Offset smoker stack size question

FB_BBQ · Jan 19, 2022

I am looking into offset smokers. One of the finalists is a reverse flow 20" diameter by 34" long "Compact Patio Smoker" by Johnson.

https://johnsonsmokers.com/compact-patio/

Instead of one stack, it has two 3" stacks (each 30" long). They do offer some customization and I am wondering about requesting 3.5" stacks instead.

It seems like Franklin and lots of others suggest about a 5" diameter stack for a smoker this size. With a 5" diameter pipe, the inner diameter would be 4.75" and the area of a cross section would be 17.72sqin. With dual 3" stacks, the combined cross section area is 11.89sqin (and two smaller pipes don't flow as well as one larger one). Going up to 3.5" pipe, the area rises to 16.59sqin. Any idea how much a dual-stack matters or that it is a reverse-flow smoker?

-OD- -ID- -Single- -Dual-
3.00" 2.75" _5.94" 11.89"
3.50" 3.25" _8.30" 16.59"
4.00" 3.75" 11.04"
5.00" 4.75" 17.72"
5.63" 5.38" 22.69"

Am I overthinking this? Thoughts?

compact-patio-2.JPG-nggid03251-ngg0dyn-150x150x100-00f0w010c011r110f110r010t010.JPG

Cat797 · Jan 19, 2022

I'd ask for a single 5" stack. Why make it complicated? I don't want to fiddle with two different stack dampers etc.

rque · Jan 19, 2022

Cat797 said:
I'd ask for a single 5" stack. Why make it complicated? I don't want to fiddle with two different stack dampers etc.

Agreed, if you’re going to have modifications made see if you can get a wider single stack.

TheHojo · Jan 19, 2022

I just sold my Johnson Smoker - same one you are thinking of purchasing, but it also had the vertical chamber above the firebox.

Mine ran like a champ - but I always kept the "trap door" open between the horizontal and vertical cook chambers - so ran mine with 3 stacks wide open - the two in the horizontal chamber and the one in the vertical chamber.

SmoothBoarBBQ · Jan 19, 2022

Proud Johnson Smokers owner here (and have a new one being built right now). The reason there are two stacks is so that the heat and smoke are pulled evenly across the cooking chamber. The stacks are spaced on each particular smoker so that everything works properly.

If you take a look at some of the larger smokers you'll see the stacks are mounted through the top, rather than the side like his smaller smokers. One of those stacks lines up with the bottom rack inside the cook chamber, and the other lines up with the upper cooking racks. This way the heat and smoke are pulled evenly across both the upper and lower grates.

I've never heard of anyone having any drafting issues with a Johnson Smoker. I'm sure he'll adjust the stack sizes if you request it, but from my experience with Johnson Smokers there's really no need to do so.

Nash · Jan 19, 2022

I can't remember what was in Franklin's book for recommendations, but his pits have a 4" stack on them, the chamber is about 20x40" I think. But that's a straight-through offset rather than a reverse-flow.

A quick look at the Johnson reverse-flow shows both stacks at the same height, which is a similar idea as a smoke "collector" or "plenum" but executed in a different way. At the end of the day, the dual stacks are to decrease one central (center of grate at exit point) spot of high convection/hotspot that you'd get with one larger stack.

Also worth noting is that the pipe diameter can vary a fair bit, depending if it's schedule 10, 40, or 80, and so on. On a recent build I sourced schedule 10 6" pipe; it's actually 6.4" ID.

FB_BBQ said:
I am looking into offset smokers. One of the finalists is a reverse flow 20" diameter by 34" long "Compact Patio Smoker" by Johnson.

https://johnsonsmokers.com/compact-patio/

Instead of one stack, it has two 3" stacks (each 30" long). They do offer some customization and I am wondering about requesting 3.5" stacks instead.

It seems like Franklin and lots of others suggest about a 5" diameter stack for a smoker this size. With a 5" diameter pipe, the inner diameter would be 4.75" and the area of a cross section would be 17.72sqin. With dual 3" stacks, the combined cross section area is 11.89sqin (and two smaller pipes don't flow as well as one larger one). Going up to 3.5" pipe, the area rises to 16.59sqin. Any idea how much a dual-stack matters or that it is a reverse-flow smoker?

