Critiques please -- compact sized rocket mass heater with rocks for heat storage

The design goal is a rocket mass heater for small spaces, requiring minimum construction work, using masonry stove body, and a 55 gallon barrel containing crushed rocks as heat storage.

Do you think the following design will work as a rocket mass heater? 55 gallon of crushed rocks is around 1000lb (450kg). The floor would need support from below.

I guess there could be a drafting problem when the rocks are cold, could be solved by adding an electric draft fan on top of barrel.

The fuel box size should be designed such that one batch of fuel will heat rocks to a relatively high temperature, such as 1200°F, then we close the throttle and let the barrel radiate heat to the room, until it cools off.

Any other possible issues and improvements to this design?

Have you built a J-tube according to the dimensions known to work? There are several factors at play, so it's a good idea to start with a tried and true configuration and then experiment to see how far you can depart from the basic design, changing one parameter at a time to figure out if it will work or not.

Just by looking at the plan, I think the feed tube is too tall and will be prone to creating a reverse stack effect and working against the desired flow. I'm not sure I understand the air holes in the barrel holding the rocks...is this a secondary combustion chamber? Sending the combustion gases directly through a bunch of rocks will remove too much heat from that zone, hampering the burn and stalling the draft.

Phil Stevens wrote:Have you built a J-tube according to the dimensions known to work? There are several factors at play, so it's a good idea to start with a tried and true configuration and then experiment to see how far you can depart from the basic design, changing one parameter at a time to figure out if it will work or not.

Just by looking at the plan, I think the feed tube is too tall and will be prone to creating a reverse stack effect and working against the desired flow. I'm not sure I understand the air holes in the barrel holding the rocks...is this a secondary combustion chamber? Sending the combustion gases directly through a bunch of rocks will remove too much heat from that zone, hampering the burn and stalling the draft.

Sorry my illustration is far from clear, the bottom holes are for flue to pass into the barrel.
Combustible gases might ignite at 500'F. Rocks need to be above this temperature in order not to kill the flames. Maybe use 500F as the start point of refueling cycle, so the next batch of fuel will always begin with relatively hot rocks. Cold-start with propane or electric heat.

Ok, but I'm still wondering if you've had a play with a standard J-tube to get a feel for how the parts work together. The heat riser is a critical part of the system and rocket builders go to lengths to keep this area hot with insulating materials. Taking the combustion gases directly from the burn chamber into heat-absorbing mass is contrary to the typical way a rocket works.

Even if your rocks are 500C that's still a long way away from the 1000C + that is achieved in a properly functioning heat riser. 500F is not even close. The results are likely to be incomplete combustion, stalled draft, and lots of smoke out the feed tube.

Hi Douglas;
As I see this from your drawing.
You are envisioning a J tube rocket.
From your drawing, as Phil mentioned the feed tube should have an inside dimension of no more than 16" yours looks much taller.
After that, all I see is an open-top barrel filled with rock and the riser sticking out the top.
That design will burn, however very poorly until your rock heats up.
You're asking the core and riser to come up to Rocket temps but you are not giving it any insulation.
Your burn will be smoky and not efficient at all.
A standard small metal box stove, with brick walls on three sides, would not require floor support and would hold and radiate heat.

If you want an RMH in a small space you must think small, as in a 4" or 5" batch box venting into a brick bell.
Much lighter than a barrel of rocks and it would be a true clean burning RMH.
A stove this size would require attention to keep filling it with wood.  
After heating the bell your RMH will be sharing that heat back with you all night!

RMH sizes have been thouroly experimented with by the innovators.   All sizes and shapes were tested.
Those innovators freely share this information with the world (there are no secrets about RMH construction)
Changing and modifying an RMH and finding a better more efficient burn is just about impossible.
Much experimentation and expensive testing equipment are needed to confirm changes are efficient.
Sadly just looking at what comes out of the chimney is not an indicator of burn efficiency.

Uncle Mud has a proven design that fills this niche, here is a thread /video that explains it: Uncle Muds Cottage Rocket On Paper


It is a J-tube and mass contained in the foot print of a single drum.

You guys might be interested in the new excerpt of the Free Heat movie by Paul and a bunch of other expert collaborators. In this particular clip, you'll see how cuts are made for the bricks so they have a better fit and how the instructor - Isaac Workman - adjusts a barrel to serve as the containment for the j-tube riser. Enjoy!