using metal in the burn tunnel and heat riser

OK
Once again we have been asked to weigh in on this subject

I will try to be somewhat brief.
The only metal I know of that will handle the temperature and corrosive gasses in the combustion unit of an RMH for very long is toungston.
Every SS and high temperature steel degrades very quickly at 1200 to 3000 degree's as is found in the  burn unit of a well built RMH.

Test after test conferms this. The metalergy conferms this the chemistry conferms this.

No matter the metal except the one mentioned burns out in ( for an RMH) a very short timeframe. Most will last a few months  some might last a year or two.

At this point I could give you all the sciannce and literature to support my claim but you can find it just as well as I can. So please do so.
The only way to protect the metallic components from failing is to place a barrier of masonry between the fire and the steel. And this only works for a short time since the steel is inside the insulation layer of the burn unit. The metals simply cannot handle the sustained temperature's.

I know it is easy to weld up a burn tunnel and heat riser, I KNOW it looks like it will work fine because all those dirty burning free standing stoves and grills work just fine. But the RMH isn't a Webber or a free standing wood stove with a 5 year life cycle. An RMH is designed to last for more than 100 years with as little maintenance as possible in its life time.

Some folks think that all of these manufactured materials must be good because they must meet industry specifications. The problem is many don't look at how they meet industry specifications. I have never seen a primary boiler that was not lined with some sort of masonry unless it was water jacketed and all of those units either burn dirty or automatically reduce temps when the pressures get high.

Those not lined are also as a rule gas or diesel fired and the chamber is very small with a heat exchanger set up to ensure the temperature never goes a over the boiling point of water.
In those systems nothing  ever gets red hot unless it is some bit of easily replaced sheet stock.

Someone is going to point out founderies that use cast iron crucibles to melt various materials. Yes they do and those cast iron vessels are very very thick and never get exposed to temps above the melting point of the cast iron.
Those Crucibles also are never exposed to actual fire either. Instead a couple of electrodes are put in that arc between And melt the materials. Not to mention   most steel's melt at a far lower temp than cast.
Even with all of this founderies replace crucibles regelerly.

To sum up you can find all the info on what steel will handle, what corrosion resistants it has at what temperature's, how long it can last in a reducing atmosphere, ECT. None of them beat a brick for durability in the environment we are building in an RMH.

There are lots of other problems with steel in the heat but I am limiting things to this one subject.

You are building a stove that has a lifetime steel's lifetime is much shorter in sustained high temperature. So unless you want to replace heat risers and burn tunnel's just about every year steel is a bad choice.

If you can afford an 8 inch toungston heat riser and burn tunnel more power to you, congratulations you have so much money you could use it as fuel..

Some few of you are boiling because all these folks are telling you what to do some have gotten angry enough to write a publication that you don't need to listen to any one on this subject and can have it all YOUR way. You can  but don't expect the folks who are dedicated to making the very best and longest lasting stove possible to be very forgiving when you start talking about how the RMH you built isn't performing up to snuff..

Any way thanks for reading.

Ernie Wisner wrote:
Every SS and high temperature steel degrades very quickly at 1200 to 3000 degree's as is found in the  burn unit of a well built RMH.

...
Yes, this is very true. However, elsewhere you have stated that temps in many of your own RMH's are estimated at 1100F in the burn tunnel and 400-700F at barrel top.  In my all steel, non-insulated and air-cooled core.... flame temps in the burn tunnel and lower heat riser have been demonstrated visibly on my other threads on this forum (by melting aluminum) to be in the 1300F range and my barrel top temps run 500-700F and yet my core temps do not go above 900F! True, my RMH is only in its first year of operation but there is no sign of spalling. It is only when cores are insulated and heat cannot be dispelled from them that their temps rise into ranges where steel corrodes.

So all I'm saying is that a steel RMH can operate quite efficiently with flame path temps of 1300F without the core temps exceeding 900F -IF the core is not insulated and is equipped with air cooling fins in the areas adjacent to initial combustion in the burn tube.

BTW, are the temps (which you mention above) in the area of 3000F found in J-tube RMH's or in batch rockets? There is a world of difference between the two, would you not agree?

There's also a variation on the Dakota pit stove with a longer chimney under the pot, sometimes called a "fox stove."
And ancient furnaces were often of the L-shaped configuration, with a LARGE firebox containing the bricks, ceramic crafts, metal for smelting, or whatever else was the target of the heat.
Even a "chimanea" might be considered as a kind of jug-stove or proto-rocket.

This has gotten kinda long, but I felt like chiming in on the definition of "rocket," and since it's an evolving term I ended up writing quite a lot.

The Permies members have started using the term "rocket mass heater" in preference for "rocket stove" since it is more specific.  
However, I would say that "rocket mass heater" is still a subset of "rocket stove."

Since we cannot force people who invented the term to stop using it for the entire range of their inventions, it seems a little pointless to demand "correct" usage of late-comers to the field.  
If you are aware of the distinction, and wish to discuss the L-style cooking rockets, perhaps "rocket cook-stove" would be a short but specific term to use.

If the heater does not have a masonry or earth or water thermal mass, it's definitely not a rocket "Mass Heater."  
Prototypes for low-mass or no-mass heaters with the J- or L-shape made from thick steel, generally very short-lived in actual use.  But I might call these a "rocket radiant heater," or a "hybrid rocket experiment."  "Rocket wood stove" just seems likely to be confusing.

