rocket mass heater risers that do NOT work

Mud and I are working on a book.  This is the part about risers that have been tried and, we think, don't make the cut.  We strong encourage everybody to share their experiences.  We hope that this thread will grow to ten pages of information about risers that fall short in some way.


steel

Steel melts at 2600 degrees, and steel spalls at 1600 degrees.  Since we are trying to exceed 2000 degrees, we can expect a lot of spalling.

I know that there are lots and lots of youtube videos showing something that is called a rocket mass heater that is welded out of steel.  And I don’t recall seeing any that were a month old.  

Peter van den berg did build one rocket mass heater here with a steel burn tunnel (not a steel riser) and was able to get it to work with two design decisions:

a 4 inch system (cannot burn much fuel at one time, so the system never gets very warm)
air is pulled into the system in such a way that it cools the steel

For 6 inch and 8 inch risers with a steel interior, those fail universally quite quickly.


portland cement

Portland cement spalls at 600 degrees - so we aren’t even getting close to what we need.

I have heard that if you modify it to be “aircrete” (portland cement that has been “foamed” until it is an open cell material instead of a closed cell material) then it won’t spall until 1000 degrees - but the material becomes really easy to crumble.

I’m told that aircrete treated with waterglass can tolerate up to 1200 degrees F.  So we still have a long ways to go.

Note that anything with cinderblock is made from portland cement.

I wonder if there could be a shippable core that is made with portland cement on the outside of the core, providing a lot of the structural integrity and a good manifold.  Maybe aircrete or portland cement with perlite as an aggregate.  It would be lightweight, insulative and pretty strong - and would be exposed to temperatures no more than 400 degrees.



stainless steel with high temp wool wrap

The great thing about stainless steel compared to mild steel (normal steel) is that stainless steel does not spall and it has a higher melting point.  

I have a stainless steel cookie sheet.  I put the raw cookie dough on the cookie sheet and while the cookies are in the oven I hear a sudden “WHACK” inside the oven.  The cookie sheet suddenly warped.  When the cookies are done, I pull them out and the cookie sheet is no longer flat.  Two, opposite corners can touch the counter, but the other two are about an inch and a half up from the counter as if they are on a mission to make a stainless steel taco shell.  Later, the cookie sheet is flat again.  

The moral of this story is that stainless steel warps when it is heated.  There are ways to get it to not warp when it is heated.  I have ideas about what those ways are, but I am not certain they will work.  

Since stainless steel doesn’t melt until 2800 degrees, you would think it would be fine on the interior of a riser.  But instead of we have reports of stainless steel

sorta wadding itself up into a ball and sitting at the bottom of the riser - it looks like a ball of wadded up aluminum foil
kinda coming apart near the bottom of the riser (where the riser temps are highest)

With the data we have on hand, we think that using stainless steel inside the riser
  - a 6 inch system with softwood fuels could, in theory, last several years
  - a 6 inch system with black locust (or other hot burning fuels) will not last one year
  - an 8 inch system will fail within the first month or two

Interesting thing about stainless steel and perlite.  Apparently, under high temperatures, when perlite touches stainless steel, the perlite corrodes the stainless steel!  So if you try to come up with your own creative solutions with these two - keep in mind that they don’t play well together.




sawdust and clay

The idea is that the sawdust will eventually all turn to charcoal or a bit of ash in a tiny air pocket - both of which are insulative.  And since clay can tolerate temperatures to 3000 degrees, it will stand.  

Overall, not a really great insulator.  And the riser tends to last about three years.



straight cob

Cob is sand, clay and sometimes straw.   Mortar (including brick mortar) is sand and clay.  

Most sand typically melts at about 3000 degrees.  And most clay melts at temperatures much higher.

Cob does not insulate.  In this way, it is the same as firebrick or clay brick.  I am going to guess that cob might last as long as a low grade clay brick.

But cob can be free.

