Recommend length for thermometer stem

4" is good...  then when you cook with a therm next to the meat on the rack, note the difference between the two ...   future adjustment can be made noting those differences.....   Never assume the installed therm reads the same as the meat zone...

Alien, the author of the "original" calculator I used 90%+ of my numbers from.....   Through a series of actual tests on real home made smokers, suggests a stack height of....... 

Taken from Feldon Central.....  Chimney Size
Enter the diameter of your chimney pipe to find out how long it should be.
  • A chimney that is too short may produce insufficient draft (drawing of air). A chimney that is too long may cause the air to cool before it exits, reducing effective draft and worse, dripping of exhaust materials onto food!
  • Many horizontal smokers have an exhaust between 30-40 inches in length, but there is no hard and fast formula.
  • If you are building a horizontal smoker with a vertical cooking cabinet, realize that the cabinet partly acts as an exhaust, thus you may greatly shorten the chimney.

From the clip above, that I do agree with, the stack needs to be a certain height for adequate draft...  Draft is created by pressure differential and thermodynamics...   If Alien says 30-40 inches is necessary, from all his studies, I go with it....  It has worked for his followers and it has worked for the folks on here that use the "tweaked" Feldon's calculator I put together...    

I could tell you that a 22" stack that has the correct volume will work...  but I don't think that's the case, everything else considered...  I would hate to have you build a smoker and after 300 hours of labor and possibly $1000 investment have it not work correctly....  and then you have to rebuild your smoker..... changing exhaust diameter size and length...  more time and more money....

Anyway, I would stick with approx. 36" height and a larger diameter is acceptable...   ease of air flow through the smoker is the target.... 

All that being said, probably the most important aspect of the smoker is being air tight...  really air tight so the dynamics of the system can work properly without being affected from adverse construction errors....

I've seen folks build smokers that doors don't fit quite right, and they fight and fight to get temps up or even across the cooking surface and burn way too much wood etc. because they felt what they built was "good enough"....   well that attitude doesn't work when building a smoker like these... 

I could go on but that's enough BS for this morning....