Barrel / heat-riser gap & CSA, RMH

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.

The riser to barrel gap is one place where more clearance is beneficial. Due to surface friction effects, a gap as small as .8" may be much too small. In air flow calculations, there is a boundary layer effectively a half inch or so that has to be deducted from the actual cross section of a duct to reach the effective cross section; .8" - 1/2" - 1/2" doesn't leave much
2" average gap is good, and more is okay.

A 5" system is near the small end of what has been found to work in practice. 4" is the smallest, and is tricky for even experienced builders to get right. 6" is a reliable common size that works well if done even close to right. 8" is a common size for heating a whole house in a climate with real winter.

What is your purpose for this heater, and what is your space, insulation, and climate like?

Hi Nathan,

One of the best innovators on RMHs (Kirk Mobert aka Donkey) suggests that the best way to place the barrel over the riser is to fire up the stove without a barrel over top, get it going good, then lower your barrel over the riser listening for the "sweet spot." You will notice that the rocketiness of the stove increases to a roar as you lower the barrel, then quiets down again as you get too close to the riser. The idea is to find the spot at which the roar is loudest, which means going through it up and down a few times, then backing it off the most roaring spot (up away from the riser) so that it's a little quieter, but still rocketing. Kind of hard to explain in words, but if you try it you'll catch on pretty quick. Of-course if you use this method, be sure to have some really good gloves on so as not to burn your hands...and a buddy to help you place supports underneath the barrel as soon as you get it where you want.

I think the principles at work with this method have to do with finding the most efficient placement for your stove, as each stove is unique...formulas give a good general idea, but i've found this to be the best way to find the right placement. Too much rocketiness means the gasses are being pushed through too fast and not burning super clean.

Curious if you decided on a rmh for the tiny house after all?

fyi, the csa consideration at the top of the riser should be conceptualized as a sort of chamber around the riser, between the top of the riser and the barrel...the first point at which air leaves the riser and enters the barrel. so to be consistent with system size, csa for this area is calculated using the length of the top of the riser (inner circle rim).
if you do feel the need to calculate the gap based on csa, the proper way to do it is to find the length of the top of the riser...so in an 8'' system with a circular riser, the length is pie x diameter...3.14 x 8 = 25.12.
the csa in a circular 8'' riser is pie x radius squared...3.14 x 4 x 4 = 50.24.
then the gap is found by dividing csa over length, so 50.24 / 25.12 = 2.
in a square riser, the length is simply the sum of all four sides...but remember that it's best to use calculations for a circle that fits inside the square...and also, as Glenn pointed out, that more space is better.

for a 5" system....

length: 5 x 3.14 = 15.7
csa: 2.5 x 2.5 x 3.14 = 19.625
gap: 19.625 / 15.7 = 1.25 (no less than)

also remember that the point at which the barrel connects to the flue (transition area/ash pit) should be at least twice csa and more if you can manage it, then tapering down to original system size to travel through the flue/mass.
was also thinking that you might want to consider a stratification chamber (aka bell) instead of traditional flue and mass if this is going where i think it's going.

really curious if this is in fact for your trailered tiny house...wishing you success in any case!

ok, just one more thing...and i know you've heard it before but it bears repeating. smaller than 6'' systems are hard to get working well...increased resistance (more surface area vs. csa) makes the draw unreliable sometimes. Peterberg (brilliant innovator) says:  

"The advantage of mass heaters is that it is not a problem to oversize them. With cast iron stoves, it becomes much more of a problem because if you oversize your heater, then you will have to make slow, dirty fires in order not to overheat your house. With mass heaters the fire always stays hot and clean, you just have to light it less often."  

"The very important point is to always oversize the heater. Of course, it is possible to fire a mass heater that isn't powerful enough three or more times a day, but this will lower the overall efficiency because the mass needs time to deliver the accumulated heat."

ok, i will shutup now.

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.