Smoker Chimney height

[bbqbrisket]

I have a Pecos. I have extened the chimney. It's not exactly the perfect height. Would it be better to be 6 inches too high or 6 inches too low?

 

[chopsaw]

This is my opinion from an in general stack length point of view .
Too long allows the smoke to cool in the stack and possibly condensate and drip back into the chamber .

I used to run mine hot , then feel the stack and find the place where it was cool enough to touch . drop an inch and cut it off .
Not scientific and maybe wrong , but it worked for me .

 

[B4H0]

I'm just gonna leave this here.

 

[1MoreFord]

I'm a believer in relatively tall stacks. More so than normal for many of the older smokers. Some of the newer builders have lengthened, and enlarged, their stacks even going to the point of fold-able stacks for transport.
My old offset had a vertical and another 2 ft or so of stack on top. Never had any problems with it. If fact, I think it could have used more diameter and/or length.
Here's some info on what is known as stack effect. For those of you who know math better than me there are some formulas to play with.

Stack effect - Wikipedia

en.wikipedia.org

 

[SmokinEdge]

Stack height doesn’t need to be longer than 3’ but the diameter of the stack is what moves the volume of air and that needs to be sized correctly for the smoker volume.

 

[Jim_C]

This is an interesting subject. I asked a similar question awhile back and did not receive a lot of feedback. Perhaps because few people are nerdy enough to get into air flow.

I also agree with the premise made in the video that a properly maintained fire will make up for deficiencies in the stack length or diameter.

I have made a number of mods to the Smoke Vault in order to run a smaller flame to gain temperature stability at lower temperatures and to eliminate flame-out.

My thought now is to run higher flames and provide more airflow through the smoke box. In theory the larger flames and increased airflow should provide a more stable temperature.

I believe a stack extension adds to the flow, but I am also wondering where stack diameter plays a role. The Smoke Vault I have has two small holes on top. I have been toying with the idea of cutting a 5" hole to accommodate a 6" diameter stack to see what role the diameter plays in air flow.

 

[SmokinEdge]

I believe a stack extension adds to the flow, but I am also wondering where stack diameter plays a role. The Smoke Vault I have has two small holes on top. I have been toying with the idea of cutting a 5" hole to accommodate a 6" diameter stack to see what role the diameter plays in air flow.

To give you an idea of the play diameter has. If you double the diameter of a pipe you triple the volume flow.

 

[Jim_C]

To give you an idea of the play diameter has. If you double the diameter of a pipe you triple the volume flow.

So what are the thoughts on better managing a consistent temperature inside the chamber......lower flame and lower airflow.......or.......higher flame and higher airflow?

 

[SmokinEdge]

So what are the thoughts on better managing a consistent temperature inside the chamber......lower flame and lower airflow.......or.......higher flame and higher airflow?

Here is how I run my pit. Every pitmaster has a slightly different approach but in the end it’s all about maintaining cook chamber temperature.

To do that I first build a coal bed, either by burning splits down or by using a pile of charcoal or briquettes. Once I have a sufficient coal bed it’s all about maintaining that coal bed and not so much a flame fire. The hot coals are the main heat source. From there it’s about managing air flow and adding a split every 30-45 minutes to keep the red hot coal bed alive. To big of a coal bed and to much cook temp, to small and to low a cook temp.

Your main temp control is the coal bed size and air flow. I never restrict my exhaust with a damper, I always let the exhaust run unrestricted. Control comes from air intake and again coal bed size.

 

[realdocBBQ]

I'm with SmokinEdge in most ways. I usually run an unrestricted exhaust.

It really comes down a lot to preference, how you want to run your fire, etc. I like a higher flow, convective cooking environment in the cook chamber. Using a damper on the exhaust does have some advantages, though, if you want to cause some restriction in the exhaust, leading to a little more turbulent flow in the cook chamber - which theoretically can lead to more smoke flavor on the meat, as the slower, rolling flow leads to more particulate deposition (i.e., smoke flavor). But a higher flow, convective cooking can be better for crisping up the bark. It's really all about your preferences.

