Hi Aki, I think you have the main reason why people use a J tube instead of an L feed. Unless there are space restrictions, cultural preferences, or just want a certain look, I can't think of any other reason why one is better than the other.
Actually, now that I think of when I build an L tube long ago, I do remember liking that it was a bit easier to clean out the ash from the burn tunnel.
That's quite a 'meaty' riser you built for yourself. It will be interesting to see how long the rock wool will last. There is a video by Bigelow Farms (web4deb) where he used rock wool as a heat riser insulation and there were portions of it that were melted.
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