Peter and Thomas -
Thank you both so much. Now to elaborate on the questions.
The riser is really close to 6" octagonal, although a bit skewed in a couple places.
This is intended to be a trial, knowing I need to be more precise in the final construction.
And the riser looks really large because I put a second layer of IFB all around.
I ran the height to excess, only because I got no roar. Very easy fix.
I'm not sure if it's an "across the pond" consideration, but I have read a lot about building the core dry-laid, allowing for easy repairs.
I'd love to hear discussions on the clay slip vs dry-laid opinions.
The standards I see for the 6" system indicate roughly 7 SI Total Air Intake with 1.4 SI of that in the secondary.
On Matt Walker's design, he noted a better burn with secondary being just over half of Total Air Intake.
Could that be due to his "riserless" build?
And I do plan on a door, I just haven't built it yet, so I attempted to simulate it with the IFB.
This is such a great place for education and information exchange. Thanks to the contributors and administrators for all their efforts!