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My concentric will flood too under the right circumstances. Double reducer style with a 1" throat. The external reflux line sounds like a great idea.

I believe I run my boka at just flooded. I may be wrong, but I hear a gurgling type boiling noise that disappears if I drop the heat or take off to fast without increasing heat.

Flooding sounds like you describe. Do I remember right, that it was you, mentioning in another thread, that you run 2" with 3.5kW?

Yup. That was me.

I do a single pass for a tasty light rum at 92% with my 2" Boka with a 5 gallon molasses wash. I prefer it to what I get out of my pot still. If I'm going for neutral I always do a stripping run.