<WARNING! Long-winded encyclopedia article ahead! >

As for the age-old wisdom of air cooling too much and leading to stagnation and condensation in the exhaust if your exhaust is too long - I've never experienced this. I prefer longer stacks, more than larger diameter. There is a tradeoff, of course, and a too-small stack can cause lots of issues. I generally control my airflow with the air inlet in the firebox - very large coal bed helps with this. If the air inlet is slightly constricted (I use pinwheel dampers on my builds, in addition to door opening standoffs), it leads to the coal bed 'sucking in' air, at a higher velocity and leading to a clean, high temp burn, and allows you to run a smaller coal bed - relatively. But, again, this goes with my high-flow style of cooking.

Let's look at this from a scientific standpoint. Air is a fluid. Not a liquid, a fluid. It's also compressible, which a liquid generally is not. Air and heat will flow in the direction of the current, and also from high pressure (and temp) to a lower pressure and temp, within reason, especially if the lower pressure and temp are at a higher elevation - i.e., heat rises. Once your smoker is warmed up (speaking mainly of offsets and cabinets), you really don't have to worry too much about 'backwards' flow - unless you're cooking in high winds in suboptimal conditions, with a restricted stack, etc., etc. It does get complicated.

If you're worried about the air in your exhaust stack 'cooling' too much and beginning to 'flow back' or be restricted, as some people have asserted for years, just put your hand over the top of your exhaust stack - if you can reach it, lol. On mine, because I favor a long exhaust, that's sometimes difficult. Anyways, if the air coming out of your exhaust feels hot, guess what? It's hotter than the air surrounding the smoker, and especially at the top of your stack. Let's say you're cooking in 90 degree temps. Not uncommon here in OK. Even 100, or 110 degree temps. If you're outside and walking around in that air temp, and you feel the temp coming out of your exhaust and it FEELS hot, guess what? It's hotter than the surrounding air! Even if you're running a relatively small fire and a low-temperature cook, say 200-225F, the air coming out of your exhaust is going to be 150-170 degrees at the least. Sooo... that's a temp difference from the top of your exhaust to the outside air of a minimum of 60 degrees, and often much much more than that.

So... do you think extending your stack by 8" or 12" or even 20" is going to significantly reduce the temp of your exhaust coming out of the stack? If you have thin-walled pipe for your exhaust, it doesn't hold much heat to keep that air warm as it moves up - but, it also heats up quicker than a 5/8" thick pipe does, too. If it's S00P3R cold out (below freezing) AND you have a thin-walled exhaust (say, 14g or 16g, you could lose some heat out of that pipe wall, yeah. But, the air is moving upward continually, and continually delivering some of it's residual BTU's to the wall of the pipe. Even in S00P3R cold weather, even with a thin-walled exhaust stack, you'll never put your hand on the exhaust and feel it being COOL. At least I doubt it, not in my experience.

Bottom line, if the exhaust temps exiting your stack are higher than the outside ambient temperature, your airflow is going to continue to want to move upward and outward, toward that lower pressure zone. Especially if it's unrestricted (by an exhaust damper, say).

Can your exhaust be TOO long???

Well, consider this picture:

Some folks may know that BBQ place - they are a little famous, they've been pumping out que successfully for a LONG time. See those exhaust stacks? Yeah, those things way up 20 feet in the air. They don't seem to have problems with the air getting too cool and their pits not flowing properly.

Now, if you have a very small exhaust and there is a lot of restriction to airflow, the movement of that fluid along the walls of the pipe increases friction and resistance. But, we're talking about a fluid, not a liquid. The resistance ISN'T the same as if water were flowing through that pipe, or if that gas were pressurized.

