Stack / chimney questions

[kryten]

Hi all,

Been a huge lurker and finally posting as I really need some newbie questions cleared up and this seems the absolute best place to ask! On other sites I find conflicting info so thought I'd come to the experts.

Been trying to search through on the site, but still not getting to the answer and have been wading through for a while. I've seen where others have asked the same questions, but then they never seem to get answered (other boards) If it has been covered here, sorry. Please put a link to the area and I'll gladly read through.

Okay, not building, but modding an Oklahoma Joe offset smoker (20 dia x 40 long). I know, FOWL! Just can't afford to build what I want at the moment.

I used the calculator online and have modded the smoker to be closer to inlet vent size needed, chamber connection opening needed and I'm down to the exhaust. Issue is length. I need 31.5 inches of exhaust stack for the smoker according to the numbers. Questions focus on length and what is counted. The existing stack comes out of the side of the smoker towards the top and then does a 90 and extends up. I purchased some 3.5 OD matching pipe and made an internal elbow that fits into the same mounting collar the external pipe mounts in. The internal pipe does a 90 and extends down to 1 inch above the grate.

I've read that all internal extensions/piping is not counted in length of exhaust. Then I read it is. So...

Does the exhaust length include the internal piping to the grate or not?

I would have thought it would be, but then someone said, on a post I read, that it was counted as part of the chamber volume only. Got me to thinking that maybe that would be so and what is counted is all external after all.

The other question comes up because of a post I read that said that if you have a horizontal pipe that comes out of the side of the smoker and then does a 90 to the vertical pipe, the horizontal section is NOT counted in your overall length calculation. The only thing counted to the overall exhaust length compared to what you need is the vertical section after the 90. Is this so?

Thoroughly confused newbie so sorry again if this is elementary stuff and has been covered somewhere a billion and one times. Thanks for any help.

[Big T]

Normally it includes the complete stack regardless if it's all outside the CC or if it sticks inside the CC. I'm not sure what effect that the double 90* bends will have on the performance, if any at all. jm2cw

[Puff]

I could be wrong...I THOUGHT I read once that it is measured from the vertical portion of the stack. This rings a bell from when I was discussing the stack elbow with Gizmo. Trying not to add to the confusion, only in the interest of getting it right

[Rodcrafter]

Yea that

[Tom_Heath]

Dohhh!!! you said Ya That. LOL Havent seen that in a while.

I am pretty sure that there is a cohelical comprehensive explanation as to if the entire stack counts.

My vote is going to be yes. Simply due to the fact that once the draft starts its going to pull air from the end of the stack and not just from the top edge or from the 90 degree elbow.

Just my thoughts.

[Rodcrafter]

LOL PD

[Pete Mazz]

I'm thinking only vertical since we're talking about the heat rising causing the draw. Suppose the stack went horizontally 3 feet then turned. I would count that 3 feet as CC, not stack. As for turns in the stack, they would tend to reduce airflow, so some extra vertical length should be added IMO. Some testing with cheap venting material I think would answer all those questions for not much coin.

JM2CW

[SmokinFF]

This applies to chimneys, but maybe smokers too. Any horizontal run or bend in the chimney restricts draw. If it's possible, come out the top and go straight up.

[kryten]

Guess I wrote too much and took too long as the system logged me out and I lost the post I was making.

SmokinFF- I looked at coming straight up with it, but the way the door is I'd have to come out towards the back corner of the chamber. Wondered about what effect that would have and also the difficulty of cutting the oval type hole.

Pete Mazz- I'm wondering if the horizontal may have something to do with it. My feeling/theory now is that the vertical counts inside and out and the horizontal counts only because it is between too verticals on my chimney, but to a diminished extent maybe. Like BigT, Puff, RodCrafter and Pit doctor have posted and brought up good info on as well.