Where cob falls short of brick is that it needs to be made and then shaped into something tall enough.  If you are trying to do this freehand, you might need to build some up, let that dry for a day and then add more, etc.


An idea:  take an 8 inch tube and wrap it with an inch of high temp wool.  Then wrap that with 3 inches of cob.  Once it is dry and hard, wrap that with an inch of high temp wool.  Slide out the tube.  This recipe could last decades.



insulated firebrick

A lightweight firebrick that can withstand high temperatures and has some insulative value.  This means that it has more insulative value than firebrick, which isn’t saying much.  So, yeah, it is more insulative, but not much.  We have tried to use it for risers and a lot of other projects - we have yet to have a win using this stuff.  

Further, the brick is really soft.  We use firebrick in our wood feed to handle all the wood being knocked about.  I suspect that the insulated firebrick might not last a week of regular use - where the regular firebrick can go ten years or more.

It is possible that we just haven’t used it correctly, or that there was something else wrong with each project where we tried to use it.  I’m open to trying it more, but be prepared to replace it when it fails.  

Mud says:

There are a variety of different grades of insulated firebrick. I have had better success with the higher temp white insulated firebrick (as opposed to the orange insulated firebrick), but even then on an 8” system I have seen some insulated firebrick crumble to bits and others show up to ⅛” of surface erosion of the whole interior of the riser in just one heating season. Insulated firebrick is more insulative (and therefore more drafty) than dense firebrick as a riser, but it just doesn’t last as long and is a lot more expensive. It is significantly less insulative (less drafty) than ceramic wool (5 minute riser) while also being more expensive. If it is what you’ve got it will work, but lots of things work better for less money.

chimney liners

Haven’t had a win yet.  They crack and shatter.

You don't mention ‘home brew mix’  extensively used in pizza oven construction and good for around 500-600c  it is a mix of fire clay lime, sand and Portland cement.
Vermiculite board seems to have improved over the last few years, as far as I can gain it is made from clay,  water  glass and powered vermiculite. I dont know if it is made in just one factory or there are different versions in different country's?

Insulating fire brick is most definitely  made from different recipes, some much better that others but the 2600f ones I use, are very good and seem quite long lasting.
I dont kwon how long they would last in the hottest part of a rocket stove as I have only used them for shortish periods, like six months.

Ceramic blanket seems to last for many years but the latest info suggest  it is  not a heathy option at all, it has been mentioned on this forum that ‘Morgan super wool’  is a safe option but… I believe the tests relate to an un heated product?
On the ‘other forum’ and on facebook there is lots of info suggesting  once ‘ any ceramic fibre’ is  super heated it becomes a dangerous carcinogenic  product! The fibers can change in chemistry and produce very fine air born particles that can even enter the body through our skin!
I dont know how dangerous that might be but  there is no doubt in my mind that ceramic fibre is not a heathy option!

I put quite a bit of effort into replicating the claims made in an old patent I found regarding the use of a castable Foamed Clay Ceramic Firebrick recipe but have yet to succeed in making a firebrick that was both insulative AND highly durable.  I'll have to build a proper kiln in order to get the samples up to known temps in a controlled fashion.

One day I intend to get back to that.

Fox James wrote:there is lots of info suggesting  once ‘ any ceramic fibre’ is  super heated it becomes a dangerous carcinogenic  product! The fibers can change in chemistry and produce very fine air born particles that can even enter the body through our skin!
I dont know how dangerous that might be but  there is no doubt in my mind that ceramic fibre is not a heathy option!

From my limited understanding following some metal casting forums, many people using ceramic fiber blankets will coat it in some sort of high temperature clay slip, refractory cement, 'water glass', or specific products from manufacturers designed to harden high temperature work surfaces and prolong the life of forges, or in this case rocket heaters.

ITC-100 refractory coating Q&A

How much area will ITC-100 cover in a forge?

