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Looking for feedback on my design

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Hello Everyone. I am looking for feedback on a plan that I have for an RMH. This will be my first build and will contain ideas and concepts that I have found online combined with some of my own. I would like to hear from the experienced builders to see if there are any holes in my plan.

The unit will be made from plate steel (not sure how thick yet...any recommendations?) and welded together. The J-tube will be 12cm square. The magazine will be 30cm from top to bottom. The tunnel will be 50cm from end to end. The riser will be 100cm from top to bottom.

The riser will contain 3cm of insulation between its walls. If I can't find vermiculite or perlite, can I use fiberglass? I read that fiberglass can withstand temperatures up to 1,000 degrees Celsius. The insulation will not be directly exposed to the fire, only to heat, as I will seal off the space between the walls at the top of the riser. The distance between the outer wall of the riser and the 'barrel' will be 3cm. The distance between the top of the riser and the top of the 'barrel' will be 3cm.

At the back of the unit and bottom of the 'barrel' a 12cm long flange will be welded at the opening. A standard size 12cm round stove pipe flange will be welded in place to to the square flange to be connected to a vent pipe.

I want to make a unit that is easy to assemble, disassemble and move. In my design the J-tube and barrel will be square. The dimensions correspond to the size of a standard brick and the entire steel unit will have bricks underneath and stacked along the sides and on top of it. As a temporary solution (while I'm testing) the bricks will be fastened in place with wire for safety and for maximum surface contact with the steel. I will play around with the amount of bricks stacked around the 'barrel' and try to find the right amount of steel to expose for 'fast heat'.

Unlike the other RMHs I have seen, the bricks will be the 'mass' for this heater and will not absorb the heat from the vent pipe, but from the barrel and a little from the J-tube and rear flange. Any thoughts on that logic? I don't expect the bricks to hold heat as long as a big mass of cob, but I think that it should radiate for a few hours between burns. It seems that most of the heat absorbed by most of the RMHs that I have seen comes from the vent pipe loop. My design will absorb heat from the hottest parts of the unit instead. Any thoughts on the effectiveness and efficiency of this idea?

I will connect a vent pipe at a 90 degree angle and it will run vertically 2.5 meters to a chimney flange.


Main concerns:

Is 12cm (4.7 inches) square, big enough for the J-tube? I think that I read that 6" is the smallest attempted, but I'm not sure if that was round or square (square having more volume).

Is 3cm between the outer wall of the riser and the 'barrel' enough space?

Will enough heat be transferred to the bricks to the unit to be a mass heater?


I hope that my description depicts an accurate picture of what I am trying to accomplish. I am looking forward to any questions and more importantly feedback so that I can get started.
Posts: 791
Location: Tonasket washington
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this is your first build of any kind of RMH and you are going to change things. Why dont you build one as we designed and then make the changes so you at least have an Idea of what you are doing. if not then build your unit out in the back yard away from everything and test it for a while.
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