Uncle Mud Tiny House Rocket Mass Heater with Pebble Bench at Wheaton Labs 2020

In September 2020 we had another opportunity to visit our friends at Permies.com and work on rocket mass heaters for their cabins. This one in the Red Shed is a combination of my CottageRocket core and Paul's pebble mass bench, plus a few new things we each really wanted to try. This heater was designed especially to heat a small space with a raised wooden floor, balancing gentle radiant heat and thermal storage with no "hot spots".

Paul was determined that we have a 1:4 height ratio between the feed tube and heat riser, while keeping the feed tube tall enough to close over the top of his standard 16" firewood. Meanwhile I wanted the pebble bench raised off the floor and lined underneath with hardware cloth to provide thermosiphon ventilation under and behind and through the heated mass to keep it from overheating the wood floor and walls.

A few things you can't see in the video: The feed tube is removeable for easy cleaning of the burn chamber. The standard CottageRocket burn chamber made of milled firebrick is lined outside and below with ceramic insulation board to keep the heat inside where it belongs. The small footprint of the heater fits in the tight space and leaves plenty of room for air circulation. Keeping the heater separate from mass allows it to be easily pulled out for inspecting and cleaning the exhaust pipes in the mass.

The standard CottageRocket uses a 2' tall "5 minute riser" design with ceramic wool compressed inside a 6" stovepipe, which works great for a 24" riser but creates too much friction with Paul's 48" Super-riser so after I ran out of time it had to be replaced with a 6" diameter cast ceramic riser, which works really well.

The 4" exhaust on the previous heater tended to get too cool to provide a good draft so this design runs the vertical exhaust right against the length of the radiant chamber to give it back a little heat to help pull the cooled exhaust through the pipes in the thermal mass. After this video was taken we added a sheet metal heat shield between the heater and the wall with a 1" gap behind it to allow natural airflow to keep the wooden wall cool.

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Nice job Chris.  
The thermosiphon ventilation you speak of in the bench sounds like a good idea to keep the floor from overheating. Wonder though if it would also cool off the mass a lot quicker once the fire is out? Akin to not covering the feed tube once burning is complete and stopping draft from carrying the heat out of the mass.

Is the feed tube secured to the body or is it just resting there by its own weight?

The tall 4" feed tube opening seems like it would be hard to get the stove started because of its small size. I guess it could be removed for starting, then replaced once going?

Greg,

The thermosiphon ventilation you speak of in the bench sounds like a good idea to keep the floor from overheating. Wonder though if it would also cool off the mass a lot quicker once the fire is out? Akin to not covering the feed tube once burning is complete and stopping draft from carrying the heat out of the mass.



The 4" is a PITA. Part of this experiment was to see if we wanted to go to 6" even in small spaces. With such a tall feed tube I would say probably. Eventually I hope to have a design for this that has a front cleanout, maybe even glass like the Gamera and my own edit of the Liberator. The brick feedtube is removeable for cleaning (and starting). It just sits in a groove on top of the heat riser.
The combination of different size pieces of stone in the pebble mass seems to help the mass "time release" its heat. More study will be necessary to see how the heat release works. Pebble mass and cob mass transfer heat in very different ways.

Thanks Chris.
In today's stove chat (episode 38), Matt Walker mentioned a lot of the same things about the 4" that you did while showcasing an example.
I've run a 6" J for many years and could not imagine going smaller unless a person was right next to it and dedicating themselves to feeding it very small wood quite often.
Appreciate your willingness to experiment though and mention your findings so that we all can learn and grow from as well.