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Building the Ultimate Rocket Mass Heater FAQ

 
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Who coined the term "Rocket Mass Heater"?

When were RMHs invented?

Why is it called a "Rocket"?  

Will that big metal rocket barrel suddenly launch off in my living room?  If so, do I need to notify the FAA?

The mizzus hates the looks of that barrel.  What other options are there?

Is there a coffee table book or directory of nice, finished, Pinterst-y RMHs?  

I see there is a "ship-able core" called the Liberator for the rocket stove part.  If I buy that, how do I add the mass part?  Are there instructions online specifically made for the Liberator?

I don't want mud in the house.  Do I have to use cob?   What materials can I use for the "Mass" part of an RMH?  

What is that ducting made of?  Can I use bricks instead of metal ducting?

Do I have to use firebricks to build one of these?  What about "normal" bricks?

What are the main benefits of, or differences between a RMH and a Masonry Stove?

Does the fire in an RMH always burn sideways?  Does it ever change its mind and smoke up the house?

Are there free clubs, non-profits, professional associations, or business organizations near me who can help teach me how to build or operate one of these?  What about helpful individuals?  
 
Rocket Scientist
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Brick flues versus standard metal ducting: It is reported that a square channel offers about the same resistance as a round channel of the same dimension, so replacing 8" duct with an 8" x 8" brick flue should work about the same. I would be generous as there is also the roughness factor added, though only a small part of the square cross section will actually contact the main flow for roughness to apply.
 
Glenn Herbert
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Who coined the term "Rocket Mass Heater"?
--?Probably Ianto Evans and his collaborators, maybe including Larry Winiarsky while they were first working together. (Check this with Ianto's book which I don't have on hand at the moment.)

When were RMHs invented?
-- ( " )

Why is it called a "Rocket"?
--From the "whooshing/roaring" sound made by a J-tube burning at full blast.

Will that big metal rocket barrel suddenly launch off in my living room?  If so, do I need to notify the FAA?
--

The mizzus hates the looks of that barrel.  What other options are there?
--Lots, from simply disguising or dressing up the barrel with custom fabricated versions or decorations, to styles which use masonry of various types in place of the barrel to surround the heat riser.

Is there a coffee table book or directory of nice, finished, Pinterst-y RMHs?  
--This thread, beautiful rocket mass heaters on permies has good examples.

I see there is a "ship-able core" called the Liberator for the rocket stove part.  If I buy that, how do I add the mass part?  Are there instructions online specifically made for the Liberator?
--

I don't want mud in the house.  Do I have to use cob?   What materials can I use for the "Mass" part of an RMH?  
--You don't have to use cob, even though it is generally free and very effective. Any material that is noncombustible and holds significant heat to release later will work. In particular, brick, stone, concrete and water are very effective, as long as any gaps are filled so there is solid material with no air spaces. Air spaces insulate, so sand is a mediocre mass by itself.

What is that ducting made of?  Can I use bricks instead of metal ducting?
--Ducting inside a mass is generally galvanized HVAC duct, as that is the cheapest new material for the job. Any smooth noncombustible tubing will work as long as the size is right and it does not have sharp corners. Flexible corrugated duct will not work well as it is too rough and will cause drag. You can use bricks as long as the round duct it replaces would fit inside the brick channel.

Do I have to use firebricks to build one of these?  What about "normal" bricks?
--You need firebricks or other highly refractory materials for the combustion core where temperatures can be 2000 degrees F (xxxx C) or more. For other parts, ordinary clay brick will work fine. Clay brick can even be used for the combustion core in a pinch, though it will not be durable there. Concrete bricks will not be durable in any higher heat areas, and may crumble or explode if used in the combustion core.

What are the main benefits of, or differences between a RMH and a Masonry Stove?
--A rocket mass heater is actually a subset of masonry heaters. The centuries-old attraction of masonry heaters is that they allow for a hot, fast fire and store the heat to release over many hours or days without a fire burning. The essential difference between a traditional masonry heater and a rocket mass heater is the super-efficient combustion core. Common differences are that an RMH is often built by the owner using local, scrounged or recycled materials instead of by a professional mason using expensive new materials.

Does the fire in an RMH always burn sideways?  Does it ever change its mind and smoke up the house?
--A properly built J-tube RMH once primed and burning will essentially always burn sideways drawing in fresh air down the feed tube. An improperly built RMH may be subject to smokeback either sometimes in unusual weather conditions or frequently due to poor design. Sometimes an unusual environment like mountains or forests will make draft reversal and smokeback happen despite optimal system design. This is why certain criteria of design and construction need to be followed, and not just freehanded or "similar" construction.