-OD- -ID- -Single- -Dual-
3.00" 2.75" _5.94" 11.89"
3.50" 3.25" _8.30" 16.59"
4.00" 3.75" 11.04"
5.00" 4.75" 17.72"
5.63" 5.38" 22.69"

Am I overthinking this? Thoughts?

FB_BBQ · Jan 20, 2022

Nash said:
I can't remember what was in Franklin's book for recommendations, but his pits have a 4" stack on them, the chamber is about 20x40" I think. But that's a straight-through offset rather than a reverse-flow.

A quick look at the Johnson reverse-flow shows both stacks at the same height, which is a similar idea as a smoke "collector" or "plenum" but executed in a different way. At the end of the day, the dual stacks are to decrease one central (center of grate at exit point) spot of high convection/hotspot that you'd get with one larger stack.

Also worth noting is that the pipe diameter can vary a fair bit, depending if it's schedule 10, 40, or 80, and so on. On a recent build I sourced schedule 10 6" pipe; it's actually 6.4" ID.

Were are you finding that Franklin's stack is 4" diameter? I have looked and cannot find that info. In his book he only offers that, for a smaller backyard smoker, 4" is the absolute minimum with a height of 3'-4'. He also says he errors on too much draw with a damper to slow it down if needed (p. 59 of the manifesto).

sandro · Jan 20, 2022

I believe Franklin's backyard pit is 4.5", Mill Scale's 4"

Nash · Jan 20, 2022

Not sure where I gleaned it from but I've seen or read it on YouTube vids, or in comments, or on forums. Wilson's bbq on YouTube mentions it's 4.5", but I'm pretty sure that's the O.D. measurement. 4" schedule 40 pipe is 4.5" O.D. and 4.1" I.D.

It also looks smaller to me, I have an offset with 20" diameter chamber and a 5" sch 40 stack, and it's definitely fatter to the eye.

busmania · Jan 20, 2022

I put a 5” (inside diameter) Stack on my 24” x 54” direct flow and it is plenty big. I think too much flow is as bad as too little but that’s just me and I also don’t build reverse flow so I’m not as familiar with that. Gotta find a sweet spot.

FB_BBQ · Jan 20, 2022

busmania said:
I put a 5” (inside diameter) Stack on my 24” x 54” direct flow and it is plenty big. I think too much flow is as bad as too little but that’s just me and I also don’t build reverse flow so I’m not as familiar with that. Gotta find a sweet spot.

I agree that it can be too big, but you can always use a damper if it is too big. Can't really adjust one larger (although i guess you can always make the stack longer).

Nash · Jan 21, 2022

Here's a couple snaps of my 20"x36" with the 5" sch 40 stack (5.1" ID) I realized once I saw it mounted that I should have went with a 4". All good though, it makes it look beefy and I added a damper, so all is well. :grin:

FB_BBQ · Jan 24, 2022

Nash said:
Here's a couple snaps of my 20"x36" with the 5" sch 40 stack (5.1" ID) I realized once I saw it mounted that I should have went with a 4". All good though, it makes it look beefy and I added a damper, so all is well. :grin:

Wow! I know the Old County Brazos has a 5.625" (OD) stack and this looks even bigger!
I have been assuming that Johnson's spec of 3" stacks means the OD is 3". If it is actually the ID, then I think it'd be fine.

Offset smoker stack size question

FB_BBQ

MemberGot rid of the matchlight.

Cat797

is one Smokin' Farker

rque

Knows what a fatty is.

TheHojo

is Blowin Smoke!

SmoothBoarBBQ

is Blowin Smoke!

Nash

Well-known member

FB_BBQ

MemberGot rid of the matchlight.

sandro

Full Fledged Farker

Nash

Well-known member

busmania

Knows what a fatty is.

FB_BBQ

MemberGot rid of the matchlight.

Nash

Well-known member

Attachments

FB_BBQ

MemberGot rid of the matchlight.

Similar threads