Pit mods so far to calculation comparison. Inlet showed too small, fixed. Chamber connection showed large, reduced it, but still left it slightly large by 3" due to "scoop" mod I did to direct heat to the top of the chamber instead of flowing underneath in the theory of the Jambo style pit. The lip of the scoop extends 1 5/8" above the cook grate. Made sense to me to try directing it to the top, have it naturally even out and fill down to the cooking grate to give efficient use of the fire in the firebox and even temps across. My 20 degree differential had me looking at the question of what is counted on the chimney again.

My feeling is (please jump in and correct me if I'm wrong or give my your thoughts everyone) that the existing pipe I have now is effectively too short even though internal center measurement would put it spot on for the 31.5" chimney needed to calc. Being too short it is drawing too much and my heat is traveling up along the top of the chamber and then getting sucked straight out instead of having time to "fill" the chamber and even out the heat more.

Know there is diminishing returns to too long or too short of a pipe, but I'm wondering if a longer section is needed to draw more to the sweet spot needed and also provide a slight bit of restriction to allow this "filling" of the heat in the chamber. I do have some aluminum expandable dryer vent I could try maybe.

Guess while I'm free thinking here I wonder if extending the internal pipe down to 3 or so inches BELOW the cook grate would help with bringing the heat down and further across the cook grate toward the firebox side.

Existing pipe measurements on internal center of pipe:
External vertical: 17"
External horizontal: 5"
Internal horizontal: 3.5"
Internal vertical: 6"

Thoughts?

[Rodcrafter]

I personally would choose to do away with all horizontal stack it is not helping in my opinion.

[Pete Mazz]

mod I did to direct heat to the top of the chamber instead of flowing underneath

Most mods to offsets consist of tuning plates to keep the flow of heat underneath the grate for as long as possible.

[kryten]

Rodcrafter: Thanks for the post! Think you are correct in the horizontal stack deal. Too much of a wild card. Was hoping to be able to use what I have to be sort of a test bed to let me know length and placement before I did any drastic cutting and welding. Asked the initial questions because I was unsure that the results I was getting were with the correct length exhaust depending on what is and isn't counted. Was going to try and do some testing today, but temps dropped again and it is raining steadily worse luck.

Just a matter of interest, well at least to me, I saw a video on YouTube with someone cooking on a Jambo. Nice internal cook chamber camera work. Realized from a picture I found and the video, that the Jambo uses an exhaust port on the side ABOVE the cook grate. My guess is that the top of the square port is 4 to 5 inches above the grate and in the picture I have, you can see the heat line extending down the side of the chamber and dissipating rapidly after it reaches the top 1/4 or so of the opening. This tells me that most of the heat is traveling across the top and mostly exiting at the top of the port and that the heat that is getting down to grate level is more diffused and perhaps more even with the inlet side of the chamber. Might experiment with that and see, but does seem like a waste of fuel/heat energy.

Pete Mazz - Thanks for the post also. I had the smoker set up that way to start with. Got fairly even temps, but seemed like a real fuel hog. That and it was a pain to clean well with all the plates and gunk that got under. The biggest deal that caused me to go the route I did was the thought of more efficient use of the fuel/heat I was producing. My existing "scoop" set-up seems to have been a winner when it comes to fuel savings. I'm using close to probably about 2/3rds to 1/2 the fuel I was using. Where I used to have to put in two splits 6 to 8" long and 3 to 4" diameter oak to maintain temps, I found on the last burn and the cook that I could often get away with just one. I did, of course, loose the radiant heat that was possible with the plates with this new design. I actually wondered about picking up a piece of 1/4" plate and welding it to the cook chamber side of the scoop to radiate a bit more heat that side. Think I'm going to figure out the exhaust before I consider that though.

[Fish and Teach]

Any chance of a photo of your exhaust?

[kryten]

Hi,
Outside of the smoker looks exactly the same as stock right now. Same collar and same factory pipe that comes out at a 90 and goes up. All held with a set screw per factory other than I loosened the set screw and pulled the factory pipe out a bit and retightened it in place. This allowed the internal pipe to slide in the same collar, but from the inside.