Bottom line, in my experience, you're much better off going with a longer exhaust than shorter. There are essentially no downsides, with the possible exception of it might take longer to get things flowing really well at the very beginning of a cook, when you're trying to get that stack heated up to assist with airflow. At that time, especially, a smaller diameter exhaust stack will restrict flow more, as the temp (again, BTUs) is being used to heat up the metal of the exhaust. Once it's hot, you're not losing a ton of that energy into the walls of the pipe, most of that flow is just moving on through, and the resistance in the exhaust is much much lower than if you were trying to push a non-compressible fluid (like water) through the length of it.

Bottom line, in my experience, I think a longer stack can help reduce the problems associated with a smaller-diameter pipe (within reason). Say, if you're like me and have trouble sourcing proper 6" thinner-walled pipe. I've built and run a 250-gallon pit with an exhaust pipe that was ~5.25" ID and it flowed marvelously - I believe in part because I made the stack longer than 'suggested' by the famous Feldon's calculator. This is because the calculator is using formulae to suggest minimum length for a given diameter - there is no real maximum, in my experience. Honestly, the longer the better - although, the caveat to this is if you use a very large diameter exhaust and make it really long, you are actually using such a huge volume of air (mass) that flow is somewhat sluggish. Say, a 10" exhaust that is 8 feet long on a 100 gallon pit.

Here is the pit in question:

But here's another thought... if you have an offset with a cabinet on the far end of it, the volume of the cabinet is actually functioning as a portion of the exhaust. You don't ever see those cabinets on the end of a traditional flow offset with a 6-foot 8" exhaust on TOP of the cabinet, do you? Nope. You see maybe 6" or even a foot, and that's about it. Because the cabinet HEIGHT is actually part of the exhaust HEIGHT (or length). And it still works.

Lots of variables in all this stuff, but the general rule I go by is to use a reasonably-sized diameter exhaust pipe, and make it as long as I want to, really as much for aesthetics as anything else. If it's over about 3 feet on a large smoker, it's going to be fine - longer IS actually better in helping generate more 'lift' - due to the heated walls of your exhaust helping generate more flow by allowing that escaping air to keep more of its BTUs, retain more of it's energy and motion (particles resonate more and bounce around faster when heated) and actually 'pull' out of the exhaust into the lower-temp, lower-pressure atmosphere surrounding your exhaust stack's exit.

Hope that helps with the visualization of some of these concepts.

<Edit> And that's without even getting into the concept of 'laminar' airflow keeping more of the escaping gasses away from the walls of the pipe, thus also conserving more of your BTUs and thus keeping your exhaust gasses even hotter. Win/Win.

<whew> Sorry for the dissertation! lol

 

[Jim_C]

I'm with SmokinEdge in most ways. I usually run an unrestricted exhaust.

It really comes down a lot to preference, how you want to run your fire, etc. I like a higher flow, convective cooking environment in the cook chamber. Using a damper on the exhaust does have some advantages, though, if you want to cause some restriction in the exhaust, leading to a little more turbulent flow in the cook chamber - which theoretically can lead to more smoke flavor on the meat, as the slower, rolling flow leads to more particulate deposition (i.e., smoke flavor). But a higher flow, convective cooking can be better for crisping up the bark. It's really all about your preferences.

<WARNING! Long-winded encyclopedia article ahead! >

As for the age-old wisdom of air cooling too much and leading to stagnation and condensation in the exhaust if your exhaust is too long - I've never experienced this. I prefer longer stacks, more than larger diameter. There is a tradeoff, of course, and a too-small stack can cause lots of issues. I generally control my airflow with the air inlet in the firebox - very large coal bed helps with this. If the air inlet is slightly constricted (I use pinwheel dampers on my builds, in addition to door opening standoffs), it leads to the coal bed 'sucking in' air, at a higher velocity and leading to a clean, high temp burn, and allows you to run a smaller coal bed - relatively. But, again, this goes with my high-flow style of cooking.

