Seeking advise on the shape of fire box for front loading RMH

Hello All, I am in the process of designing a Rocket Mass Heater for our 30’ yurt. I have built several batch box style RMH based on the design created by Lasse Holmes, using angle iron to create a rigid form, as well as give something for the door to hang on. They have all worked really well, but I am sure there is room for improvement and added efficiency. I would also like to use the door sold by Firespeaking ( ) which would create a slightly taller and more narrow fire box then I have build in the past.
My two main questions are, can anyone point me to any resources discussing the pros and cons of the different size or shape of fire boxes for a front loading, batch style RMH?
And how do folks recommend attaching the door if I do something different and dont use the angle iron frame?
The angle iron frame also provides the strength and ability to make an arch of the top of the fire box with fire brick, and curious what other people use for the top of the firebox?

Welcome @EarthenShelter and thanks for posting!

It could be helpful for other readers if you posted a photo or two of the angle iron frame that you are talking about.

My understanding, as you describe, is that the frame @Canyon developed serves the purpose of buttressing the long arch (or vault) on top of a batch box style firebox as well as providing a structure for a door to mount to. I believe Lasse vaults/ed the top of the firebox in part because refractory tile (12" x 24" x 2 1/2", 12" x 12" x 2 1/2") which are like larger firebrick slabs were not available or very expensive in Alaska. You can call a Harbison Walker distribution center to source these.

As for mounting the door without the frame, I have been meaning to write a door mounting article for some time… hopefully this winter. The standard way to do it to a masonry opening is with ceramic wool gasketing and masonry screws. If you are planning on facing your project with cob or sculptural earthen mix, one interesting technique I would recommend is the use of motorcycle spokes which are stainless steel and have threaded screws at the end of them. You can mask the door, thread the spokes through the mounting holes, and build the door in as you go. A pile of bricks or other armature is sometimes needed to keep the door steady as the sculptural mix is being applied and settling.

Have you reviewed @peterberg10’s documentation?

Definitely a useful resource also.

@Tallgrass , do you have thoughts to share?


Thanks so much Max! Here is an image of the frame, unfortunately it is from the back, and not the place the door attaches to, but gives an idea.