The 20" is fine and will work. Often times too tall an exhaust results in your draft stalling because the heat cools and becomes heavy. I would come up one more inch on the fire grate.
My first attempt at a build. - Page 5
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kam59 mentioned he was one of the folks that developed the "Feldon Calculator".... That being noted, I have copied and pasted a note from Feldon's calc...... specifically about the exhaust... I follow Feldon's recommendation when choosing exhaust length.... BUT... build the exhaust any length that suits your desires.....
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.
Morgan put that statement in his calculator hoping folks would use some discretion in choosing too tall an exhaust or too short an exhaust. Morgan designed the online calculator for folks to use with Aliens permission he does not build cookers. When I build a cooker or run numbers for folks I choose an exhaust diameter I know will work as far as balance to the cooker. Some common sense is needed when choosing an exhaust diameter because any calculator you use can only figure the volume needed based on the numbers you plug into it.
A 24" x 60" cooking chamber with 22" square firebox using a 6" exhaust would be 19" tall I would bump it to 24" and the cooker would work well.
Take the same cooker and put a 4" exhaust on it and you would be sitting at 42" tall
A 3" would be at 75" tall.
Neither of these are viable options in my opinion as they are too tall to create a decent draft and burn clean.
I hope this helps. :)
I'm using a 4" inside diameter stack. My plenum hole on the inside of the cook chamber is 5 1/2" X 4 3/4". Im using 1" angle for my cooking grate track. Here's a pic of how the 1" angle will cover a portion of the hole. The tape measure is where the track will be. . Do you think the track will mess up my draft? I made the hole bigger than 4" thinking that would compensate for the rail being in the way.
yes....YES....definitely do this...(don't ask me how I know this ;) )
By the way, that poster board is actual size. Like a dummy though, I did it upside down. The rounded side is actually the bottom.