What is not a rocket?
If it does not contain any insulation of any kind, and if it does not have a vertical smoke-burning part of the firebox (internal chimney or "heat riser"), someone has radically mis-understood the rocket concept.
I allow a little bit of leeway for Matt Walker's "riserless core," since it derived from rocket experiments, however I think he is heading back away from rockets toward baffled wood stoves and cook stoves.
I don't imagine other tinkerers can make clean-burning adaptations of his designs without your own emissions meter to confirm success.

"Rocket" does not simply mean clean-burning stove, or firebox for a masonry heater, or bouquet of flames.  It refers to a specific set of design paths.

As I understand it, the word "rocket" came into use among the researchers playing with this stuff in the 1970s, both as a reference to space-age concepts and insulation materials now available, and to the "whooshing" sound the fire makes when traveling fast through the constrained tube.  (You can get similar sounds from a bonfire but it takes a lot more fuel.)  Jet engines actually use some similar concepts, only with an air scoop instead of a thermal siphon.

The common denominator for rocket stove design seems to be a vertical, insulated "heat riser" or internal chimney, which helps to burn the smoke and concentrate the heat of the flame before it is directed toward the functional target.  You can have a rocket that heats water, cooks food, heats a thermal mass, warms an oven, or even one that just incinerates things and wastes a lot of heat.
Most rocket cookstoves worthy of the name include some kind of insulation, as do rocket mass heaters and rocket masonry stoves/cookstoves.

To work a different angle, if we are only talking about masonry, mass, or "accumulation" heaters, would we use the word "stove"?
Masonry heaters are often called masonry stoves.  In Europe the category is sometimes called "accumulation stoves" or "accumulation heaters," or some variation.  There are many masonry heaters that also are used for cooking, or especially for baking, including an oven above the heater's fire box.  Because even the large-mass accumulation heaters generally heat in line-of-sight or by direct contact, they are sometimes considered stoves or radiant heaters rather than "furnaces"... a category described elsewhere.
With these mass/accumulation heaters, the exhaust from the fire will be passed through heat-exchange channels or bells (stratification chambers) for heat collection.  It is not just ecological or efficient, but CRITICAL for safety that the fire burn very clean.  A dirty, smoky fire will cause the mass to "accumulate" creosote as it stores heat, becoming a fire hazard, and necessitating frequent, expensive, and destructive cleaning.  

We have now heard about 2 "rocket mass heaters" that were designed outside the proven parameters, which accumulated creosote.  The owners reported two chimney fires in a single heating season, and are now troubleshooting their heater for unintended design errors.  Luckily, both owners had used relatively good chimney construction, which prevented the chimney fire from burning down their house.

If you want efficient heat, and especially if you want to store that heat overnight, the smoke MUST be burned as completely as possible.
Therefore with most masonry heaters, mass heaters, or accumulation heaters, including masonry cookstoves, you see a big emphasis on a clean-burning firebox, and on proper balancing of draft and mass so that the fire runs at roughly the speed it was designed to run.

In the context of metal used (inappropriately) in the firebox, the reason the ambiguity of "rocket stove" pushes some people's buttons is that many people get the metal firebox idea from "rocket stoves" designed for cooking.  
People who design "clean-burning" or "efficient" rocket cookstoves often have lower standards than we cold-climate designers would consider acceptable for an indoor heater.  
Rocket cook-stove gurus may declare success if they reduce the smoke even slightly compared with an open hearth, campfire, or barbecue pit.  The goal of smoke-free or soot-free cooking may be sacrificed in favor of other goals like extremely low cost, portable/camp stoves, convenient cooking height, etc.  Because these outdoor cookstoves often have not even a chimney, the accumulation of soot and creosote, or the occasional "chimney fire" where this creosote burns along with the fresh wood, is not a big concern for the builders.  
 The smallest of these "rocket stoves" are toys or models rather than stoves - made with two steel cans, to demonstrate the chimney effect, or perhaps 3 or 4 cans to create an insulation gap which may be filled with air, perlite, or wood ash.
While these micro "stoves" can burn slightly cleaner than a smoldering stick of wood, they are rarely smoke-free enough for indoor cooking.  Fire and insulation simply don't scale well down to that small size.
Larger, institutional models of rocket stove, and better-insulated camp stoves such as the "jug stove" or termite-mound tea stoves, can burn a bit cleaner.  
I have not personally seen one that burns clean enough that it would never leave soot on the bottom of the pot or cook plate, but it is possible they may exist somewhere.
 
I have also not personally seen an L-style rocket firebox that burned clean enough to power a creosote-free heat-exchange for a mass heater, without also dumping smoke into the room.  
Batch box "rockets" without a proper door and secondary air tend to smoke out both ends.  Hybrids between two different designs can get ugly, quickly.

It has become very common to see people learning about these relatively primitive, cooking "rocket stoves," and then hear about the "rocket mass heater," and simply try to attach a mass to their cookstove's exhaust.  They often do not realize they are leaping from one design path to another, or that there are a LOT of prior examples and information available to avoid repeating known errors.  
This is kind of like living with bicycles, hearing about cars and semi-trucks, and trying to put together a trailer hitch that will allow a rickshaw "engine" to pull a semi-tractor's trailer load.
In the case of the bicycle, the most likely failure mode is that you will not go anywhere, and will do a lot of extra work for no gain.  You might manage to crush yourself, but it's unlikely you would crush others.
In the case of mis-matched firebox and heating load, the most likely failure modes involve smoke in the home, creosote in the mass, and significant dangers to health and longevity for both the occupants and the building.

It is possible for one bad failure with a DIY indoor heater to wipe out a whole branch of the family line.  

It is also possible, and far more likely, for a problem to smolder undetected by the original designer, only to pop up and create a problem for subsequent generations.  Or in the most awkward season; most masonry materials can't be properly built or repaired in below-freezing temperatures.

