That looks like a good start. My notes on an 8" J tube says 10 to 12 # of wood with a 45min burn time, with a 30min feed interval, would put you right @ 100K btu. I sounds like that is very close to what you have just short of the 125Kbtu you need for a good boil on the full 8sqft. Lookin at your pictures I like the layout, out and back on top of an insulated chanel. Are you sure your cross section at the top of the riser is large enough to make a unristricted 90° turn? Also did you try running it without those metal baffles? It sounds like you only need a little bit more burn rate to get the heat you need.
Tuning experiments:
1: Lift the cover over the riser about an inch
1.5: And/or notch the top of the riser to add flow left and right
2: Increase the opening down into the lower return. 90° turn @ least 10"x8" opening.
3: Add 4' to the top of the chimney, more pull.
4: Bevel the intrance to the chimney, that wall penetration should be at least 9" in diameter with 45° bevel. Easy way to do this is a 10" x 8" chimney adapter. Or just shape the insulation into a funnel.
5: Raise the chimney penetration off the floor at least 2" (1/4 diameter) will increase flow.
Note: airflow into a hole in a plate comes in from the side of the plate not straight into the pipe.
Fyi: A round hole in a plate only flows about 62% of what a tube of the same diameter flows. That's why i think you need more cross section at those transitions. Flares in and out of pipes help flow alot. Tom
