The flue can be a little bit smaller. In general the optimal speed of gases in the chimney is 1.5 m/s, so smaller flue will increase the speed of movement. Because the movement of fumes is already blocked by ceramic pieces being fired it would make sense to increase the cutout in the bottom if larger core is being used. In my opinion the bottom opening
should be centered, and the kiln posts put around the opening.
What are the dimensions of your kiln?
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