Will Cavity Under Stack Affect Draft?

[Manatawny]

I'm re-designing my block arch, this year, and the concept leaves a 16" deep cavity (approx. 10"x10") directly beneath the juncture of my firebox and my 8" stack.

In other words, this cavity is directly beneath the stack, but my fire will meet this cavity 16" up from its floor.

Will this mess with my draft? Specifically, I'm wondering whether the draft moving up will make that cavity a low pressure zone, and I'm wondering if that low pressure will in turn create a small amount of suction which slows the upward movement of the draft?

I suppose that I can just fill the hole with dirt, come to think of it...

Thoughts?

[Haynes Forest Products]

The gasses will swirl around in that area but if you have draft and the cavity is past the pan then it should not effect the heat under the pan. Now on a windy day it might yank and snap it around. I say fill with sand. you dont want dirt flying out onto the pans.

[maple flats]

No problem. In fact my evaporator is a raised flue and the space under the flues is less than 1/2", then at the back, by design, directly under the base stack there is a space of about 10" x 33" inside the firebricks. The exhaust will go up the stack, notice I say exhaust and not smoke, because the fire burns so hot there is no smoke (at least you can't see it).

[Stu007]

I found that having a space behind the pan and under the stack reduced the amount of sparks exiting the top of the stack. I am assuming the low velocity zone allowed more time for sparks to combust as they swirled around the open space.