Paul Delaet :Let me take a small piece of your Post and I am sure you will hear from several more of your Fellow Members . If the Constant cross sectional area C.C.S.A.
of a 8''
Rocket Mass Heater RMH is 50 sq.in. ( too big ) and the C.C.S.A. of the 6''RMH is 27 sq. in. (still to big ) Then the C.C.S.A. of a 4'' RMH is 13 sq. in.
Note that when we went from 8'' to 4'' we reduced size by 1/2 but decreased the C.C.S.A. to 1/4.
That means that we barely have enough Volume in our smaller piping to promote a Good draft Which is critical to give us the "Rocket effect'' we need to create a High
temperature Efficient good clean burn.
The usual practice is to build one of the larger 6'' or 8'' Rockets and live with them for at least 1 heating season. Generally the smaller 4'' RMH is considered a more difficult
or an Advanced build Because you are already working at the smallest size that can work, Plus the amount of horizontal run is also very limited. Because the resistance to flow
of 1 elbow Counts against your permitted length the same as 5' of horizontal piping - you Must limit the elbows on this size Rocket !
This is where I ask if you have been to
Rocketstoves.com to download your copy of the brand new 3rd Edition of Evans' and Jackson's
Rocket Mass heaters ?
This should have answered most of your questions an guaranteed you can come back here and use the same words to discuss the Correct sizing, Length Ratios, and Parts
Orientation both to the whole and each other ! And don't worry about the math it is really all 5th grade/Elementary stuff ! Remember every single Rocketeer here started
from where you are now , For the Good of the Crafts ! Big AL