I had such a hard time getting people to understand the value of the upper vent, I'm happy to see more and more builders applying it and realizing the added control it gives. Ive been playing with duo intake vents for a while on large pits, where I put a pie vent on the bottom of the fire box and a slide style on the door a few inches above the fire grate. Actually on those, I will actually switch completely over to the upper vents towards the end of the cooking cycle, so I recommend making both vents fairly large.
I'm still fine tuning this on the smaller builds, mainly due to size limitations of the firebox, and the necessity of a patio size cooker needs to be cleaner to operate and clean the ashes out than a pit.and you can not have an ash pan and a bottom pie vent at the same time. To tell the truth, I count on my road trip with the pit to blow out the remaining ashes to keep from having to sweep it out when I get home. But one time on one of my vertical cookers, I didn't get the fire out all the way, and didn't latch the firebox door good, and the fire started roaring going down the road. This was a full concession style trailer with screen enclosure, sinks, etc. and the fire basket actually slide out of the firebox and landed in the middle of the floor with roaring fire going. I learned real quick the value of having a fire extinguisher on board.
Anyway, on my small pits Ive decided on a two hole pie vent that the radius is set at fire grate height, with the upper hole 40% and lower hole 60% of recommended intake volume. That is what seems to work the best on the round firebox. My three hole design (#22) did not get enough air below the grate with out raising the grate too high.
