# importance of chimney length (height) ?



## spen

Question --

How important is the length of  the chimney ?  

How long (tall) should it be?  

I was looking at the cow boy hot tub heater things:

http://www.cowboyhottubs.com/

They are a small box and have a really tall chimney, and the draft effect of the heat rising seems to suck the air through the box.   Wondering how the chimney length effects a smoker.  

I am about to start building a UDS like smoker, instead of mounting the top vent / chimney in the lid, I want to mount a chimney in the side near the top.  

Shorty


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## mballi3011

Well there shorty you bring up and interesting question there. I don't know what the height should be but on a UDS I would think that it just has to be on the top of the smoker. Now I have seen abunch here and they really don't have to draw the smoke up like a house chimmey. So if you want to make it feet tall then I guess you could make it as tall as you want it to be.


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## spen

So the thing is:

- the little kettle grill I am using has no chimney at all, it just has a slotted vent in the lid.   

http://www.smokingmeatforums.com/fo...modified-my-small-kingsford-grill#post_443041

- I have seen the tall chimneys which I would think would create a faster draw through the smoker, so it might consume more charcoal and wood because it is moving the air faster

- you guys have the reverse flow setups where the exhaust pipe starts at the grill height, and that would seem to do the opposite, it would slow the air rate down because the air flow would have to push it's way out.   

I am trying to look through previous threads to find out about the air flow dymanics, but I can't figure the right keywords to use.   Read something about stale smoke, don't understand that concept, and haven't found a air flow comparison for dummies guide.  

Shorty


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## caveman

spen said:


> So the thing is:
> 
> - the little kettle grill I am using has no chimney at all, it just has a slotted vent in the lid.
> 
> http://www.smokingmeatforums.com/fo...modified-my-small-kingsford-grill#post_443041
> 
> - I have seen the tall chimneys which I would think would create a faster draw through the smoker, so it might consume more charcoal and wood because it is moving the air faster
> 
> - you guys have the reverse flow setups where the exhaust pipe starts at the grill height, and that would seem to do the opposite, it would slow the air rate down because the air flow would have to push it's way out.
> 
> I am trying to look through previous threads to find out about the air flow dymanics, but I can't figure the right keywords to use.   Read something about stale smoke, don't understand that concept, and haven't found a air flow comparison for dummies guide.
> 
> Shorty


Hi there Shorty.  Maybe this link will help.  http://www.wedlinydomowe.com/smokehouse-draft.htm

See if that answers some of your questions.  Good luck.


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## bbally

I think this is what you are looking for, the engineering behind a RF is complicated, no calculator that I have seen really depicts a true reverse flow running correctly.

Though the spreadsheet does make a unit that cooks.  And most units built will cook food.  The true reverse flow with a complete zero dead volume exchange is a very complicated beast.

http://www.smokingmeatforums.com/forum/thread/94214/exhaust-pipe-damper#post_497931


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## spen

Did a bunch of reading last night, lots of confusing and contradictory stuff, but I think I understand a bit more now.   Since there are so many different types of air flow configurations, for this smoker I am building right now, I am going to make the intake and  exhaust from multiple pieces, each with dampers, so I can experiment with stack heights and fiddle with the flow rates etc.   

So....  the point I am at now, what are the signs to see if the fire is burning properly, and at the right air flow rate ?  

I saw that thin blue is good, thick white is bad -- is that it?  Are there other indicators to watch for? 

Thanks

Shorty


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## matts

I am sure there are differences between a short or tall stack on a smoker, but I put a tall stack on my smoke vault just to keep the smoke out of my face while I was around it.


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## fourthwind

The length of the stack will alter the back pressure in the smoke chamber..  Too long or too narrow of a stack could cause an airflow problem.  Most people go by the excell sheet that tells you what size stack you need depending on the size of firebox, inlets, and smoke chamber sizes.  I doubt the excell figures would work for a UDS, but my guess is since 99% of the UDS I have seen don't have a stack that it is not needed.


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## alelover

When you calculate the pipe length do you include the length of pipe going down to the grill surface inside the smoker too?


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## bbally

No the pipe length is calculated by the pipe length exposed to ambient temperatures only.  Inside the smoker is a depth of smoke control, but not part of the length that sets the natural induced draft.


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## tonyshotlickbbq

Try this calculator:

http://www.feldoncentral.com/bbqcalculator.html


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## brownpeter335

Nice topic! I got some knowledge about importance of chimney length here. Thanks for this.


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## davecs1

Has anyone put the dimensions in for an OK Joe Highland? I just did and it tells me my firebox is 150% to big and my chimney is about 60 inches short. The intake is way to small, but just about right if you use the whole door ,which I have been doing. The inlet to the smoke chamber is to big, but my lavalock baffle corrects that, it seems. 

This has confirmed my thoughts the chimney is inadequate. If I put a 5 inch stack on I can get away with a 36 inch chimney, so I guess I will give that a try.


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## pete mazz

On any vertical smoker the CC acts as the chimney so all that's needed is an exhaust .


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## davecs1

The Highland smoker is an offset Horizontal smoker.


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## pete mazz

Sorry. All the talk about UDS's got me.


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## davecs1

No problem, the thread is almost 3 years old. I was doing research on chimney dimensions and it came up In a search. I thought I would post as I am a bit puzzled by my understanding of what I believe I measure my OK Joe dimensions to be and what the smoker build calculators say about those dimensions. My guess is they built the smoker to look like a typical smoker, but possibly with no regard to performance.


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