This has been done a few times with success, but is definitely risky.
First, due to mathematical/physics factors, if you divide the flow into smaller channels, you need to make the sum of areas much larger than the single channel. There are friction effects with the channel walls which reduce the effective size of the flow area. To simplify, you could consider that the outer inch of area is useless for flow, so if you divide the flow into smaller tubes, the outer inch of each of them is useless, and that is a much larger fraction of the total area than with the larger tube. If you go down to say 3" tubes, that leaves only one inch diameter for flow. Not strictly accurate, but the idea is correct. So you might use four tubes each 6" instead of 8", and all with exactly the same length and number of bends before recombining. It has been said that in cases of multiple pathways, one will always predominate, not necessarily predictably, though if evenly arranged they would likely all take a reasonable quantity of the flow.
A much easier, simpler, and probably more reliable method of heating a large bench area would be an open bell or stratification chamber, essentially a hollow masonry box with the entrance at one end and the exit from the floor of the other end. Hot gases would rise to the top, spread around, fall as they cool, and exit after giving up most of their heat to the mass. You can place brick columns evenly spaced inside to hold up the top surface and still allow even circulation. I have gotten 18" x 24" and 24" x 24"
concrete pavers for this purpose; anything much larger than common bricks would probably work fine.
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