In my opinion good diffusers between the stone and the flame are a much bigger factor in an even bake then the side walls. I think the sidewalls function more to eliminate hot spots on the sides of the oven, but am open to conversation of that.
Jeff, if you take a cordierite stone with an equally sized deflector and lit a smoke bomb below it, the smoke would travel up and around the stone, and, because of friction, the smoke would curl in a bit and contact the outer top edge of the stone. Hot air will follow a similar path. The heat from the edge, via conduction, to a small extent, will drive up the temp on the rest of the stone (as well as create undesirable hot spots on the edges). The sidewalls prevent this.
I'm not necessarily saying that sidewalls are mandatory for making bottom heat scenarios work, but they are part of the reason why gas deck ovens generally have good heat balance.
I am also trying a different stone that I had cut down to 15" this week to see if there is a difference.
Mark, are you using a different stone by itself, or are you swapping out the bottom stone in the 2 stone? If it's swapping, the newer stone will most likely be more conductive, and thus exacerbate your top bottom heat issue at higher temps. There are no known commercial stones with less conductivity than the Fibrament stones that ship in the 2 stone grill insert. If you want to get into firebricks, there are certain brands of firebricks with lower conductivity that will handicap your bottom bake and allow you to push the grill to higher temps. At the same time though, the thickness of the firebricks will extend your preheats substantially.
Yeah, these little stoves put out a ton of heat. I've made my best blackened salmon on one of these. I would not underestimate the oven until you try it. I must get one of these. Thanks for sharing this. 700 degrees rated.
Rich, as I told Mark, I don't doubt that this is a great little stove with loads of power. I am highly suspicious of the oven, though. Many forum members have attempted LBEs with far more powerful burners and have run into heat balance issues. Spending this kind of money and getting a product with a heat balance that can't outperform a home oven, as Jeff pointed out, would be pretty horrible, in my opinion.
This being said, this is a very competitive price point for another 2 stone clone. If you want to risk your money on what I think is a pretty big gamble, if you were fortunate enough to end up with something that will give you a viable balanced New York style bake (Neapolitan is out of the question with a ceiling of that height, that material and these temperatures), it would be nice to have a less expensive 2 Stone competitor.