Are there free clubs, non-profits, professional associations, or business organizations near me who can help teach me how to build or operate one of these?  What about helpful individuals?
--This is highly localized and cannot be answered in general. There are lots of helpful individuals who will give free advice here on permies, and some around the world who may be available to hire or consult. List of Rocket Mass Heater Builders
 
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New guy here, been lurking for a few years, but this thread has me excited.

I have been following rocket stoves for years, but have never pulled the trigger.

I have gathered an old house chimney's worth of bricks, a whole bunch of metal ducting, and I have access to steel barrels.

The hurdle that has stopped me so far is two things.

#1 there is no standard formula on how these things work, that works every time, and is idiot proof (I qualify as an idiot sometimes). I see a bunch of hand wavy "this should work well" type of statements, and things like "well you'll have to experiment, and fire it a few times as you are building it to make it work best" which makes it very hard to commit to dragging all this what my wife considers garbage in our home, and playing around with it in the basement (weight makes it impossible to place it anywhere else) lighting fires, and seeing if it will work, meanwhile winter in Wisconsin gets to -30, and if it doesn't work right, we are in big trouble. All the while, she's tapping her toe with her arms folded, because she is justifiably skeptical that this is going to work.

#2 I went to great expense and effort to convert my home to radiant water heat, I love it, I love not having hot air blowing around my house making my allergies flare up and the boiler takes up a small section of wall where it hangs in my basement. If I were to do a rocket, somehow I have to get the heat from the basement to the 1st, and 2nd floor of the home, which can easily be done with water, but people make statements like "boom squish" which basically dismisses my ability to work with this. I installe dthe boiler myself, as well as the radiators, and piping, and know how to install a radiant water system. The water never boils never goes above 170F, in the pressurized system. Now, I did have a wood fired outdoor heater before, which consumed literal tons of wood per winter, my entire summer was used up with cutting chopping, stacking, and driving my truck back and forth to places where people wanted fence line cleared. With the amount of hours, and fuel I spent I saved nothing doing this. Anyhow, the reason I bring it up, is that was an open system, that was how they prevented "boom squish" very simple, really. Why can't a person do an open system with a heat exchanger to the closed system for a radiant heat setup?

Thank you for your time. Solve these 2 issues, and I will have one in my home.

Another suggestion.

I want to heat my shop with one, and I have access to waste oil, is there a way to make one that we can inject waste oil to suppliment the wood/trash burning?

Honestly, this may be a first built one, before I do one in my home.

But yeah, I also have an issue with cob. I just don't see it as very durable. I fear it will crumble, and be messy, and crack, and break. Even concrete doesn't hold up to some abuse, I would feel better about entirely iron, or steel.

See what y'all do with these issues I run int that prevent me. Like I said initially, I've been sold on the CONCEPT, I have just been discouraged by the seemingly experimental nature of it. I do'nt have time to experiment, I have other things I need to get done in life, I want it to be something that does not take all of my time a way putzing with it to make it just right, I want to spend some time, build it, then basically forget about it, til I need to feed it every so often.





 
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Hi Christian,

I’ll just speak about cob.  It’s best if it’s learned from someone who knows, not because it’s complicated, but because of the variables.

If you do a web search on cob workshops, eventually you will find one close enough.  In the meantime, why not get a copy of Kiko Denzer’s book “Build your own earth oven”, and get started?

Make some test batches and see if they crumble, if so, increase the amount of clay.

For materials, I went to a place that sells dirt from various piles from various locations, and got a few gallons from various piles, so that I could see which I wanted to buy for my project.  It’s not an uncommon request at that type of business.  The book will tell you about soil tests.  Do NOT use topsoil, you need mineral material.  Usually people find dirt with more sand, and mix it with dirt with more clay, to end up with the right proportions.  When there’s enough sharp, (not rounded) sand, you will be able to hear the sand grinding against itself, per Ianto Evans… and this is where being directly taught comes in.

And the more you know BEFORE you attend your class with a master, the better questions you will be able to ask, and the more useful the class will be to get you moving on the undertaking, or you’ll decide not to use cob in your life.

Good idea to want to know what you’re doing before bringing the mud into your home😊

Have fun with it!  And best wishes!
 
