Help designing a mass heater

Hi everyone!

Long time lurker first time poster.
I recently purchased a property with some building skeletons on it. One of the buildings is ~200sqft and I'm using it to experiment with several techniques before pivoting to the primary abode.

One of the things I'd like to play around with is building a small rocket mass heater in the space using clay bricks, stove pipe and cob - all of which I have readily available.

I read the Ianto Evans book ~ and I have a few followup questions I'd like to pose to the permies community.

The main thing I'd like to address is the barrel. I'd prefer not to use a barrel (or at least have it exposed) for aesthetic reasons. The Ianto Evans source says you can cover the barrel with cob, but the Ernie + Erica book (which I have yet to finish) specifically says you shouldn't cover the barrel with cob completely, but instead leave some exposed. Can I use brick for the heat riser instead of a barrel and cover it completely in cob if the barrel shouldn't be covered?

Another thing is that there are some gorgeous examples of what I believe would be referred to simply as "mass heaters" online where there's a fire viewing port instead of the feed pit typical of the R.M.H and no apparent barrel, but I can't find any instructions on how to go about building them. I've attached some pictures - this would be an ideal direction for me even if it crosses a line away from a conventional R.M.H. Is anyone aware of the technical term for this design or can point me in the right direction to begin building one in this style?

Lastly, I'm attaching a photo of the corner of the space I'm looking to install one in. I'm thinking fire in the center and benches/mass flanked on either side, one along the wall and another halfway underneath the windows. I'm planning a simple sub-floor using clay slip insulation in the bays and cedar scraps for the wood floor, while installing cement board instead of wood over in the area I'm going to install the mass heater, cobbing over anything I don't fill out with stove. Do you think this is a fine idea for safety reasons?

I also have some questions about the cob, pigmentation, carving the cob for those nice wavy accents etc ~ but I suspect that's a whole world in and of itself. First things first!

Thank you everyone for sharing your wisdom. I'm so excited to get started!

Hi Holly;

Yes, you can surround and cover  the riser with bricks and then you could cob those bricks if you liked.

You would want a much larger gap from the top of the riser.  Traditional J-Tubes used 2-3" as a gap.
I recommend having as much as 12" gap and then utilizing a stratification chamber (empty bell)
One thing I think you should do is stay away from a piped solid mass.
They are old school and can require periodic cleaning to maintain flow.

For your little area a 6" J-Tube would be plenty large enough.

Here is some lite reading to get you started
https://permies.com/t/270559/Stratification-chambers-Bells-explained
https://permies.com/t/272271/distinctions-RMH-type

This page has a great explanation of what you have to keep in mind when designing the "bell" portion of the heater: https://batchrocket.eu/en/building#belltheory

The advantage of exposed metal is that it conducts heat into the surrounding air very quickly. Besides the obvious effect of getting heat into the room as fast as possible, this also means that it dramatically cools down the exhaust gases inside the bell and therefore causes a strong downdraft, which keeps air moving through the system towards the chimney rather than stalling out and causing smoke to exit back into the room. Other materials will conduct heat more slowly, so you may have to compensate for that somehow rather than exactly copying a barrel-based design but using different materials.