Rocket Mass Heaters
How does Aircrete lend itself to the idea of Rocket Mass Heater construction?
As you know the idea of the "Mass" component in such a heater is to absorb heat from a "Rocket" type stove, acting as a battery to release it after the fuel is all consumed in the stove.
Typically people are using "Cob" and I have seen others use a box filled with pebbles with a rock or granite top. I would post links and/or pics but I believe this crowd would be familiar with this concept, if not google offers extensive discovery.
Aircrete would be the ideal material for building the riser.
To keep this topic useful, I'd like to point out that the terms "Rocket Stove" and "Rocket Mass Heater" are used to describe quite an array of stove and heater designs and concepts from a surprisingly wide range of viewpoints. I personally subscribe to the CobCottage school of Rocket Mass Heaters. I own their book, have read it a few times. I have used a few Rocket Mass Heaters of this class and even built one.
A Rocket Mass Heater consists of a rocket stove and a mass heater, which are two distinctly separate parts of the system. The mass heater part is merely a long horizontal exhaust encased in dense thermal mass, such as concrete or cob. Aircrete would be useless for the mass heater portion of the system.
One of the critical components of the rocket stove is the riser. Aircrete would be the ideal material for building the riser because the whole point of the riser is to be insulated (and, uh, fireproof). The riser I built was made of clay and perlite which worked well enough. The riser is where the super heated gas rises up with such power that when it hits the top of the drum it then forces exhaust down the outside of the drum and through 20' to 40' of lateral exhaust.
I would beg to differ as regards aircrete being ideal for the riser. Portland cement and soap are not refractory materials- even concrete cannot be used in high heat situations as it crumbles. I’ve built three rocket mass heaters, the first two I designed from watching YouTube but the designs didn’t work well once the stove was fired up. Third time’s a charm- I bought a book on rocket mass heaters written by a group that has built over 700 of them. The riser is fire brick to withstand the severe temps in the bell and it’s insulated with refractory rated rock wool. Normal bricks just crack apart in the heat. Most people use a steel drum for the bell but I used a 60 gallon well pressure tank, much thicker steel. Got it from a local scrap metal yard.
It works well with nearly constant feeding. The wood burns quicker than in a traditional wood stove and the firebox is small because it’s engineered to have the same dimensions as the exit pipe, which according to the book I have, says 8” pipe works better than 6”
Please keep us posted, sounds like you are on the brink of success!
@annesley Thanks for the reality check about using aircrete for the riser. The rocket mass heaters I used required me to stoke every 10, 15, or 30 minutes depending on the firewood available. I would do this for an hour or two during which an incredible amount of heat was stored into the mass and then released gradually into the cozy living space over the course of the next 24 to 48 hours.
I have a lot more to learn about Rocket Mass Heaters but what thru me off this guy,,,
Honey Do Carpenter youtube that you are referring too,
is that he throws a lot of heat out the stack pipe rather than keeping it in the home
stored in "Mass".
I might not be understanding him correctly, but I have tried to study what he is doing.
He fixed a stove, tore it apart and his fix was to insulate the stack pipe so that the pipe stayed
hot and drew out the heat better.
I will stay tuned, appreciate the conversation.
The best built rocket mass heaters I've seen have super hot fires in insulated boxes to burn the fuel as cleanly and effectively as possible (hotter fire = more mass is converted to heat energy = less smoke emissions). Fireboxes that are wrapped with thermal mass actually cool down the fire - not the best place/time to extract the heat. The insulated riser powers the intake and exhaust flow so that the super-heated gas has plenty opportunity to then run through a thermal mass storage bank before exiting the building cool and clean. The best system I ever used would burn so clean yet extract so much heat that when the exhaust finally came out of the house (knee high off the ground) I could put my face directly into the exhaust and breathe it in*. The air exited the building at about 95ºF after running through some 40' of lateral cob benches.
That's what i'm talkin bout! Bring it on home brotha!
In my opinion, the goals with a rocket mass heater are to:
- Extract as much heat energy as possible from the fuel
- Release the cleanest exhaust possible
- Store as much of that heat in the home as possible
These goals can be realistically achieved in the upper 90th percentiles using known techniques as established by CobCottage and others. Releasing clean exhaust is a BIG deal when living in a tight village setting.
Sums it up nicely!
This is my understanding as well.
That is why I was confused with the YouTube guy,, Honey Do Carpenter,
when he insulated his exhaust stack just before exiting the home so that it would draw better.
From what I have seen,, designs loop the exhaust then bring it back up to
rise by the original riser so that it gives it a bit of a heat boost to draw on out further.
According the rocket mass heater theory as I understand it, the best place and time to get power for your system is the riser itself. The force of movement generated in the riser will draw in the fresh oxygenated air and push out the exhaust. After the exhaust gas has cooled, it is considered counterproductive to direct it upward against gravity. The power of the riser is significantly increased by every inch of its height and by each additional 100ºF. The more power a riser produces, the longer it's horizontal exhaust can be, the more heat can be extracted into the mass.