Two design glitches of the Rocket Mass Heater: Cold Plug and Secondary Air Tube Pipe getting too hot

  https://www.youtube.com/watch?v=jpD-yKw2mNM I've been investigating Rocket Mass Heaters for a few years now - and there are always hilarious new issues arising. Here is Paul (the main inventor) on a problem of the "Batch" RMH (meaning no J-tube feed for the logs but rather a standard wood stove door). You put in a secondary air pipe:

I do have a concern that could turn out to be fully mitigated.  The metal tube is gonna get freaky hot.  A stainless steel tube set in a steel holder.  The temps will never get hot enough to melt stainless steel, but the temps will get hot enough to make stainless steel soft and slumpy.  But this could be fully mitigated by cooler air moving through the pipe at that critical time - possibly cooling it enough to keep it from slumping.

The mild steel that holds the stainless steel could spall at 1600 - but there will be ash on top of it that will insulate it from the fire a bit - plus the cold air moving through it could help.

  Mud mentioned that 8 inch batch box systems saw the pipe get soft and would need to be replaced.  But 6 inch systems did not have that problem.

So basically the smaller the system, the less hot the temp...

  My 6" and my 7" batch secondary air tubes are still solid after two years of extreme high-use burning.
I expect them to last indefinitely.

I did create my own stainless steel secondary pipe set up - that I used on my wood stove hack of ceramic fiber.

I record 800 F. plus in the wood stove - but the top of the wood stove steel (not stainless) was glowing red - and thus starting to spall. hahahhaa. Also the burn chamber was too small so I had to constantly feed the stove. But I would just keep the stove door open and would have amazing 800 F. heat kicking out into the hermit hut - thereby warming it up very fast. hahahaha.

I asked about using a Stainless Steel heat riser and the other main inventor, "Uncle Mud" said it can cause catastrophic failure. But if I have the smaller six inch diameter then maybe the temp would not be too much!! I posted a comment on that.

https://www.youtube.com/watch?v=Ii4voG5d7fU

So here was have the infamous "cold plug" problem - meaning if your room is too cold relative to the external air temp when you cold start the RMH then the exhaust is not hot enough to create negative pressure to suck out the house via the colder external air temp. So you get back smoke exhaust as a "cold plug." oops. So instead they put the exhaust stove pipe as close as possible to the "heat riser" chamber - thereby preheating the exhaust stove pipe....

So the key factor is to first make sure the house is very well insulated but then you have the problem of needing enough intake oxygen for the fire. What one winter hut camper designed is a stove pipe going through a hole in the floor just next to the wood stove - so that is the external air source while the rest of the hut is very well sealed up and insulated....so that the internal room stays warm enough to prevent cold plug - with the external air temp being low enough so that the exhaust heat is hot enough in relation to the external temp for the negative pressure to suck up the exhaust. 

Steel Barrel Batch Box design for 4 times more instant heat - no cold plug - vid 

 Six inch Batch box design thread