Nathan Prince

Brilliant yet so simple Donkey!  I wouldn't have thought to physically adjust the barrel while running.
This will be a mass heater in a 300sqf trailered tiny house.  The mass can be substantial because it will be removable/replaceable in a split level trough just below the floor boards, could be similar to Paul Wheatons pebble bench.  Sizing is very important because this will be insulated similarly to my current 120sqf house that takes 1900watt max at -35°.  

The unit itself will consist of two faced and welded 8" sch40 oil&gas pipe tees forming the outer case of the lower half of the j-tube.  The interior cast of air-crete refractory mix around a plywood square form.  

The barrel is 14" STD weld caps top and bottom around a STD 14" pipe, the mannifold will be a sch40 10"-6" eccentric reducer.  All radius cut and welded, with a break for service access of course.  

The heat riser will be independetly cast of air-crete refractory mix.  

The most consistent advice pertaining to csa is to keep it even through the tubes & avoid restrictions & leave ash catches.  My calculations show a standard 8" system having a 50.2" csa j-tube and a standard 22.5" barrel would mantain a consistent csa with a horizontal gap between the barrel and heat riser at .8".  This seems narrow compared to plans out there.  
What is the best gap based on experience?
If there are functional tollerances, what would they be & the effects on heat diffusion off the barrel and system siphon?
What needs to be considered with a smaller system?
Im shooting for a 5"D / 20"csa unit.

Ok, this is a trailered tiny house, regular RMH construction wont float for weight and durability.  Steel was the obvious choice for durability, but it sounds like high carbon is a no no for safety and service life.  Perhaps a set of refractory tubes similar to a heat riser for the feed and burn tunnel nested into a pipe casing (for rabbiting) (the burn tunnel opening could be blinded off for servicing)
The bench mass will be a split level sub floor ondol, unceremonious and removable in an engineered trough so gross weight doesnt prevent trailering, but the heater could still be used as secondary heat while uninstalled.
Im not commited to any style of construction, I just have a-typical parameters.  As beautiful as they are, I dont want to disassemble cobb art every time I move.  Thanks for the insight, the link to alloy operating temperatures was very helpful.  

Sourcing materials for a RMH, cabin is 250-300sqft, r-20 walls, r-30 roof&floor, in hardieness zone 3.  As a primary heating source,   what would be an ideal flue size and bench mass volume.  Other than the heat riser and bench mass, the construction will be heavy steel (at least 1/4").
Because common pipe is in whole inches, what consideration should be given to interior diameter through the tubes?