Let's look at this from a scientific standpoint. Air is a fluid. Not a liquid, a fluid. It's also compressible, which a liquid generally is not. Air and heat will flow in the direction of the current, and also from high pressure (and temp) to a lower pressure and temp, within reason, especially if the lower pressure and temp are at a higher elevation - i.e., heat rises. Once your smoker is warmed up (speaking mainly of offsets and cabinets), you really don't have to worry too much about 'backwards' flow - unless you're cooking in high winds in suboptimal conditions, with a restricted stack, etc., etc. It does get complicated.

If you're worried about the air in your exhaust stack 'cooling' too much and beginning to 'flow back' or be restricted, as some people have asserted for years, just put your hand over the top of your exhaust stack - if you can reach it, lol. On mine, because I favor a long exhaust, that's sometimes difficult. Anyways, if the air coming out of your exhaust feels hot, guess what? It's hotter than the air surrounding the smoker, and especially at the top of your stack. Let's say you're cooking in 90 degree temps. Not uncommon here in OK. Even 100, or 110 degree temps. If you're outside and walking around in that air temp, and you feel the temp coming out of your exhaust and it FEELS hot, guess what? It's hotter than the surrounding air! Even if you're running a relatively small fire and a low-temperature cook, say 200-225F, the air coming out of your exhaust is going to be 150-170 degrees at the least. Sooo... that's a temp difference from the top of your exhaust to the outside air of a minimum of 60 degrees, and often much much more than that.

So... do you think extending your stack by 8" or 12" or even 20" is going to significantly reduce the temp of your exhaust coming out of the stack? If you have thin-walled pipe for your exhaust, it doesn't hold much heat to keep that air warm as it moves up - but, it also heats up quicker than a 5/8" thick pipe does, too. If it's S00P3R cold out (below freezing) AND you have a thin-walled exhaust (say, 14g or 16g, you could lose some heat out of that pipe wall, yeah. But, the air is moving upward continually, and continually delivering some of it's residual BTU's to the wall of the pipe. Even in S00P3R cold weather, even with a thin-walled exhaust stack, you'll never put your hand on the exhaust and feel it being COOL. At least I doubt it, not in my experience.

Bottom line, if the exhaust temps exiting your stack are higher than the outside ambient temperature, your airflow is going to continue to want to move upward and outward, toward that lower pressure zone. Especially if it's unrestricted (by an exhaust damper, say).

Can your exhaust be TOO long???

Well, consider this picture:

View attachment 721017

Some folks may know that BBQ place - they are a little famous, they've been pumping out que successfully for a LONG time. See those exhaust stacks? Yeah, those things way up 20 feet in the air. They don't seem to have problems with the air getting too cool and their pits not flowing properly.

Now, if you have a very small exhaust and there is a lot of restriction to airflow, the movement of that fluid along the walls of the pipe increases friction and resistance. But, we're talking about a fluid, not a liquid. The resistance ISN'T the same as if water were flowing through that pipe, or if that gas were pressurized.

Bottom line, in my experience, you're much better off going with a longer exhaust than shorter. There are essentially no downsides, with the possible exception of it might take longer to get things flowing really well at the very beginning of a cook, when you're trying to get that stack heated up to assist with airflow. At that time, especially, a smaller diameter exhaust stack will restrict flow more, as the temp (again, BTUs) is being used to heat up the metal of the exhaust. Once it's hot, you're not losing a ton of that energy into the walls of the pipe, most of that flow is just moving on through, and the resistance in the exhaust is much much lower than if you were trying to push a non-compressible fluid (like water) through the length of it.

Bottom line, in my experience, I think a longer stack can help reduce the problems associated with a smaller-diameter pipe (within reason). Say, if you're like me and have trouble sourcing proper 6" thinner-walled pipe. I've built and run a 250-gallon pit with an exhaust pipe that was ~5.25" ID and it flowed marvelously - I believe in part because I made the stack longer than 'suggested' by the famous Feldon's calculator. This is because the calculator is using formulae to suggest minimum length for a given diameter - there is no real maximum, in my experience. Honestly, the longer the better - although, the caveat to this is if you use a very large diameter exhaust and make it really long, you are actually using such a huge volume of air (mass) that flow is somewhat sluggish. Say, a 10" exhaust that is 8 feet long on a 100 gallon pit.

