Doesn't that double chimney, with the air gap slow down the heat transfer to the copper tube?
Seems like you would get hotter water by installing a coil between the two chimneys.
If I was going to make one of these cookers to heat water,
I would be tempted to try putting a coil inside the main chimney wall,
right in the flame.
Maybe just on the top half, so the bottom section could heat up properly,
to create the draft needed for a hot fire.
I wonder how your chimney would work, if you filled the air-gap between
the chimney walls with sand or gravel?
It would take longer to warm up and really start drafting, but the sand
would conduct the heat to the copper coil.?. Maybe?
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