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How well it holds its heat
Without fire brick there is a risk of spalling due to heat. Refractory mortar will help it hold together better. I used heat stop50. What he has is ac cool design but it probably won't last or will have issues.'
In theory it's a good design, but materials are where there is s difference. The bricks used are not designed for that kind of heating and cooling cycles. Then there is spalling, air pockets that can explode due to heat. Ask any masonary expert and they will tell you the same thing. If you are going to build one spend a little extra so it will last longer and be safer.