Brian Hall : Ianto Evans found a special place on the west coast, with a very stable micro climate, in a valley that enhanced the very stable
wind direction year-round!
Then he build his houses to that location, none the the structures build by Ianto in the
Intentional community called Cob Cottage and built by him are over 15' tall, I would
strongly suggest your vertical chimney extend above your roof line aways ! If you plan on building it that way and budget for it, and if your location for your outside chimney is
perfect you too may be able to use less. It will take a couple of heating seasons to tell if you were that Lucky.
Obviously you want to make this build attempt, The standard recommendation is to build it full scale out doors, and when you can get it to work there, bring it inside and try it !
My strong recommendation, based on the large number of failed sub-six inch systems, especially in a First Build still stands, I personally have had no need for that small a
system! While borderline due to your systems use of elbows, it just squeaks through on interior Dimensions (as you have found and reported so far !)
Many people have reported good luck with using booster fans, especially if they did not need to run them continuously, Living in my location I find it necessary to have plans
for winter storms, and power outages, I have a plan and a back-up plan, and a back up plan to that plan, a 12 volt system with 2 car type batteries and a trickle charger
should easily get you through two days of forced off-grid use !
You Do not want to go square, an effort to make your interior passages as smooth as possible with a reline/patch of any rough places with a combination of a clay/sand mortar
mixed with Refractory Cement to allow you to have the smoothest walls possible, with the largest dimensions possible will put you inside the Constant Cross Sectional Area
that every one needs to follow. Whenever possible it is recommended to use round pipe because of the increased turbulence and friction inside your pipe that only increased
height of your external chimney can overcome !
Have you been to
rocketstoves.com for a PDF Copy $15 of Evans' and Jackson's great book
Rocket Mass Heaters there is
STILL No other book in any language with more Rocket
Mass Heater information, (and I don't make a dime !) This will allow you to understand the need for Constant Cross Sectional Areas through out your build, and the rather strict
ratios that must be followed regarding the Depth of Feed Tube to Heat Riser height, And the Burn Tunnel/Combustion Chamber length and The Heat Riser height, And where to
measure from, it will also allow you to use a common language to describe what you have/see to other builders !
Good luck with your build, come back here often, All Of your fellow members would like to see another Successful RMH build ! For the good of the craft !
Think like fire, flow like a Gas, Don't be the Marshmallow! As always, your questions and answers are welcome and Solicited ! PYRO - Magically Big AL !