I have witnessed a few near misses, where Grandpa's old wood stove installation was slowly charring roof purlins behind its metal shield.  If Grandson had simply insulated the roof (as he intended), instead of also replacing the stove with an incidentally larger chimney, he may have lost the whole building within a year.  
Steel well-casing is not a chimney "upgrade," for many reasons.  Neither galvanized nor steel ducting is a proper chimney liner.  
The word "rocket" does not IN ANY WAY relieve you of responsibility to take precautions. Such as appropriate clearances and heat shielding, against the possibility of a chimney fire.

It is an unfortunate artifact of English usage that the words "you can" are often used generically, to mean "it is possible" or "someone once did."  They can be misleading if taken personally, since "you" may not have favorable skills, materials, understanding, or the right situation.    
"You can" build a rocket stove with metal (as in rocket cookstove, assuming you have the metal skills to operate a can opener, and don't mind re-building every few weeks/months of heavy use).
"You can" build a rocket mass heater which will give clean, efficient heat for decades (if you personally take the time to learn a few basic masonry skills, and research and follow a proven design.  "You" careful farmers may actually have better results than some of "you" trained engineers, because "you" farmers may be less tempted to alter the proven design with no experience).  
However, I doubt "you can" build a durable, efficient, smoke-free, safe-to-operate rocket mass heater with metal in the firebox.  
Might happen some day, but the amount of energy that will go into obtaining and working those metals may well exceed the entire lifetime energy savings of the "efficient" stove design.

"Someone once did" build a few rocket mass heaters with very short chimneys, and others with a horizontal exhaust pointing downwind.  But this does not mean that "you can" count on this party-trick design working adequately in your actual situation.

So "you can" build with metal, and "you can" call a rocket mass heater a rocket stove.  
But we would very much prefer if you didn't call random metal stove experiments a "rocket mass heater," especially if it is based on unproven ideas, or on some other non-mass-heating prototype which has never proven itself over a full heating season.

I still like Paul's term for new experiments in this field: "Freak Show of Flaming Death."
I have made some of my living these past 10 years by entertaining crowds with dare-devil fire tricks and my husband's hard-won ability to handle hot objects with his bear hands (pun intended, he has caluses that frustrate carpenter ants).  
So we do resemble this remark.

Good judgement comes from experience, and a lot of that [experience] comes from bad judgement.*    

Ernie and I have put in a lot of experience-time prototyping with metal, replacing crapped-out metal, repairing masonry damaged by crapped-out metal, troubleshooting a chimney fire due in part to crapped-out metal, and inspecting other people's labor-intensive metal prototypes (which generally crapped out within one season of use, or were discarded before then).  
The charts' predictions about the type and temperature of metal failure seem to correspond with our experience.  We have had some embarrassing repeat visits to clients whose stoves needed repair due to our early optimism about high-grade metal parts.

We have also seen other kinds of failure from metal/masonry hybrids, due to incompatible thermal expansion differences between most kinds of metal and most kinds of masonry.

So if you care to take the benefit of our experience, you may save yourself some trouble.

If you LOVE working with metal and want to help solve these problems in a productive way, I have a LOT of project ideas for pairing high-quality metalwork OUTSIDE the firebox with high-quality refractory materials inside.

Thanks for reading.

Yours,
Erica W

*'experience' quote sometimes attributed to Will Rogers; I think another version was penned by Mark Twain.

Hi Erica,

Thanks for your lengthy and detailed response above.

I must not have communicated well in my previous comment.

What I had in mind was the major difference between 6" - 8" "J style" RMH's (many of which you have built and describe as running at approximately 1100F in the burn tunnel and 400-700F at the barrel top) and batch boxes which burn a much higher volume of wood in a much larger combustion chamber and thus produce much higher temperatures (up to 3000F) for shorter periods of time.

My only contention is that a properly designed air cooled, non insulated, steel "J style" core can run with flame path temps even hotter than 1100F without the steel core exceeding 900F. Such a unit can efficiently heat a mass or masonry bell without scalling of the steel core.

If  an "insulated core" or a "non-metallic core" is a determining factor which eliminates an air-cooled, non-insulated steel core which heats a mass from the designation of "Rocket mass heater", so be it.  It just seems to me that it's a pretty restricted definition!

Thanks again for your input.
Bruce

Bruce Woodford wrote:Hi Erica,

Thanks for your lengthy and detailed response above.

I must not have communicated well in my previous comment.

What I had in mind was the major difference between 6" - 8" "J style" RMH's (many of which you have built and describe as running at approximately 1100F in the burn tunnel and 400-700F at the barrel top) and batch boxes which burn a much higher volume of wood in a much larger combustion chamber and thus produce much higher temperatures (up to 3000F) for shorter periods of time.

My only contention is that a properly designed air cooled, non insulated, steel "J style" core can run with flame path temps even hotter than 1100F without the steel core exceeding 900F. Such a unit can efficiently heat a mass or masonry bell without scalling of the steel core.

If  an "insulated core" or a "non-metallic core" is a determining factor which eliminates an air-cooled, non-insulated steel core which heats a mass from the designation of "Rocket mass heater", so be it.  It just seems to me that it's a pretty restricted definition!

Thanks again for your input.
Bruce

Hi Bruce, I think we may have cross-posted.  I wasn't responding directly to your comment, but to one that I had seen a few posts prior when I started typing that lengthy reply.