Here is the pit in question:

View attachment 721020
But here's another thought... if you have an offset with a cabinet on the far end of it, the volume of the cabinet is actually functioning as a portion of the exhaust. You don't ever see those cabinets on the end of a traditional flow offset with a 6-foot 8" exhaust on TOP of the cabinet, do you? Nope. You see maybe 6" or even a foot, and that's about it. Because the cabinet HEIGHT is actually part of the exhaust HEIGHT (or length). And it still works.

Lots of variables in all this stuff, but the general rule I go by is to use a reasonably-sized diameter exhaust pipe, and make it as long as I want to, really as much for aesthetics as anything else. If it's over about 3 feet on a large smoker, it's going to be fine - longer IS actually better in helping generate more 'lift' - due to the heated walls of your exhaust helping generate more flow by allowing that escaping air to keep more of its BTUs, retain more of it's energy and motion (particles resonate more and bounce around faster when heated) and actually 'pull' out of the exhaust into the lower-temp, lower-pressure atmosphere surrounding your exhaust stack's exit.

Hope that helps with the visualization of some of these concepts.

<Edit> And that's without even getting into the concept of 'laminar' airflow keeping more of the escaping gasses away from the walls of the pipe, thus also conserving more of your BTUs and thus keeping your exhaust gasses even hotter. Win/Win.

<whew> Sorry for the dissertation! lol

SmokinEdge and realdocBBQ, this is a lot of great information.

I fully agree with not limiting the exhaust and controlling temperature from the fuel source. I like the idea of more, rather than less, airflow. I also like the idea of running a 3' stack.

One of the comments SmokinEdge made earlier; "double the diameter of a pipe you triple the volume flow" makes sense. The upright smoker I have has those two small holes on top with the adjustable plate. Maybe the equivalent of a 2" diameter pipe. I'm thinking of a 5 1/2" hole with a 6" diameter stack.

I have a few followup questions........

I will need to use sheet metal basically. I have read that the standard HVAC duct "will kill you" because of the chemicals released from the plated zinc at high exhaust temperatures.

1. I am uncertain HVAC duct will cause chemical issues with the chamber if there is a positive airflow? Albeit there may be some drip back into the box. Thoughts?

2. Steel pipe is out of the question because the upright smoker I have is a stamped steel box that will not support the weight. What about the double-walled chimney duct? Same issue as HVAC duct?

3. If the above two ideas are out of the question, I'm assuming 24"x36" stainless steel sheet metal is the answer. The stuffs not cheap..........

 

[Up_In_Smoke]

snip...

I have a few followup questions........

I will need to use sheet metal basically. I have read that the standard HVAC duct "will kill you" because of the chemicals released from the plated zinc at high exhaust temperatures.
... snip

Zinc doesn't vaporize until about 1665 degrees F.

If you're concerned, build your stack with standard HVAC pipe then fire up your smoker and get it hotter than hell (empty of course).
Do it twice if you're really concerned.

 

[realdocBBQ]

Got one of these?

Can put this on top of your cut-out hole to 'simulate' a chimney/exhaust. See how it flows.

I've got one of those Smoke Vault 24s as well, I know where you're coming from. It is definitely a 'low-flow' cooker.

 

[Up_In_Smoke]

Got one of these?

View attachment 721024


Can put this on top of your cut-out hole to 'simulate' a chimney/exhaust. See how it flows.

I've got one of those Smoke Vault 24s as well, I know where you're coming from. It is definitely a 'low-flow' cooker.

