Over the past 6 years I have built a handful of tiny-home rocket mass heaters, capable of keeping a 200sq.ft. dwelling steadily warm with one or two brief fires per day.
With the first few builds I wrapped masonry fire brick with fiberglass mesh embedded in earthen plasters – with the caveat that clients to call me when they are planning to move so I can ensure the heater is properly braced before transport. But sure enough someone went and moved their home without telling me. And although the heater survived the journey without damage, I have come to realize that the vagaries of someone else’s random decision-making (even with a clear verbal or written agreement) were bound to be constantly unsettling.
So, as beautiful as the cob-skinned heaters can be, my approached has evolved to add a thin metal skin from 18 guage steel, into which the heat riser and fire brick can be rapidly installed
The J-feed fuel chamber extends about 5″ out from the front of the heater body and the whole unit is isolated from combustible flooring with 2″ of continuous calcium silicate board. The fuel feed chamber and burn chamber have another 1″ of ceramic fibre board under them, with hardi-board base and cob as leveling shim.
The ceramic fibre board heat riser directs hottest flue gasses into concentrated heat under the cook top. The gasses then downdraft to exit (in this case) out the base of an internal chimney.
All in all the stove-building process involves about 3 days of gathering and off-site preparations, with only 2 days on site. (flue and chimney work is another matter). This is much simpler and faster than better than 7-10 days onsite for a pure masonry build
The heater has a glass front plate so you can watch the burn; and a light weight cowl that can be placed atop the fuel chamber to prevent smoke back if you want to burn longer sticks of wood (eg, 18″ cedar…)
With a few plaster touch ups and some metal trim around the cooktop this heater will be completed for less than $2000