But i seem to remember that you go for the bell's ISA, in the case of a flue. So for a 6 around 50sqft. or 31ft of straight pipe. Peter was saying that you shouldn't account for the lower 1/4 of the pipe. So you could get away with a smidge more. And the old consensus for bells (well, not that old) was that you could have a good 6m² so that 64sqft. Bit more tolerance for a longer flue.
If you want a bench. You can also use half barrel bells.
From what I've read by Peter van den Berg, the output of a 6" batch-box is very close to that of an 8" J-tube.
My own 6" batch-box is driving 40 feet *equivalent* length of 8" horizontal flue, encased in "claycrete" style cob and faced with solid brick, but otherwise classic bench style RMH layout. *Flue run consists of 20 feet of steel pipe sections plus 4x 90-degree horizontally oriented elbows for an equivalent drag of 40 feet. Vertical chimney is 15 ft. of class-A insulated pipe.
I used 8" flue because the 6" batch doesn't play well with back pressure (exhaust restriction). The half barrel "bell" bench Satamax linked to is another way to effectively and inexpensively construct low back pressure thermal mass benches.
best regards, Byron
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