ITC-100 will cover 6 to 12 square feet per pint, or 3 to 6 square feet per half pint.  If you apply a basecoat of Satanite to your forge first, you can get by with the larger number for square feet coverage.  An additional benefit to doing this with Satanite first, is that Satanite is cheap and by building up a 1/4" layer of Satanite over you Inswool liner before applying the ITC-100 your forge will be more robust.

With inswool Liner being an "Alumina-Silica fiber blanket for high temperature applications." I'm pretty certain, as mentioned above in the portland cement section, you could probably use cement, steel, or other materials as a skeleton. Then insulate it and use refractory cement or some other coating to strengthen the wool or other insulating layer like insulating bricks. If you build it yourself and keep leftover materials then you can do your own repairs. It would seem very wise to use proper safety equipment when building or repairing anything using ceramic fiber blankets.

And let's touch on another aspect of hardening the surface of the blanket - airflow. If you went up to a house and stripped away the interior and exterior walls so all that was between you and the outdoors was a roll of unbacked fiberglass insulation, that insulation wouldn't do you much good with its ability to breathe. Especially if the wind picks up, without layers to block airflow the insulation is almost 100% worthless. Would this not be the case for loose ceramic fiber in a rocket heater riser designed to contain combustion gasses? It isn't able to insulate if it there is nothing to stop it from breathing. At that point it's just a carcinogenic blanket not doing much good in my eyes. A skeleton covered in insulation and coated to be relatively air tight seems like the way to go when using ceramic fiber blankets. They also make ceramic boards which wouldn't allow gasses to permeate and might be better for certain applications as well as quick experimentation.

"Interesting thing about stainless steel and perlite.  Apparently, under high temperatures, when perlite touches stainless steel, the perlite corrodes the stainless steel!  So if you try to come up with your own creative solutions with these two - keep in mind that they don’t play well together."

 GREAT to know! I would have never thought this was the case...I guess I better do more homework. Thank you for the information.

Hello.  Thanks for listing the fails.  This can save a person lots of time.  On the theme of cement materials, have you tried papercrete?  If you put sand into the mix, I've read online that it's fireproof.  I haven't tested this myself, but I have been building a few things with papercrete.  I am planning on building a rocket stove pizza oven with the basic structure made of papercrete and the inside oven just a used metal barrel (one that contained food items before being resold to me).  This won't happen for a few months so I'm afraid I can't offer any personal experience of using papercrete in a rocket stove structure.......yet.

How many season does it need to last to be acceptable?
Does the cancerous potential of the ceramic fiber blankets put them on the naughty list?
I wonder if adding rockwool to the clay/sawdust mixture would extend its working life.
Water glass on the inner surface might help.

I work with ceramic fiber products quite often as I build Pizza ovens, apart from taking the obvious safety precautions, the end product is encased in cement and not exposed to any naked flame or super intense heat.

So I feel fairly safe about not polluting the environment long term,  however, even when wearing a suit and a mask and also being careful to gently handle the fiber, fibers get air born in huge quantities, you can  easily see them in sun light!

Just taking off my suit will cause lots of airborne fiber dust but, I am led to believe this dust is not half as dangerous as the fibers that have been super heated and crystallized  (probably not the correct term) as they are then a more permanent structure that will not dissolve in the body and can remain dormant resting in lungs and other organs.

Now I am no expert, I am just abbreviating what I have read into layman’s terms.

I have used ceramic fiber in 5min risers and there is no doubt  that it works extremely well, I have tried coating it with water glass, that is not easy on the matting but quite easy on the board, but that only seems to have limited success as it eventually crumbles off.
There does seem to be some new or products coming on the market that might be more suitable for coating  ceramic wool, but I have not tried these.

paul wheaton wrote:
straight cob

An idea:  take an 8 inch tube and wrap it with an inch of high temp wool.  Then wrap that with 3 inches of cob.  Once it is dry and hard, wrap that with an inch of high temp wool.  Slide out the tube.  This recipe could last decades.