I've seen an air-cooled steel core for a 4" system.  There are some guys working very hard on this concept, and they are getting ready to experiment with very thin refractory coatings, because the steel is at its limit.
It's neat to hear that you have a lower-temp metal J-style rocket design that is holding up well.  Have you had a chance to test the emissions for smoke, or run the exhaust down to masonry-heater chimney temps (200-300 F, below 350 F which is required for smoky woodstoves) so you can check for creosote = unburned volatiles?

I'm not sure where you get the figure of 1100 F for J-style.  That's a minimum for clean combustion in my experience. You might be conflating it with 1100 C, which is about the max burn without burning nitrogen = 2300F.

If the fire box burns that warm consistently all the time, with variable natural wood fuels, there is a good chance the fire will burn much hotter at the peak of the burn rate.  Especially in climates with extreme seasonal temperature differences = larger differences in draft.
We have a lot of evidence that suggests temperatures of 2300 F are exceeded in 6" or 8" rocket cores, on the interior surface of the firebox, depending on the insulation and length of burn (melted perlite nodules, pottery cones, irrigation and color changes and ceramic fusing in clay/firebrick parts).
We have guess-estimates from people who've worked with hotter industrial applications that with these temps at the edges, the center flame path may be in excess of 3000 degrees F, possibly in excess of 3600 F on some of the super-insulated riser versions.  At these temperatures, nitrogen in the air burns along with the fuel, and we are into a different kind of pollution.  (but also certain other industrial possibilities...)

It may be possible to keep a higher-mass, brick-lined stove, or the air-cooled metal stove you suggest, closer to the target range.   But I think trying to protect a metal interior firebox drives your target range unnecessarily low, and increases the chances of a cool fire and smoke/creosote pollution.  If your metal sides of the firebox never get into the clean burn range, there's a certain implication that you are always feeding a "sleeve" of smoke along the outside of the flame path to protect the metal.  This sleeve-like smoke layer would likely deposit creosote if you were going to put it through a cool chimney, or cause health problems if the exhaust was being released into primitive kitchens as with some rocket cookstoves.

I would say 1200-1800 F would be my ideal temperature range, always hot enough to burn clean, but never burning nitrogen.  
950-2300 F is your actual acceptable outlier parameter, allowing for weird weather or fuel conditions that may swing outside the design/testing conditions.  1200-1800 F is your everyday working target with your common local fuels.
Unfortunately, this turns out to be a small window compared with the naturally variable fuel values between natural wood.  Even the outside parameter temperature range is less than 2x the difference in Kelvin.  Natural wood fuels can have 3x or 4x difference in caloric value per volume between different kinds of dry wood, let alone wet or dry-ish wood from different woodpiles and seasons.  It's hard to keep the minimum and max. temp that close together with natural fuels and grid-free, solid-state 'controls.' Adding gizmos just creates more possible fail points.  Letting the stove have natural draft, where more draft pulls in more air, can be almost self-regulating without a lot of moving parts.

It is possible that some people would call an un-insulated, air-cooled metal thermal siphon a rocket.  The "pocket rockets" and some of the camp cookstove demo rockets are just plain metal, and there are people working on permanent versions of these things using thicker metal and workarounds.  (The thin metal tends to burn out rapidly; the thick metal spalls and blisters on the surface but may last proportionally longer if the scale happens to adequately protect the remaining thickness; refractory "paints" and coatings may be able to drop the temperature and oxygen exposure by protecting the metal surface.)

However, since the rocket name came from a team that was working to clean up unnecessary smoke, and the insulation and hot burn temps are a critical part of cleaning up that smoke, I think it's worth including in the definition.   People can still argue how their firebox works around this performance characteristic in a clever way.  Sort of putting uninsulated "rockets" in air quotes.  

For example, when I say insulation, I don't mean you're required to use a specific material.  All materials have an R-value; enough brick or clay would have some insulation value, even if it was a lot thicker than the equivalent fire blanket or rock wool or perlite insulation.  So if you put say a couple mil of refractory fabric, but it had the effect of reflecting back most of the heat and keeping the metal substantially cooler than the fire itself, that would effectively be insulation value.  Trapped air has insulation value.

I'm also not ruling out the use of metal entirely - it certainly has its uses in many parts of the rocket experimentation world.  
Just asserting my opinion that affordable, low-eco-footprint metals (or even fancy factory-fresh ones from the chart) are a poor match for continuous performance in the target temperature range for clean fire, especially with variable natural fuels.
That's why I said that a metal that can serve as the interior lining of any clean-burning firebox will be pretty impressive.  Titanium or tungsten or ultra-performance steel, or foamed metals, seem like they might carry higher energy costs and pollution loads in manufacture and disposal, and they are also rarer and thus more expensive.  All of this leads to a bigger financial and ecological "debt" to work off, embodied energy vs. energy savings over the lifetime of the stove.  

Peter van den Berg's air-cooled secondary air intake in the batch boxes may be an exception or case in point.  Perhaps you can cool one particular area enough that metal can survive there, while enhancing the overall function of the stove.  Perhaps that metal will burn out too, just slower.  Some of the secondary-air feeds I've seen for sidewinders are deliberately sacrificial, easily-replaced parts, because air cooling doesn't protect metal that much.
Likewise, our barrels burn out far slower than the Mother Earth News' barrel-stove designs.  We have barrels still in use after 10-30 years; most barrels-full-of-fire last about 3 or 4 seasons of regular use, tops.  
But I would still expect the barrel's steel to be a weak point if you look at the lifespan of masonry heaters in centuries, rather than seasons.  I'm interested in stories of successful rocket-bell-stove hybrids, where the rocket J-tube is surrounded in a masonry bell instead of metal barrel.