Glenn Herbert
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Welcome, Christian! I had radiant floor heat for 15 years before I built my RMH six years ago, still have it for the basement as the heater is on the main floor. I love the radiant heat, but don't miss it with the RMH operating. The comfort is very similar. If you have a zone for the second floor, you can probably keep that running as needed while the RMH takes care of lower floors. Having a full backup system is valuable if you are ever away in winter.

I have found that the lofts above my main floor are warmed well too, even the one farthest from the heater. As long as your house is relatively compact and decently insulated you should be okay. I would have to see your setup before guaranteeing anything, of course. All the "shoulds" and "probablys" you hear are due to the many variables in houses; nothing works in all situations. Identify all features of your house that might be different from ones known to work and you can get a good idea of how yours would work.

For the basement installation, there are a few issues to consider.
Draft: What is the insulation and sealing level of your house? If the upper floors let hot air out, that might compete with the chimney and pull smoke back into the basement. You want the house well sealed to retain heat anyway. Is your chimney inside the house? That will have better draft than an external chimney (and also not waste heat to the outdoors.)
Convenience: Do you already use your basement regularly? If you don't have any other reason to go down there, you may have trouble keeping the fire tended. A J-tube needs to be fed every half hour or so for the couple-few hours per day required to build up heat in the mass. A batch box can give the same amount of heat with fewer loadings. Of course, some people have found that once they had a warm comfy spot in the basement it became a favorite hangout.
Heat distribution: If you have openings in the floor above or an open stairwell, you may get plenty of warm air migrating to the first floor. Another possibility is a tall bell in the basement that extends through a framed opening in the floor to give a radiant source in the first floor. You would have warm floors from the basement and direct heating. This depends on your ability to find a workable location for both floors simultaneously. A double bell on two different floors has also been done, but that is a more advanced project for an experienced builder.

Boom squish is a serious issue, but there is a well-tested style that is safe. An open (unpressurized) tank is heated, with a pressurized coil running through it and feeding the heating load. As pressurized water has a higher boiling point than atmospheric water, even if the tank boils the coil will not. A makeup water supply with float valve is needed to make sure the tank does not run dry.

 
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I fit very much in the category of "Could be converted", so I hope I speak for a lot of people when I explain what my hold-outs are. I don't know much about them, yet, (mostly the sales pitch), but I'm busy, so I only want to deep research things that are worth my time.

To start with, let's talk about one piece of the resolving misconceptions part. The misconception you listed was "They're hard to build." and the answer:
"You can build them in a couple days with a small crew, or hire an experienced builder - maybe for less than the cost of conventional heating installation"

So rather than resolving my "misconception", you amplified it:
"Maybe" less than the cost of conventional heating installation tells me it's a 5 digit price tag. Prices are usually reflective of the work, materials, and expertise necessary for a task, so I figure this is going to be huge whether I buy or build.

"A couple days with a small crew" suggests that after I take a lot of time to build the knowledge necessary. And find other people with the skills. And possibly buy some tools. And we all take 2-3 full days of work (after all the time already taken), we might find success. And do I even have the skills? Do I need to be good at welding or other metal work?


Thus, even though I love the idea of the RMH, it looks like I would spend hours researching it, look at the task in front of me, and say "Maybe some day".

Now that I've given you some thorough negatives, I'll tell you what might still convert me: Minimum useful components.

I don't know enough about RMHs to know if this is a possibility, but are there components that are independently useful? As a hypothetical example, if 20% of the work of building a RMH could result in a great little device for cooking perfect Naan bread, that sounds achievable to me...and then I'm 20% of the way toward building my RMH.

Basically, if the message is "Rocket Mass Heaters take 10 steps...and if you don't do all 10 correctly, you have nothing." That's intimidating. On the other hand, if the message is:

"Rocket Mass Heaters take 10 steps, but there are several great stopping points where you will have something cool that's worth the time and effort," that's exciting.
 
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Has anyone designed/built a rocket mass heater that heats water, has a stovetop and oven, and warms the room with whatever heat is left? I'm looking for the best-ever rocket stove! ;)

Beau Davidson wrote:The people who really know about Rocket Mass Heaters believe they can solve a huge number of global problems.  The problem is - not enough people know them well enough to speak with authority, and there a heaps of misconceptions.  We aim to change that - but how?  

The goal is to develop a giant FAQ for Rocket Mass Heaters.  In so doing, we aim to dispel these misconceptions, paint a picture of all that Rocket Mass Heaters offer to individuals and the world, and give away this invaluable knowledge.

We have a good start here: https://richsoil.com/wood-heat.jsp#faq.  Are there questions you would add?  If so, post them below, s'il vous plaît.