Pipe is 3.5" OD and is from a piece of exhaust pipe I bought from Summit Racing online. Matches the factory pipe exactly. I cut the pipe at a 45 degree angle and welded the pieces together with my cheapo flux core mig I've had ages. Please excuse the terrible welding, but I have no training and no real practice with welding.
Hope that helps!

[Big T]

I wouldn't include this piece in my overall length due to the two 90 degree bends. I would measure from the top of the stack to the short side of the 90*. jm2cw

[The Czar]

[kryten]

Thanks for the post BigT, DCMan

Yes, think the horizontal is too much of a wild card in the whole deal.

Have not made any real progress on it as I did just a burn with the smoker and some testing with different length pipes both outside and inside the cook chamber, but then after spending all that effort I find that the temp probes I was using have started to give false readings. So, changes that I'd noted in the temp from side to side with the different length pipes etc. were worthless. I believe the culprit is the cable piece running to the probes. So, ordered new temp probes and they won't be here until this Saturday some time.

I also ordered some steel pieces to make a sort of angled box off the side of the cook chamber that I'll mount the straight exhaust chimney too. Sort of aka the jambo exhaust. Only going to count the pipe coming straight up off of this as the length compared to the calc. Going this route with the boxing on the side as just seems like it would be a bit more stable what with my lack of welding skills.

My only real deal is placement on the side of the chamber so that I get the best use of the heat/smoke provided, but also have a design that will even the temps out more from side to side.

Basically comes down to three spots in the running.

Mount like the Jambo which places the entire opening right above the cook grate (bottom lip of the boxed opening is even with the top of the cook grate and the upper part of the opening is 4 or so inches above the grate). Down side I see to this is that a lot of the main heat energy will exit the cook chamber at the top section of the exhaust opening and thus 3 inches or so above the cook grate. This seems a waste of fuel. On the plus side, the fact that the main heat exits above the grate like that, might mean that the temp on the firebox and cook chamber side might be more even. Basically, instead of the concentration of heat being down at the grate to exit and thus a higher temp on the exhaust side of the chamber, the main heat exits and more residual heat is what reaches the grate and so perhaps is closer to the temps measured on the firebox side. Don't know if that makes sense how I've written it.

Second is to place the opening with the top lip at 1" above the cook grate and the remainder below. I've seen a few pits built this way when I've been dreaming about a more "pro" pit. I'd think that I'd probably remove a section of the angle support on the cook grate that was in front of the opening to ensure good flow without obstruction. Plus on this placement I'd imagine is the better use of the heat produced at the grate level. Down side may be that I end up with a high margin of temperature difference from side to side similar to the 20 degrees I'm experiencing now.

Last consideration is to place the entire exit opening below the cook grate (maybe even a couple of inches below). Plus on this is I would think it is the most efficient use of the heat produced as it will need to fill the entire cook chamber to below the grate to be able to exit. I envision that the grate temps would be more even from side to side also. The thought is that the heat would flow up and across the chamber. Fill downward with an eddied (sp?) concentration on the exhaust side, but the exit would be far enough down the chamber side that the heat would then pull down further across so that it reached the firebox side more so I'd have even temps. Know I'm not explaining that well.

If you envision the heat concentration like a wedge door stop with the flat (floor part of the stop) facing up. The part that then extends down further would be my exhaust end heat with the heat extending down to the exit port. The heat would then be "pulled down" like the tapering of the door wedge, over to the firebox side. My thought on this would be the heat would be "pulled down" further and thus the firebox side grate would now be more in the main heat band (tapered part of the wedge) and thus the heat would be more even from side to side. Down side to this may be the "stale smoke" that I've seen people mention.

So, further testing needed I guess. Get the temp probes and see what happens.

Any thoughts all?

[kryten]

Don't know if anyone is still interested or that even it means a hill of beans as each smoker will probably react differently and mine is modified as listed in previous posts, but thought I'd not fall off the page as it were and post up results during a test run.

Finally had good weather so did a long test burn to see what I came up with. I'll list below and list temp difference I measured with the new temp probes set at the grate 1/3 of the way from each side (exhaust side and inlet side). Tested the probes first for accuracy with boiling water and they are bang on. Burning Kingsford blue bag charcoal mostly (to not burn through as much wood) and some oak added. Waited at least 30 minutes between changes before recording temps.