Even modern masonry heaters often design their firebrick fireboxes so the brick can be fully replaced every 20 years or so.  
Clean wood fire is about as hot as flowing lava, and about as destructive to most materials.

Just my opinion.  I didn't invent the terms, and I would love to think they'll still be growing and changing long after I'm gone.

-Erica

All that said, with engineering you have a design load and then a safety factor.
If your design load and your fail point are too close together, there's no room for error.

When I am choosing refractory liner material for inside the firebox, I am generally willing to shell out more coin for something rated for 2400F or hotter.  Fire brick is generally rated between that and 2700 F.  
If I have to pay a lot more to go from 2100 to 2400 to 2800 to 3100 F, I will generally go to 2400 F before I even start testing, and upgrade from there if I have a failure.

If you don't care about clean burn because you're running a hot chimney (350+ F like for woodstoves, to avoid creosote deposition), you might not need to burn as clean as I do.
But unburned smoke = wasted fuel.  I think the higher-temp, cleaner-burning fireboxes will still win on efficiency when most other things are equal.

This is the area where Ernie and I spend our time; working toward a firebox that gets hot quick and stays hot throughout the burn cycle.  We do this with a balance of insulation and thermal mass.
I have a hard time imagining a metal firebox whose engineering attention has mostly gone into protecting the metal, operating just at the flicker point between smoky fire and metal decay, never dropping below the clean burn point and letting the wood cool and smolder.  

Brick with insulation around it will store and re-radiate heat to keep the wood burning bright and hot toward the end of a fire session, which is a feature I love, especially when I get started typing and neglect to re-load my stove in a timely manner.

Thanks for listening.
I do respect the efforts of people who are trying to push this experiment further than it's been taken before.  
After getting burned a few times using "high-grade" metal components, the lesson I learned was that it wasn't worth trying. "I know better now."
But that might not be true for someone with a greater love and skill in metalworking.  I try to keep an open mind, always interested to see new variations.  

If I sound discouraging, it's because I don't want new folks to go down this path without a clear understanding of the serious practical difficulties involved, or the risks of catastrophic failure.  These are experiments to do in non-combustible areas, with good safety clearances, etc.

I really appreciate the full-year-later reports, they are way more valuable than baby pictures and initial enthusiasm alone.
If you can go past 5 years of seasonal heating without warping a metal firebox, you'll be doing better than a lot of conventional wood stoves.

I so wish I had seen this thread four years ago, when I built my rocket mass heater.  I had Ianto's book, and he allows that a thick steel tube can be used for the inside of the heat riser, so that is what I need.  I don't even know the nature of the steel.  Something over 1/4"thick and heavy from the metal salvage yard that happened to be exactly the correct length.  I thought I was so lucky.  Then I read of imminent failure.  It has been 3 years of use, now. Probably about 400 hours or so of burn time (zone 7a), and here I am about to enter the burn season.
So, the question is, WHAT SHOULD I DO?  Should I tear it apart for a complete rebuild?  I'm leaning towards trying to make it through another winter before such a big project.
Thoughts?

Hi Laura Kelly.

Well, can you remove the barrel easily? If yes, check your heat riser. If it's a J tube, you might be at less risk. than a batch rocket. Have you ever seen metal flakes in your ashes when cleaning? If the barrel is sealed at the bottom, with no removable cap. Another trick to check the heat riser. Is to use two little mirrors in the burn tunnel, like a rear view periscope. Depending on what you have around the heat riser, you might even not notice that your metal tube has failed. Fired clay holds it's shape rather well in some cases.

Hth.

Max.

If you can reach a camera far enough into the burn tube and point it up the heat riser, you can "see" what is going on with your riser. That's what I do with mine (a metal riser).

Hi all,
Not really sure where to post this. On the surface it's just another RMH build.
But check out the guy's custom refactory material.


Sodium Silicate:

Sodium Hydroxide: 200g
Silica Gel: 300g
Distilled Water: 500ml

Sodium Silicate Wiki

Here is a follow up analysis of the refactory material:

I saw that.   I have lots and lots of concerns about that particular build.   The important thing is:   will it hold up for a couple of winters?   I suspect it won't make it a couple of weeks, but I am interested in seeing how long it will hold up.

Are you suspicious of the refactory mix degrading, or the design in general?

Nick Kitchener wrote:Are you suspicious of the refactory mix degrading, or the design in general?

Yes.  

Naturally, I would do it a very different way.  Mostly because I have seen too many metal cores spall out or melt out.  And I have seen portland cement spall out at such a freaky low temp.  So I just feel like metal and portland cement in the core will not last.   And you can make it last longer with some tricks - but I kinda think those tricks are not enough.     But maybe this guy knows all this stuff and his tricks are trickier than the tricks in my head and it will work great.  If nothing else, it seems like he is doing a great job of documenting his experiments - so if it all fails, we will get details on how long it lasted.

Maybe a refactory of simple perlite / vermiculite glued together with Sodium Silicate. Do away with portland cement entirely...

We are currently seeing failures with perlite and vermiculite.   These materials are fine where the temps are lower than 1400, but no longer for the inside of the core.

You're getting into Aluminum Oxide ceramics territory then...

paul wheaton wrote:We are currently seeing failures with perlite and vermiculite.   These materials are fine where the temps are lower than 1400, but no longer for the inside of the core.

You know... diatomaceous earth is essentially Silicon Dioxide. I wonder if an "aircrete" made from DE bound with Sodium Silicate (water glass) would perform better. It would be much harder than perlite or vermiculite.

Nick Kitchener wrote:

paul wheaton wrote:We are currently seeing failures with perlite and vermiculite.   These materials are fine where the temps are lower than 1400, but no longer for the inside of the core.