Wonder if he could put his HVAC pipe over the chimney and do the burn-in that way?
It would be a lot hotter.

 

[Jim_C]

Got one of these?

View attachment 721024


Can put this on top of your cut-out hole to 'simulate' a chimney/exhaust. See how it flows.

I've got one of those Smoke Vault 24s as well, I know where you're coming from. It is definitely a 'low-flow' cooker.

realdocBBQ - I have been using a 24" piece of 6" HVAC duct. It just sits on top of the chamber. I have removed the adjustable plate on top of the smoker, but have not cut a larger hole. I noticed an improve airflow just setting the pipe on top. Next step is to make a more permanent installation.

Up_In_Smoke - Thanks for the information on the temperature zinc vaporizes at. It eases the concern. I'm on a wood deck so I'll need to pass on using the briquet chimney to do a burn-in. Being on a wood deck is why I run a gas burner for the grill and smoker.

 

[realdocBBQ]

Yup, so you've already applied the principle I discussed above - putting a duct on there, allowing it to heat up and seeing how that actually provides draw. Excellent. Opening that hole can only improve things!

<edit> BTW, I didn't mean a LIT chimney of briquettes! I just meant slap that thing on top of the hole and allow it heat up to function and help create draft. I do this sometimes on my drum smoker.

 

[Up_In_Smoke]

snip...

<edit> BTW, I didn't mean a LIT chimney of briquettes! I just meant slap that thing on top of the hole and allow it heat up to function and help create draft. I do this sometimes on my drum smoker.

I meant a lit chimney.
LOL :)

 

[SmokinEdge]

SmokinEdge and realdocBBQ, this is a lot of great information.

I fully agree with not limiting the exhaust and controlling temperature from the fuel source. I like the idea of more, rather than less, airflow. I also like the idea of running a 3' stack.

One of the comments SmokinEdge made earlier; "double the diameter of a pipe you triple the volume flow" makes sense. The upright smoker I have has those two small holes on top with the adjustable plate. Maybe the equivalent of a 2" diameter pipe. I'm thinking of a 5 1/2" hole with a 6" diameter stack.

I have a few followup questions........

I will need to use sheet metal basically. I have read that the standard HVAC duct "will kill you" because of the chemicals released from the plated zinc at high exhaust temperatures.

1. I am uncertain HVAC duct will cause chemical issues with the chamber if there is a positive airflow? Albeit there may be some drip back into the box. Thoughts?

2. Steel pipe is out of the question because the upright smoker I have is a stamped steel box that will not support the weight. What about the double-walled chimney duct? Same issue as HVAC duct?

3. If the above two ideas are out of the question, I'm assuming 24"x36" stainless steel sheet metal is the answer. The stuffs not cheap..........

The chimney should be 3’ above the top of the cook chamber and not from grate height. Also depending on the area volume of your smoker is what determines stack diameter. If the chimney is to large in diameter you will be dumping to much heat from the cook chamber, if to small diameter it won’t move enough air. The diameter, again, of the chimney is directly related to the cook chamber size in volume. Also think about the fact that you can not exhaust more volume than you can intake.

So intake, and exhaust should be balanced and based on CC volume. More isn’t always better.

 

[1MoreFord]

My old offset had a piece of EMT conduit for an exhaust stack. It's galvanized, just like the HVAC stuff you're talking about. I seasoned it real good and never had any troubles.
Stacks just don't get hot enough to be dangerous unless you have a pit fire and then you have other things to worry about.

 

[Redicans]

I saw a video a while back from Aaron Franklin. He said stack length should be right about the length of the smoke chamber. So I extended my smokers stack to 40" as my chamber is 40" I used a piece of the HVAC ducting. It works perfect for my setup. As far as the zinc coating I am not worried about it as it never gets that hot. I can remove the extension with my bare hands in the middle of the smoke as long as I do it quickly. But I never do. Also after a few smokes the inside of the extension is seasoned nicely and you can't even see the zinc coating.