So, setup as listed above and with the internal extension as pictured above that brings the pipe down to just above the grate.
Temp variance was 20 degrees side to side. Exhaust end hotter.

Added a 5.25" extension pipe to internal pipe to bring the outlet to below the grate level. Basically used magnets to hold it in place. Slid that side grate out of the way.
Temp variance was 15 to 20 degrees. Exhaust end hotter.
Have two temps listed as I took one reading and got 15 degrees and then took another a bit later and got 20 degrees again so basically no effect for the extension pipe.

Took the 5.25" extension off the internal pipe and added it to the external exhaust pipe top instead.
Temp variance was 15 degrees. Exhaust end hotter. Seemed to start to increase in the chamber heat level with no addition of fuel. Temperatures were in the 275 range and went up to 330 range. If this can be attributed at least in part to the addition of length on the exhaust pipe external then that was interesting to me. The 15 degree difference side to side may be within a margin of error (gusts of wind etc.) and so may not be significant enough to consider a distinct change perhaps.

Adjusted inlet vents smaller. Placed them both to half open instead of completely open.
Temp difference measured twice at 10 degrees. Exhaust side hotter.
Temps dropped to 300 range.

I then considered the Jambo design which basically exits the main part of the heat from the exhaust above the grate I'd estimate 4 to 5 inches above the grate level for the top of the exhaust opening. Simulated this with an internal L pipe similar to the one pictured above, but shorter to bring the exit(outlet) of the pipe to 4 inches above the grate.
Temp variance was 20 degrees side to side. Exhaust side hotter.

This was interesting to me as all say to bring the exit down to 1 inch above the grate level. I measured no difference with having at the 1" to having the exit 4" above the grate.

I then went crazy and kept the short L inside the smoker, but added a 21.5" (did not want to cut my spare bit of pipe) external exhaust extension.
Variance temp 20 degrees side to side. Exhaust side hotter.

Last test I did I threw everything at it to make the longest pipe. 21.5" external extension, long internal L pipe (the one I started with and pictured above) and the 5.25" extension pipe placed on the internal L to extend the outlet to below grate level. Should mention for those that don't want to calculate that the outlet would be approx 4.25" below grate level.
Temp variance measured at 12 degrees. Exhaust side hotter.
On this measurement I perhaps did not wait as long to measure as I needed to get something done in the house and may have rushed it a bit, but I would not have thought by that much.

Came back out to the smoker after probably an hour and temps were 340 exhaust, 337 inlet side.

As I said, info may be useless and if anything I had a good day playing around with the smoker and am posting for entertainment value. What I did find interesting is that for the most part, FOR MY PARTICULAR SMOKER, most of the changes had no real effect and could be perhaps covered under standard temp fluctuations you may get anyway. Only ones I really take a little note of is the 10 degree difference perhaps.

I did find the extension of the external pipe seeming to raise the temps overall interesting. I also found the last test interesting in two ways. The temp was pretty much dead on even. My thought is that the pipe length overall and exit point placement had something to do with this as did the extreme length between readings for this last one. Overall I think there is something in it.

So, my thoughts are to remove the internal extension, remove the factory pipe, plug the hole, box out a new exhaust exit (aka in the style the jambo is made) in the side of the smoker below the grate and then have a straight pipe come off of that to be my exhaust. That way the boxed section is basically counted as being part of the cook chamber and I can extend the straight exhaust pipe to match the calculator length needed. At least that is the current theory.

Hope the post has been interesting at least.

[Puff]

I read this with interest as the best way to learn is to experiment. I did a bunch of experiments with my smoker to find the sweet spots. I still think, and this may not be so, that the longer length of stack increases heat efficiency in the draft and helps bring up temperatures. I'll have to experiment more in this vein when our weather warms up more.

I read SOMEWHERE... That the pit calculator is geared to RF smoker measurements

[Rodcrafter]

Good job, and interesting data.