You know... diatomaceous earth is essentially Silicon Dioxide. I wonder if an "aircrete" made from DE bound with Sodium Silicate (water glass) would perform better. It would be much harder than perlite or vermiculite.

I think that a six inch core made from duraboard and sprayed with the foam cement would work excellently and last decades (assuming the wood feed is fire brick).

I posted a couple months ago about my steel pipe core RMH.  I did as Bruce Woodford suggested and used a mirrors and lights to peek up into the tube.  Couldn't see any differentiation in the dark, smooth surface.  Decided to cross my fingers and head into another heating season.  I have never seen metal flakes when I clean the pipes, and we really don't burn that much.  Maybe 3 rick a season.  Usually short burns, like hour or two at most, as it just doesn't get cold enough here, so I'm hoping that I've actually got two more seasons before I take apart and rebuild.  The barrel is pretty embedded, so its a job I'm not anxious to begin!

Hi Laura;  When the time comes for a rebuild , you will be pleasantly surprised at how easy cob comes off and then reconstitutes for reuse with water !    

A bit of video on this topic in our 4-dvd set

I have been researching and studying RMH designs for home heating use. I have seen many designs using metal in the core. When you take metals to high temperatures, they tend to melt, but before that, they give off metal vapor. The clouds you see coming of a welding tip is just that, metal in the form of smoke. It is one of the reasons welders develop respiratory problems "metal fume fever", from the metal vapors condensing inside their lungs when they inhale the smoke. People tend to think of metals as solid and immutable, but many metals we use daily, wear off and are ingested, absorbed, or inhaled. If you regularly cook tomato sauce in an aluminum pot, the insides will eventually become pitted. Where do you think that aluminum went? It's why your sauce has a metallic taste. I prefer sauce not flavored with aluminum.

In conclusion, if you are going to use metals in your core, you should probably be a little more careful with your exhaust.

Be warm,
L.cho

3 years burning in the firebox of my workshop rocket.

Bottom of the barrel. First two photos.

Stainless steel down bit, of the p channel.

Ans it's brand new replacement.

i made a video to show and example of steel spalling

This is what you get when the temperature of the steel is getting high enough in an oxygen-rich environment. Anything above 1500 ºF and the steel is corroding away in a very spectacular way. As such, the damage is done waaay lower than melting temperature. What you see is a channel that's being blocked on purpose or accidentally during quite a lot of burns.

Since it is a p-channel shown in the video I feel I need to comment on this.
It's all about temperature, so when one would be able to keep the temperature of the steel part low enough it wouldn't spall away. That's the idea of the p-channel and floor channel arrangements. By streaming colder air through such a duct it is possible to keep the temp down. An overhead p-channel, like this one probably, does need to have an overhang over the top of the port in order to suck air in with a sufficient velocity. The lowest pressure and thereby the greatest suction is just behind the narrowest point in the venturi. In this case right at the p-channel opening which is helping immensily, making a difference like between day and night.

The floor channel is different in this respect: the horizontal part is sporting a csa that's about 1.5 to 2 times larger as the csa of the vertical part. Which means the vertical part is a temporarily restriction, resulting in a higher velocity according to Benoulli's law, keeping the most vulnerable part cooler. Not cool enough to ensure the steel would last forever, by the way. My heater, consuming about 1.6 cords of soft wood scraps and so on per season, is going into its fourth season now while the floor channel is still in one piece but visibly battered.

But then, car tires won't last forever for example and almost everybody thinks it's a fact of life. So a secondary air channel out of normal steel is actually a replacable part. An acceptable lifespan is required but other than that there's nothing wrong with the philosophy.

Judging by the video, you've lost a lot of weight Paul!

Well, update on my heater.




This is part of the firebox top on this heater. After 3, and a smidge, heating seasons.


https://permies.com/t/44806/Cobbling-workshop-heater

Yeah, sure, metal can resist fire! This 1.5mm steel.

I'm no metallurgist, but my understanding is that despite the hot side of the burn chamber and the heat riser being "low" oxygen, all metals to include the refractory will oxidize starting at around 400* C if there is any atmosphere (i.e. not a vacuum). The spalling seen in the metal chambers so far is evidence of sublimation and dissipation which reinforces the fact that there is an accelerated oxidation process occurring and it is this oxidized surface that is peeling or spalling off into the chambers. Add the fact that most fuel sources will create an acidic gaseous environment and you are essentially anodizing the metal at extreme temperatures and then spalling it off as thermal expansion occurs. This is basically like running cheese through a grater rapidly shredding it into small bits. Metal fatigue and spalling in the cores of these RMH is simply the exothermic physics of rapidly aging metal hundreds of years in mere weeks or months. The second challenge with using metal as the core of an RMH is the process of annealing which occurs in a well designed and insulated RMH. The metal gets super heated quickly and cools VERY slowly resulting in a metal with increased ductility, decreased hardness and decreased thermodynamic integrity resulting in a softer metal more susceptible to rapid oxidation and fatigue. This is a vicious loop that would appear to make the use of metal in the cores of our RMH ill advised.

Normally this is a non-issue as Insulated fire brick, refractory cements, etc. are all relatively inexpensive solutions without this oxidative drawback. Really if you are building a stationary RMH in a traditional housing situation, you're a fool to employ any other construction method than insulation wools, clay, cement or IFB. Physics, experience and anecdotal evidence has demonstrated that in the middle of the cold season your RMH will fail, at the very least leaving you with no heat, but at worst leaving you or your family dead from waste gas pollution in your abode. Every year people die from heater related fuel and gas polution. So again, why gamble with your heat source and life?

