importance of chimney length (height) ?

Discussion in 'Smoker Builds' started by spen, Jun 17, 2010.

  1. spen

    spen Newbie

    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:

    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.  

  2. mballi3011

    mballi3011 Smoking Guru OTBS Member SMF Premier Member


    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.
  3. spen

    spen Newbie

    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.

    - 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.  

  4. caveman

    caveman Master of the Pit SMF Premier Member

    Hi there Shorty.  Maybe this link will help.

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

    bbally Master of the Pit OTBS Member

    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.
  6. spen

    spen Newbie

    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? 


  7. matts

    matts Smoking Fanatic

    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.
  8. fourthwind

    fourthwind Master of the Pit OTBS Member SMF Premier Member

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

    bbally Master of the Pit OTBS Member

    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.
  11. brownpeter335

    brownpeter335 Newbie

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

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