[Pete Mazz]

Personally, I wouldn't worry about a 10 - 15 degree difference.

[kryten]

I read this with interest as the best way to learn is to experiment. I did a bunch of experiments with my smoker to find the sweet spots. I still think, and this may not be so, that the longer length of stack increases heat efficiency in the draft and helps bring up temperatures. I'll have to experiment more in this vein when our weather warms up more.

I read SOMEWHERE... That the pit calculator is geared to RF smoker measurements

Really glad the post was of interest and also interested in your similar thoughts on the longer stack. Without it, looks like my temps averaged in the 240 - 275 range with the inlets full open. When I extended the exhaust, the temps went up to the 340 range and I basically had to adjust the inlets down some to bring the temps more to the level I wanted. I'm thinking that the external pipe length made the majority of difference in temperature I saw. Seemed to start jumping up pretty well right away when I put the external extension on.

I'm thinking that my existing external pipe may be too short, so I may be just getting it closer to the, as you said, "sweet spot" length that it needs. Having said that though, compared with the calc measurements, I ran the test with less and more pipe than was called for (while still being an extension to the external pipe) and had similar temp increase results.

I'm sort of looking at it from the other side of the coin (again, certainly no expert and have absolutely no science to back it up), what I was wondering was if you have a longer chimney pipe, that is closer to the correct length, but still longer, it will still have an effective draw to pull heat from the firebox, BUT at the same moment it will allow the heat to remain in the cook chamber a bit longer as it is perhaps drawing at a slightly reduced rate. Again, no science behind this just a gut feeling. My guess was this is maybe why I am seeing a temperature increase like I did. A similar result I observed when I used just the factory external short exhaust pipe, but adjusted the damper on the exhaust closed a bit. Temperatures in the cook chamber increased. What do you think? Make any sense?

Just observation/gut feeling (again no data support), but with the factory short pipe wide open it seems to really expel the heat and smoke out in a hurry. I'm thinking that it is perhaps drawing too much and pulling my heat from the firebox, straight across the cook chamber and then the majority is getting sucked straight out even with adjusting the inlets. This is what first caused me to experiment with the exhaust outlet damper and that is where I got a temperature increase in the cook chamber that makes me think the above about the longer stack length. Probably wrong, but I always like to learn.

Either way, the longer pipe temp increase looks like it would be a more effective use of the heat produced from the fuel provided. That interests me for sure! Any time I can produce the same cook heat need from less fuel is a good thing. Perhaps might even extend the burn time within temp range. No test data on that as still in early stages yet, but perhaps.

Same boat as you weather wise though, so my exhaust build and further tests will have to wait a bit. I'm old enough now to be too much of a wimp to get out there when it is 16 degrees and snowing to work on it.

Really interesting too on the calc being geared toward RF design maybe. When I start the exhaust build I might just make the box piece off the side of the chamber, put the hole for the pipe and then do a test burn. That way I can put varying lengths of chimney pipe on and test to try and find the "sweet spot" that looks like it will give the most even heat and make the most use of the heat produced by the fuel.

Thanks again for the post!

[kryten]

Good job, and interesting data.

Thanks!

[kryten]

Personally, I wouldn't worry about a 10 - 15 degree difference.

Was thinking the same thing, but then I know me. I can never leave well enough alone. I get caught up in tweaking and experimenting until I get the result I want. Enjoy learning and trying to figure things out. If I left it alone I know every time I cooked with it I'd be irked that the temps were not closer. Goal may not be realistic, but if I could get it to around a 5 degree variance I'd be okay with that. I may just be doing this

Thanks for your post!

[Puff]

For what it's worth, my 275 RF smoker can be 10-15 degrees a art and during the same cook may be 4-10 degrees apart. Since it has not impacted the food quality, I just shift things around a bit to get it settled. Even a constant 15 seems to have little effect based on the maverick ET733 with probes all over the smoker. If it's running a more dramatic spread the weekend, I may jus move the meat around fore equal heat exposure. But mostly not.