What if however, you wish to employ an RMH in a situation where the weight or fragility of an IFB or refractory cement furnace makes these solutions equally unviable? For instance, I wish to build an RMH on a city bus I'm converting into a tiny house RV. How do I then address the design weaknesses of cement and metal? My hypothesis is two pronged in its solution. We must address both oxidation and annealing, but how?

First, I propose using a nickel based alloy such as Inconel, Hasteloy, or Incaloy as our core metal as these will be highly resistant to the observed oxidation and annealing experienced with all other alloy blends.
Second, to further protect our robust metal cores, applying ceramic coating layers inside and outside our metal cores. ITC makes several products that I believe will work well for this purpose.

Description:
Alloy 625 is a nonmagnetic , corrosion - and oxidation-resistant, nickel-based alloy. Its outstanding strength and toughness in the temperature range cryogenic to 2000°F (1093°C) are derived primarily from the solid solution effects of the refractory metals, columbium and molybdenum, in a nickel-chromium matrix. The alloy has excellent fatigue strength and stress-corrosion cracking resistance to chloride ions.

INCONEL alloy 625 has good resistance to oxidation and scaling at high temperature. Its performance in an extremely sever tests is shown in comparison with that of other materials in Figure 15. In this test, periodic weight-loss determinations indicate the ability of the alloy to retain a protective oxide coating under drastic cyclic conditions. 1800°F is a temperature at which scaling resistance becomes a significant factor in service.

Some typical applications for alloy 625 have included heat shields, furnace hardware, gas turbine engine ducting, combustion liners and spray bars, chemical plant hardware, and special seawater applications.
The hardening effect that takes place in the material on exposure in the range centered around 1200°F (See Mechanical Properties section) is due to sluggish precipitation of a nickel-niobium-rich phase, gamma prime. This phase gradually transforms to orthorhombic Ni3 Nb when the alloy is heated for long times in the intermediate temperature range.  Extensive investigation of the stability of alloy 625 following exposure for extended periods in the 1000° to 1800°F temperature range has shown complete absence of embrittling intermetallic phases such as sigma.

Corrosion Resistance:
Alloy 625 has withstood many corrosive environments. In alkaline, salt water, fresh water, neutral salts, and in the air, almost no attack occurs. The nickel and chromium provide resistance to oxidizing environments. Nickel and molybdenum provide for resistance to non-oxidizing atmospheres. Pitting and crevice corrosion are prevented by molybdenum. Niobium stabilizes the alloy against sensitization during welding. Chloride stress-corrosion cracking resistance is excellent. The alloy resists scaling and oxidation at high temperatures (SPALLING).

Preferred Alloy 625 heat treatment:
(1)High Solution Anneal - 2000/2200°F (1093/1204°C), air quench or faster.

Weldability:
Welding can be accomplished by the gas-shielded processes using a tungsten electrode or a consumable electrode. INCONEL alloy 625 is readily joined by conventional welding processes and procedures. INCONEL Filler Metal 625 and INCONEL Welding Electrode 112 are nickel-chromium-molybdenum products designed for
welding INCONEL alloy 625 to itself and to other materials. Postweld heat treatment of the weld are not necessary to maintain corrosion resistance. Heavy restrained sections can be welded and the weld's mechanical properties follow the same trends as base metal properties. Standard practices such as clean surfaces, good joint alignment, U-joints for thicker sections, etc., should be followed.


As for the ceramic coating, ITC 213 Ceramic Coating for Metals (3500* F operating temperature) as a base layer to 1/16-1/8 inch thick would act as both vapor and thermal barrier limiting both oxidation and annealing. Then, adding an additional 1/16-1/8 inch top layer of ITC 296A Ceramic Top Coating (5000* F operating temperature). This top coat should theoretically protect the initial coat and further prevent oxidation and annealing. This coating process would include all internal and external metal core surfaces. According to their published literature: Coated 1/8” thick plain carbon steel w/ITC 213 & 296A on both hot face and cold face results in 2500*F hot face, 1450*F cold face, with a 1050*F differential! The operating temperatures of Inconel combined with this barrier should prevent the catastrophic failures we're seeing in metal core RMHs.

I'd love to hear people's thoughts and for an adventurous tinkerer or two to test my theory.

thomas rubino wrote:Hi Laura;  When the time comes for a rebuild , you will be pleasantly surprised at how easy cob comes off and then reconstitutes for reuse with water !    

Ditto. Done it myself 4 times in my journeys wrapping a woodstove with cob.  The chimney pipe completely burned out.inside of a year, but the cob on the chimney held up and kept it going just fine.  And I agree above w how nasty cement can get after a while, flaking and crumbling off. Cob is much more forgiving imho and nofumes or chemicals involved.  That's just my unedjumakayted 2 cents.

thomas rubino wrote:Hi Laura;  When the time comes for a rebuild , you will be pleasantly surprised at how easy cob comes off and then reconstitutes for reuse with water !    

I'm taking the plunge.  Demolition has begun.  I have ordered the superwool for a 5-minute riser to replace the steel pipe used originally.
I will post photos of how the interior has held up when I get there.  After removing a few wheelbarrows of cob!

