my name is Andreas and I live in France.
I want to build a RMH and have two more questions.
How do you seal the stainless steel pipes at the joints?
How can I calculate the distance from the heat-riser to the lid?
Andreas, if you cob your pipes, you don't need to seal them. Otherwise, chimney pipe aluminium tape.
And as for the top gap, You have to understand CSA or cross sectional area.
The csa is the number of square inches or centimeters the cross section of your pipe has.
And in a normal J tube, you shoot for 1.5 to 3 times that number, in ring projection.
To explain ring projection, the top of the pipe makes a ring, at which point, it's the narrowest point between top of the barrel and pipe, like if the pipe continued to the very top, touching it. In reality it is not. But you can calculate the circumference of the tube, by the top gap, and that will give you a surface, which should be at the very least 1.5 times the CSA of the pipe, and a good idea would be to have a goo 3X the csa. , if you don't care about cooking on top of the barrel. Maximum gap doesn't exist. Well, if does, when your internal surface area (isa) reaches the point gases cool down against the walls of the barrel, (or bell) and stall the drafting of the stove.