An anecdote to provide some framework, and food-for-thought:

My Marketing Experience with Red Bull Energy Drink, as an analogy to the challenges and possible solutions we face with Rocket Mass Heaters.

In a former season of life, I worked for the energy drink Red Bull.  I was one of those college kids that drove around town in a mini cooper with a giant can on top.  (Don't judge.  It paid well for a flexible college gig and let me quit my job at Wal-Mart.  Also note, most of these methods are similar to, or even identical to commonly employed evangelistic tactics.  Don't go cider-press on me with this point, just food for thought.  Evangelism, by definition, is "the spreading of good news by public declaration or personal witness," and I contend that this is applicable for Rocket Mass Heaters - way more so than energy drinks, at least.)  

What was interesting about the Red Bull gig (it wasn't the drink - I never drank it) was the methodology and the training we went through to effectively communicate about the product.  Once a week, the whole guerilla marketing team (we were called the "wing squad") gathered and did role-playing exercises so we could practice communicating an internalized script, regardless of the context or the person we were talking to.

The goals of these interactions were simple:

1) Dispel Myths and Common Misconceptions about the product.  
2) Highlight the Virtues of the product, particularly those that are most relevant to the given situation
3) Provide an Experience with the product, as a gift.



1) Dispel Myths and Common Misconceptions
For Red Bull, there was one primary myth.  Namely, that it was an alcoholic beverage, due to its prevalence in the bar scene.  
Therefore, near the beginning of each conversation, we would ask if people had heard of Red Bull, and if so, what they thought of it.  Many would say "isn't that beer?"  Tee-up for, "well, actually, no.  It's an energy drink."

For Rocket Mass Heaters, there are many myths and misconceptions.  
So we could perhaps start like we did with Red Bull.  "Have you heard of Rocket Mass Heaters?"  And, if so, "What is your impression?"  That will likely tell us what we need to know to dispel any misconceptions.

  • It has to do with rockets.
  •     Actually no, the name "rocket" is derived from the whooshing sound of the incredible draft.
  • They aren't reliable.
  •     Actually, due to the ease of construction of the J-Tube style RMH and the incredibly clean burn, they are among the easiest-to-operate, safest, and most reliable wood-burning stoves in existence.
  • They're hard to build.
  •     You can build them in a couple days with a small crew, or hire an experienced builder - maybe for less than the cost of conventional heating installation
  • You can't build one in a city.
  •    Not only are codes coming around, but there are also now multiple UL-certified Rocket Stoves on the market that can be purchased and delivered, ready-to-install.
  • You can't build one in a rental.
  •     The Liberator and the Pebble Style RMH are just two examples of RMH systems that are easy to install AND uninstall, should the need ever arise.
  • You can't build one in a conventional home.
  •     There are RMH systems suitable for any and all weight limitations and building systems.
  • Aesthetic misconceptions (i.e. cob, barrel, footprint)
  •     RMH don't have to use barrels, or cob.  Some of them are simply beautiful, and some of them are hardly noticeable in the room at all.

    The list goes on.



    2) Highlight the Virtues of the product, particularly those that are most relevant to the given situation
    For Red Bull, we had an easily memorizable bullet list.  It has been 20 years, and I still remember them!  If I could take that brain-space back, I would.  But it goes to show you how rehearsing these things works, from a psychological perspective.
  • About the same amount of caffeine as a cup of coffee
  • Less sugar than a cup of cheerios
  • Supplies Taurine, a non-essential amino acid that the body can't produce (forget that it's not actually an amino acid, but an organic compound) responsible for muscle recovery and the elimination of lactic acid


  • For Rocket Mass Heaters, the virtues are incredibly well-documented and compelling.
  • Carbon neutral or negative
  •    Far smaller carbon footprint than electric, natural gas, or other wood stoves.  When used in conjunction with proper woodlot management, can represent a truly carbon-negative home-heat solution.
  • Cheaper to operate than any other form of conventional home heat
  •    Some people use just 10% the amount of wood that they used to use with their "high efficiency" wood stoves.  Some people heat their home with sticks that fall out of their trees.  Some people heat their homes with junk mail and amazon boxes.
  • Cleaner burning and safer than any other wood stove
  • Cheaper to install than other home heat systems

  • et cetera


    3) Provide an Experience with the product, as a gift.
    For Red Bull, we'd give the person a can at the end of our interaction.  Interestingly, we were very intentional to never use the word "free," but rather "gift," in order to communicate the value of the item given.  We would give them a chance to respond, and then ask them for a lead, or a reference to someone they know personally who could use the same gift.  They would call their friend and tell them they were sending the wing squad to them with a gift.  Instant raport and credibility with audience #2!