Barrel is Off! I know some of you understand the relief I feel to finally see what has been happening inside the house heat machine. Early morning light is too dramatic for clear photos, but I can easily see reasons for losing rocket power.  As you can see from my previous post, the original design had a very wide insulation area, leaving only 1 1/2 to 2 inches clear space around the barrel. An Ianto Evans idea, as I recall. This didn't ever completely clog, as I was able to clean it annually with muscadine vines from the transition area clean-out. However, the space on top of the insulation was so wide and the clay/perlite mix of such a texture that ash piled up over the last five years out of my reach. I also had put the barrel fairly low, close to this insulation cap, as I use the rocket to cook for most of the winter and I wanted a hot top. (My only other cooking facilities are outside--two L-burners, a cob oven and a Sunoven. so I adore cooking on the indoor rocket.  No soot on the pots!)
Now, with the new superwool riser, I will have very little insulation thickness for ash to collect.  I've seen an image on this forum of the superwool left "raw" on top, and I've read of someone covering it with foil.  My instinct is to cover it with slippery, sloped foil cap so as to be less enticing to ash. Are there other ideas?   I will have a removable top, now, so I will be able to open to clean, though that idea is so new to me I can hardly fathom.
The stainless steel sheet I used to enclose the perlite seems to be in fine condition for re-use, and the steel pipe inside my heat riser shows no sign of spalling.  I'm guessing this means I haven't been burning my fire hot enough over the years since its 2014 construction.  I knew the pipe was thick, and in a previous post I had guessed 5/16", but now that I can measure again, I see it is about 1/2" thick!!  So heavy.  
 So here is the volatile question:  What are the PROS/CONS for re-use of this pipe surrounded with superwool, and then wrapped in my used stainless?  
The advantages for me would be:
1.) Not having to move around this incredibly heavy thing!
2.) There has been virtually zero ash build-up on its surface, so it seems slicker than the wool, encouraging faster air movement
Excuse the cat.  I also have photos of him inside my outdoor oven as I build the weekly fire. He hasn't been singed yet. . .

Hi Laura;  Congrats on getting your barrel off!  And more so for getting a barrel with removable top! Your going to like it!

So your question.   Yes, you must take your heavy pipe out!
Here is why , you are losing heat trying to heat up that big pipe and your combustion is not nearly as complete as it will be with a 5 minute riser.
What makes a 5 minute riser so good is the fact that it has no mass at all and funnels that heat directly to your barrel top and the into your mass.   Much less ash will get down into your system.
As the whole riser is only just over an inch thick very little ash will collect on top. I would just leave it as is.  

I have been studying this forum for a few weeks and I can see that metal is not a recommended material for the hot spots !
However outside of this forum I can also see that by far the most popular rocket stoves designs are made from metal!
For every one cement stove there seems to be hundreds of metal ones at least that seems to be the case if you look a Facebook or YouTube!
I must conclude that metal ones without insulation are somewhat air cooled, they don’t get that hot and they don’t last very long?
Perhaps if these more basic metal stoves are only used occasionally they might last quite well, lots of people seem to be happy to use their little portable rocket stoves.
Anyway I saw a few weeks back that there is a new design rocket stove made in America and just about to go into production, I read about it on Facebook but it is to far down the page for me to find it again... however I did grab a couple of pictures and it Is made from metal!

For the most part, you are correct in your findings. Metal is generally not recommended anywhere near the core as its life expectancy is very low when insulated. A lot of videos show these stoves being built with no insulation so will shed the heat very quickly thus helping to preserve the metal, but then you don't get the high enough temperatures to completely combust all of the exhaust gasses. Just because there is no visible smoke, doesn't necessarily mean all the invisible gasses have been consumed.

There is another metal stove out there called the Liberator. I know "Uncle Mud" has been using one these to heat his home during the past few winters and has enjoyed it very much. Uncle Mud Liberator Rocket Mass Heater Review

Matt Walker has been having great success with a metal called RA330. He has been using it as a liner for his cookstove and secondary air feed for a few years now also with great success. Very expensive, but proves the point that metal can be used in the firebox if you are a serious rocket scientist.

I suppose, now that the metal core is removed, that this post belongs more in a rebuild thread. I find this image interesting, as the barrel shape can still be inferred, and one can see that curious transition from barrel to "ash pit"  The whole structure is built from cob, and I can remember having trouble figuring out how to construct such an abstract void when i was working with a blank slate.  Thus the funny little bump in the middle of the passage  I get the feeling that not many folks lean so heavily on cob in the transition area.  I see a lot of cut stacked barrel parts and bricks.  This freeform cob shape seems to work well, though.  The stove has been very "rockety:

If your not having any problems with draft, than your "funny little bump" might as well stay for added support, but as you have seen, cob is quite strong and can form fairly strong archways once dry.

You will find that your 5 minute riser will seem small in comparison to your perlite/clay riser and maybe even wonder if it can do the job. Oh yes....it will!

thomas rubino wrote:
Here is why , you are losing heat trying to heat up that big pipe and your combustion is not nearly as complete as it will be with a 5 minute riser.
What makes a 5 minute riser so good is the fact that it has no mass at all and funnels that heat directly to your barrel top and the into your mass.   Much less ash will get down into your system.

Finally some cold weather so I can run the RMH for a few hours at a time.  I do love the rebuild, of course, and the quick-heat cooktop. However, I do miss the way my old barrel stayed hot so long. The tea kettle always hot.    I also am amazed at how quickly my firebox coals cool down now. Coals used to continue to powdery ash in my former, more massive design. In the morning, the ash would be reduced to a cup or two in volume. If I'd had a bigger burn, I may still have a few embers underneath. Now, the morning firebox is full of a gallon of chunky cold coals, as if I'd doused them with water the night before. So many coals! I'm getting used to it, but wow!

Laura, you might be missing some mass behind, and your draft is too strong. Maybe. Your firebox and burn tunnel still are made of bricks?

There is another thing which can do that, wet wood.