Excellent thread - clearly posed question and the reasoned (and not pontifical) responses I have come to expect from Allen and Erica. This speaks to my condition, as my workshop foundation was started with passages for thermal exhaust before I even heard of rocket mass heaters, and both the stove and the exhaust will necessarily be on the windward side of the building. I am planning to attach a pent roofed woodshed on that side of the shop to house the stove and wood supply, but haven't quite figured out how to capture heat primarily in the workshop's thermal mass, rather than have the barrel emit a lot of heat in the shed. I'm mindful that there needs to be heat transfer in the barrel to pump the exhaust through the mass. More details on all that when building proceeds and I can post pictures and ask detailed questions. For now, I am planning to use a vertical chimney, although on first reading of Ianto's book I was attracted to the idea of a horizontal exhaust. My aim is to have a system that works reliably under a variety of conditions, rather than one that works at maximum efficiency under "perfect" conditions. Nevertheless, I'd like to hear more from Peter as well. Phase change is worth paying attention to, IMO.
Like Vicki, I have parts collected because they might be useful - in my case a lot of flue blocks that went into the foundation to form horizontal channels and a couple of sections of triple-wall stainless flue pipe that I now envision sticking out above the shed roof. I wonder how many RMH designs have been constrained at the outset by the builder's desire to find a good use for materials at hand.
Again, thanks for this fine forum. Allen, I like your "practical zeal". Erica, your elucidation of potential variables is literally a breath of fresh air.