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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