! Quite a few reputable builders don't put drafts in the firebox door, and while I think a door draft offers better control without a doubt, any that I've used without door draft run just fine. Cheers

Interesting idea on the "throat" dimensions! Looking at your pictures compared to what the calc spits out, is yours at the exact size required? I like how you cut yours back to the inner wall of the tank, but if I were to do that the throat would be way to large

Nothing wrong with too large throat opening, too small on the other hand is detrimental. Mine is probably 30-50% larger than calculator numbers, but for the throat it's basically a recommended minimum.

[Brewmaster]

With regard to your stack, it's kind of too large to be affected by "stack effect" that much, so cutting it down probably won't make much difference, it's still a huge hole in the smoker. Try a simple wood-stove pipe damper install, only requires a small hole drilled and 2 minutes to install.

[tfrank778]

With regard to your stack, it's kind of too large to be affected by "stack effect" that much, so cutting it down probably won't make much difference, it's still a huge hole in the smoker. Try a simple wood-stove pipe damper install, only requires a small hole drilled and 2 minutes to install.

7in stack damper.jpg

First things first... I'm a moron... The stack I.D. is actually 6" not sure what I was thinking of.... With that being said it is still large

Alright Fellas... Here's the modifications I made....

Opened up the throat adding an additional 32 in^2. Since the "shelf was removed" adding some round stock to hold the water pan above the opening

Installed damper on the exhaust to allow control of air flowing out the smoke stack

Still working on cleaning up the welds/cut so it'll look better once it's completely finished, but this is a start.

Going to hopefully fire it up tomorrow to test out the changes.

Next up are vent on the door if this doesn't help

[Brewmaster]

Looking good tfrank, best of luck on the test run!