RMH submerged in earthen floor

I've been researching rocket mass heaters for some time and had a few questions for those with considerably more experience than me!

My plan is to build a RMH sunken into an earthen floor, with only the barrel of the heat riser and the feed tube opening visible, and to have the floor act as the thermal mass.
This is mainly due to space considerations (the structure will be 12 x 8 feet), so a huge thermal mass bench may not be the best option here.
Anyone out there with RMH's in a small space like this - would it be too much heat for such a small insulated room? Is there a better alternative?

Can the metal duct typically used within the thermal mass be replaced with a brick tunnel sealed with cob? This would be under an earthen floor so I am worried about the strength of the ducting. The metal ducting seems to be quite expensive in my area, maybe I am looking for the wrong material.

What sort of protection would a wooden wall need, near to the barrel?
Should the burn tunnel be insulated with perlite/ceramic insulation despite being submerged in an earthen floor?

This is all relatively new to me so any help at all would be really appreciated!

It will be probably more complicated than the building itself. For such a small room it may be an overkill. 4" system can generate a lot of heat. Where are you located and how long you are planing to heat during heating season?
Ducts can be made of bricks - they are just heater channels laid flat.

Hi Cedric;
I second Christibal!
A 4" batch box with a small brick bell would be plenty to heat your space.
Much easier to build than an in-ground RMH.
The nice thing about a bell is it can be shaped to fit your space, tall and thin, short and fat as long as your "ISA" internal surface area is correct it can be any shape at all.

Piped masses are quickly being replaced by bells.
You could build a 6" J-tube rocket and plumb it into a bell.

I am based in Ireland, quite near sea level where the winters are rarely cold enough for snow.

I do like the sound of a smaller footprint bell, though the designs I have seen are all front loading rather than J-tube, and usually have a metal "p-tube" to deliver secondary air - is this the design you are talking about? Or is there a simpler design? As simple as possible would be ideal - this isn't a big space, after all.

I would love to do a simple design like this, but I am much less familiar with the bell concept than the traditional rmh.

Can I simply build a standard J-tube with heat riser, but replace the barrel with a brick mass to form the "bell," and have the chimney exhaust from the base of the bell so that the coolest gases leave from the base of the chamber?

If anyone has reference to any guides or examples, that would be amazing!

Hi Cedric;
Here are links to my shop build.
An 8" J-Tube into a large brick bell.
I chose to use a barrel but I could have enclosed the riser with brick instead.
Obviously, this is way larger than you need but the idea works fine.


https://permies.com/t/94980/Brick-Bell-Shop-Heater
https://permies.com/t/99519/Baby-Dragon-Roars

Your idea of burying the core is ok but the concept of heating the floor might not work so well,
For your J tube to perform properly you need to insulate the outside so the fire can burn really hot, the insulation will stop the heat radiating into the soil. If you dont insulate the core the soil will be a heat sink and fire wont get hot enough to work very well.

"Can I simply build a standard J-tube with heat riser, but replace the barrel with a brick mass to form the "bell," and have the chimney exhaust from the base of the bell so that the coolest gases leave from the base of the chamber?"

That is exactly what I did, though on a scale to heat a whole house. I wouldn't go smaller than a 6" J-tube; this would likely need to be larger overall than the size you could get with a 4" batch box powering the system. The heat output and bell size would be similar in either case, while the J-tube feed would need to stick out from the bell where the batch box would have only a door in the side of the bell.

Fine homebuilding ,some many/decades/years ago showed a Karl Marcos building  his version of under floor masonry stove called a K!ang , its available as a pdf somewhere , seems a heap of work to do --well for a lazy like me anyway and well beyond my masonry skills.