My thinking on this is that only some high quality cob mixtures (really good firing clay, good sharp sand, etc) would withstand this treatment, and even then, would likely be quite crumbly. From my experience, using my "as found on-site" quality cob mix (a bit subpar by anyone's measure!), even the temperature of gasses reaching the bottom of the barrel cause the clay to fire and become unstable/crumbly without adding ash and grog to strengthen it. Perhaps taking that into account and building with this in mind, along with a thicker layer of insulation on the inside (2 inches wrap inside, 1/2 inch outside wrap) would improve the durability.

Also, as noted in the 5 minute riser thread, the laminar flow issues of fiber blanket directly exposed to the interior of the heat riser seem to call for a clay slip layer to add smoothness, reduce permeability and improve durability.

I think I just might have to rebuild myself a heat riser with this thinking. Only major issue I'm seeing is the thickness of this riser - 2" interior wrap, 3" cob, 1/2" exterior wrap...that's 5.5" of riser thickness. Too thick and we wont be able to fit a 55 gal drum over that thing! ;)

Perhaps we can call it the "forever riser"

paul wheaton wrote:Peter van den berg did build one rocket mass heater here with a steel burn tunnel (not a steel riser) and was able to get it to work with two design decisions:

a 4 inch system (cannot burn much fuel at one time, so the system never gets very warm)
air is pulled into the system in such a way that it cools the steel.

Sorry, this is not entirely correct. The top half of the feed tube was made of steel with a built-in secondary air provision. Which allowed it to be cooled below spalling temperature. The hottest parts, being the lower half of the feed tube, all of the burn tunnel and lower part of the riser were all made out of refractory concrete.

During experimenting in the years before, I welded a complete steel 4" J-tube. The bottom part of the feed tube spalled heavily, so I reckon that would be the most vulnerable spot. Mark this was a small system, it might be different in larger systems.

Thanks for the clarification Peter.

You know that I am talking about minnie mouse, right?  I didn't know that minnie mouse had refractory cement inside.

Yes, it's about Minnie Mouse. It's the only one J-tube I did at permies.

I used insulated firebrick for the heat riser of mine.  It's a 6 inch system and I believe the firebrick was rated for either 2600 or 2700 degrees F.  This has been my 4th heating season with it.  While I haven't yet opened up the barrel to clean and inspect after this winter's burn I haven't noticed any problems.  When cleaning prior to the start of this heating season the bricks all looked to be in excellent shape.

I can say the firebricks were expensive.  

Insulating firebricks are made to line the inside of pottery kilns which can get to 2300F for several hours. They last for many years in this use (including gas or wood firing). You do need the right grade of IFB, as lower quality ones will soften or distort at those temperatures.

Cob from local clay may be able to stand lower ranges of RMH riser use depending on the locality. Mine fires hard and strong at around 1800F, shrinks and distorts some as it vitrifies around 2000F, and melts by 2300F. Clay in traditional stoneware pottery regions can be good straight from the ground to 2300-2400F, maybe more (that is the temperature the pottery is fired to while remaining perfect.) If your local clay is silty or has other drawbacks, it may not even be good to 1800F.

Glenn Herbert wrote:Cob from local clay may be able to stand lower ranges of RMH riser use depending on the locality. Mine fires hard and strong at around 1800F, shrinks and distorts some as it vitrifies around 2000F, and melts by 2300F. Clay in traditional stoneware pottery regions can be good straight from the ground to 2300-2400F, maybe more (that is the temperature the pottery is fired to while remaining perfect.) If your local clay is silty or has other drawbacks, it may not even be good to 1800F.

Hey, this is my first message here. I'd like to thank you for all the detailed contributions you've made so far!

The idea to built a heater so sufficient, low cost and kind on wood is very appealing.  Especially as I would like to extend this blessing to poor Lebanese and the Syrian refugees, who are burning plastic, clothing and black motor oil...

