Insulating a raised flue wood arch

[PerryFamily]

Working on getting my new 3x8 raised flue arch set up.
It was all re tinned and bricked 2 seasons ago. However, it was poorly bricked in the flue pan section. The ramp went nearly 3' into the flue pan section. Then had about 1' that was just open, then the next ramp and a huge area for the exhaust to enter the stack.Resulting in very a lot of lost heat.
Question: what is the optimum area in the front and back for the heat to enter the raised flues and escape?
My back stack is 11" so that is the number I was going to use and vermiculite the entire center section?
All thoughts and ideas are appreciated.

[maple flats]

On my 3x8 I raised the back wall to 5" then ramped up to the flues in about 12" length. Under the flues I only have 1/2" clearance, forcing the heat up into the flues. At the back I put an upright row of firebrick tucked under a lip on the rear cross block, then the section drops down to the floor of the arch. There I have a layer of flat bricks laid there. In the area under the raised flues I filled it with vermiculite (bought at a commercial greenhouse supply house) and that is capped with a layer of half bricks. On top I troweled about a 1/4" of dry refractory to smooth it out and fill in any voids. Then I used a fine mist spray bottle to barely moisten it. I repeated that about 3-4 times over the next several days. This seems to have worked well, I have not needed to fix anything since I installed it 9-10 years ago.
I bought the vermiculite at Milikowski, but they have since been bought out by Griffin. Vermiculite will be far cheaper there than buying a whole pile of small bags from Home Depot or Lowes. If memory serves me I think it was going to cost almost $500 at Lowes, and I bought 3 bags at 6CF each for just over $60 at Milikowski's.

[PerryFamily]

If I am understanding you correctly, your ramp exposes approx 12" of flues on the way inlet side and only 5" on the outlet side going to the stack?

[maple flats]

That is how mine is and it works great. Before I added the high pressure air, the ramp followed the slope of the tin on the arch, which exposed a lot more. I customized it to force more heat up into the flues faster. I now get a much harder boil. The roughly 5" in the back is full width. To come up with that I calculated the area of the 5", which is 5x36=180 sq. in.. The area in my 12" stack is pi R squared, or 3.14 x 6 x 6=113 sq in., the 180 gave me plenty area , with a margin of about 1 to 1.6. You likely could get away with 4" but I didn't try it.
Your stack is 11" so you would have even better draft or (less restriction).
With my original set-up with a much longer ramp my best boil didn't start until the ramp go close to the flues. Now I get a hard boil from the beginning to the end of the flues.

[n8hutch]

I don't know the correct Answer but I have a 30" by 10' Arch and I was going to ramp 10" into the flue pan & start my ramp out at 10" from the back of the flue pan. I have a 30" x8" base stack so that gives me 240 sq inches of at the exit point of the Arch that goes into a 12" stack which is roughly 114 square inches, I figure you need that extra area when you are trying to change the direction of air flow . So with 10" exposed at either end that will give me 300sq inches of area . but its actually about half of that 150sq because of the flues, this is still more than the volume of my smoke stack so I am thinking it should work well, on A side Note I was wondering if I could check out your operation some time, my mother inlaw lives in Charlestown and I am down there quite often.

[PerryFamily]

I followed the original ramp until I got to the 11" mark and then went straight up. The rear is an easy fix. I can just move the wall to get around a 5" gap.

Anyone is welcome to visit. Just message me first.