    For Rocket Mass Heaters, this experiential component is more difficult to attain, but perhaps more crucial for conversion.
    We need to get folks' bodies onto the mass in the dead of winter to truly communicate the experience.
    We need physical representations of various fuels that provide winter heat:
  • a massive pile of coal representing electric and, well, coal
  • I don't know, uranium and a tumor or something, representing nuclear
  • a massive pile of plastic and gasoline jugs representing even "renewable" electric like solar and wind
  • a massive stack of firewood for a conventional wood stove
  • a teeny-tiny, neatly stacked half-cord of kindling, and/or a pallet of waste paper, representing the fuel required to operate a RMH for 1 cold winter
  • Perhaps we should aim to document the installation and usage of a dozen or a hundred in every major cold and temperate-climate municipality in the world so people can start to experience them first-hand.



    With this in mind,

    Considering the Myths and Misconceptions facing Rocket Mass Heaters,
    Bearing in mind their Virtues in our modern context
    And the desire to provide people the gift of a RMH Experience

    What do you feel should populate this Ultimate Rocket Mass Heater FAQ?

     
    Glenn Herbert
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    Matt Walker has almost what you asked for: Tiny House Cook Stove and Heater

    You could easily add water heating by combining this with one of the projects that does that. I would have to look around a bit to find a good one in the forum.
     
    pollinator
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    FAQ quickly devolved into evangelism and brainstorming, but that is OK.

    In my opinion, a masonry heater, while a bit more complicated in concept, is every bit as effective and offers much greater appeal to most conventional homes.

    So how to expose to the masses? A few years back, I did some checking locally to see if any bricklayers had experience with such a stove. Only one did, and it was to install a brick facade around a pre-fab masonry heater kit. By coincidence that was in a home that is only a mile from my house. I went to see the guy and got the full dope on his unit and how well it worked.

    But as for how to get one? In my opinion, what would to the trick here is to go to the offices of the local brick store........offer to do a workshop in that store (build a masonry heater right on the showroom floor), and do it with help of at least 5 different bricklayers. They need to be taught the principles. Once the thing fires up and everyone sees how well it works, it will sell itself. A masonry heater would be only marginally more expensive or difficult to build into a new home build than the conventional units going in now.

    Get that ball rolling and it would be a short step into convincing the brick kilns to build any special fire bricks needed.  Also a short step to putting together RMH kit packages the brick store can sell to bricklayers or homeowners alike. Get the core right, then let them adapt bells, etc, to those.

    Do that with every store in US that sells brick and you are on your way.
     
    Eugene Howard
    pollinator
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    BTW, would include one more  tidbit for the FAQ. When I first got interested in the RMH / masonry heater concept, first thing I did was to call my insurance agent re: coverage. His comment was what they would be looking at would be distance to combustibles. Basically there are parameters to follow and as long as you met those, it would be no problem. He didn't see at as being any different that most wood stove installs. Ultimately, it would be up to the insurance company inspector to bless or condemn.

    Same would seem to be the case with building codes. There are plenty of one of masonry fireplaces still being built. Those are not UL approved, but do follow a set of specs.
     
    Thekla McDaniels
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    I agree, Eugene, education for the craftspeople and the code enforcers are at the heart of the challenge to get acceptance of this “new“ concept.

    In my experience, code enforcers are the more problematic.  In the county where I have experience, the building inspectors do not face any kind of accountability.  They can be wrong, require expensive changes and retrofits for the builder, then, if somehow the builder can bring reason, logic, and appropriate interpretation of the code as written, the building inspector is assigned to other projects, replaced by another one.  Kinda like in the days when a priest was abusing children, they didn’t address the problem of the child molesting priest, just sent him to a community where he was unknown, so he could continue his heinous activities “unmolested” himself.

    I think as we slowly move into a saner future (in the USA), accountability for county building departments and their inspectors is something to bring to public awareness.  It would be supported by almost everyone, keeping the biases against marginalized people (wood hippies, tree huggers, greenies, environmentalists  etc) out of the conversation.  It’s not just people wanting to build rocket stove mass heaters who face difficulties with building inspectors.
     
    If I had asked people what they wanted, they would have said faster horses - Henry Ford. Tiny ad:
    Rocket Mass Heater Jamboree And Updates
    https://permies.com/t/170234/Rocket-Mass-Heater-Jamboree-Updates
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