Limitations are therefor money in general. Also, everything is built with 4inch / 10cm pipes. Those are openings in the wall of apartment buildings to which diesel heaters or wood heaters are attached.

So, I'm looking to built a prototype 4inch system, little mass to prevent draft problems.

Since those can't get really hot it seems, a cheap cob riser would suffice?
How would I insulate the riser however? Since I don't want to use ceramic wool (maybe hard to get here, possibly toxic and expensive for locals).
Same goes for the burn chamber, it should be insulated to get to high temperatures.
Basically I'm wondering if I can get away with mostly using cob and still get that second burn.
I will inquire about what local pizza bakers use and how hot it gets in those ovens.

It would be amazing if poor people would be able to use twigs to heat their homes!!!

Hi Cedars and welcome to permies! Given the situation, how big of a deal would it be if you had to possibly repair or rebuild a riser occasionally? What I'm thinking is that a cob riser with some insulating aggregate, such as scoria, would be good enough for a 4" system and if it was treated as something that was only expected to last one or two seasons then the need to replace would just be part of the maintenance routine.

Cedars of Lebanon wrote:

Glenn Herbert wrote:Cob from local clay may be able to stand lower ranges of RMH riser use depending on the locality. Mine fires hard and strong at around 1800F, shrinks and distorts some as it vitrifies around 2000F, and melts by 2300F. Clay in traditional stoneware pottery regions can be good straight from the ground to 2300-2400F, maybe more (that is the temperature the pottery is fired to while remaining perfect.) If your local clay is silty or has other drawbacks, it may not even be good to 1800F.

Hey, this is my first message here. I'd like to thank you for all the detailed contributions you've made so far!

The idea to built a heater so sufficient, low cost and kind on wood is very appealing.  Especially as I would like to extend this blessing to poor Lebanese and the Syrian refugees, who are burning plastic, clothing and black motor oil...

Limitations are therefor money in general. Also, everything is built with 4inch / 10cm pipes. Those are openings in the wall of apartment buildings to which diesel heaters or wood heaters are attached.

So, I'm looking to built a prototype 4inch system, little mass to prevent draft problems.

Since those can't get really hot it seems, a cheap cob riser would suffice?
How would I insulate the riser however? Since I don't want to use ceramic wool (maybe hard to get here, possibly toxic and expensive for locals).
Same goes for the burn chamber, it should be insulated to get to high temperatures.
Basically I'm wondering if I can get away with mostly using cob and still get that second burn.
I will inquire about what local pizza bakers use and how hot it gets in those ovens.

It would be amazing if poor people would be able to use twigs to heat their homes!!!

Might be a good idea to move this to its own thread for further discussion and guidance, but my thinking is that insulating, since it's probably the most important aspect of getting a good clean (and therefore efficient) burn, needs to be "done right". Perlite and vermiculite are serviceable insulating materials for the outside of the riser and core, and both are likely available "locally" in your region from mining operations (so not overly expensive), but going cheap and light on the insulation is where I myself have gotten into trouble in the past (smokey inefficient burns, loss of draft / reversal of flow, and so on). Insulation is what makes these critters work

Thank you Phil and Tristan!
I've started another thread as suggested.
The answers to your suggestions and extra information has been added there.
I hope you can find the thread, the header is: "Rocket heater from cob and cheap insulating materials that poor Lebanese and Syrians can make"

Hello people,

Another for the list… porcelain ‘plank’ tiles, looked attractive as they come in the perfect length (900mm).. and just a little too wide, allowing them to be assembled in the ‘sundial’ fashion …
I built up a nice looking riser and insulated with 1” ceramic fibre blanket

Long story short, worked for a week then (as predicted by many)  POP, next day, cracks are obvious …

It was worth an experiment (I thought)

Hi Nick, welcome to the forum, you will find some mixed opinions on this site but, generally speaking you will get far better advice than facebook can offer.
We all like to see new projects on the go so so dont hold back posting you stoves journey on the form.