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How Long Does Ceramic Fiber Board Last in a RMH?

 
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So how long does ceramic fiber board last in a RMH,  in both the combustion chamber and riser?  Judging from photos I've seen it does not appear to be very durable.  Any feedback and or photos from folks who have been using this for years would be much appreciated.  

I'm about to start a RMH batch box build I have been prepping for the last year or so.  Until recently I was going to use a ceramic fiber core, but after some research, consultation and a lot of head scratching I'm switching to full fire bricks.  Mason stove builders have cautioned me against relying upon the CFB in the burn chamber and advised me to use full fire brick.  I realize there is a lower efficiency due to the heat absorbed by the brick, but the mason builders tell me this is not a big deal, the stove still functions wonderfully, especially once it warms up, and  do I really need 2000 F in my stove anyway?  I want a stove that's going to hold up to years of stoking and the inevitable abuse when someone other than me uses it.  I've seen photos of CFB in pretty rough shape.  I've puzzled over ways to line the core with brick splits, but they seem to be just floating in the combustion chamber, dry stacked, with no anchor or attachment, and it seems inevitable they will, sooner or later, split and/ or fall.  

One place I am still considering using CFB is the riser - it seems reasonably safe there, away from the wear and tear of loading firewood etc, but does it hold up long term to the heat?  I see some photos where it looks like CFB has just melted from the heat after prolonged exposure. I like the idea of a CFB riser, but I'd rather not have to tear my RMH apart after 5 years just to replace a riser that i could make out of a more durable material.

Any feedback welcome, especially from folks who have used CFB.

Thanks so much, I love this forum!

Mark
 
Mark Dumont
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I should clarify that this RMH is a long term build for daily winter use in my home, not an experimental, shop kinda thing...
 
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I used to own a glassblowing studio, building all my own furnaces, glory holes and the like. http://www.wesbond.com/products.htm is a company that makes the supplies for ridgidizing and protective coating ceramic fiber for industry.  The one I always used (with terrific success) was their  Wesrock RFC#17. This was also used  at temps way above what a rocket heater will see. Wesbond is a family operation, they're primarily an industrial supplier but they are REALLY nice folks and will sell smaller quantities of most products. The quantities available are down at the bottom of each product page. No online ordering or anything like that. The way it worked when I had the studio (a few years ago now) was that you call them up, tell them what you want and they'll ship it and then  bill you. Very trusting folks here so don't take advantage....not to mention nobody else makes anything close to what their stuff will do so it would be very unwise to take advantage of them.  They also have what's needed to ridgidize ceramic fiber shapes. I used to make my own fiberboard and shapes from the rolls of ceramic fiber with their chemicals.
 
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a friend gave me a gallon of refractory "mud" its like a paste. maybe coating the fiberboard with something like this stuff would make it fire/heat proof?
 
gardener
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Mark DuPont wrote:Mason stove builders have cautioned me against relying upon the CFB in the burn chamber and advised me to use full fire brick.  I realize there is a lower efficiency due to the heat absorbed by the brick, but the mason builders tell me this is not a big deal, the stove still functions wonderfully, especially once it warms up, and  do I really need 2000 F in my stove anyway?  I want a stove that's going to hold up to years of stoking and the inevitable abuse when someone other than me uses it.  I've seen photos of CFB in pretty rough shape.  I've puzzled over ways to line the core with brick splits, but they seem to be just floating in the combustion chamber, dry stacked, with no anchor or attachment, and it seems inevitable they will, sooner or later, split and/ or fall.


You've probably seen a lot of Matt Walkers videos on the subject and know that he swears by cf board. He often says in the Stove Chats that they do wear but to him is worth replacing when needed (I think yearly) due to the superior insulation they give and not burning your hands/arms from loading wood into the firebox. In one of his videos Ceramic Fiber Rocket Stove Build Discussion he says he's been using cf board for 5 years. Now I know since that video, he has inserted a metal liner RA330 which is a very high heat resistant metal that he feels is a lifetime investment and does away with having to replace the cf board. Certainly not for everyone but another option to look at.
As far as how hot you burn, if the temperature isn't high enough, certain gasses will not combust to give a complete burn and you might as well just be building a regular wood stove that burns much cooler (and often dirtier).

My batch box is made from dense firebrick splits and adhered with fire cement. Half a season and are still intact and not shifting though I am very careful loading and am the only one who uses the stove so that can make a huge difference.  

Mark DuPont wrote:One place I am still considering using CFB is the riser - it seems reasonably safe there, away from the wear and tear of loading firewood etc, but does it hold up long term to the heat?  I see some photos where it looks like CFB has just melted from the heat after prolonged exposure. I like the idea of a CFB riser, but I'd rather not have to tear my RMH apart after 5 years just to replace a riser that i could make out of a more durable material.


I have no experience with using cf Board but have a half seasons worth of experience with cf blanket made into a 5 minute heat riser. I can say that its holding up very well with no signs of degradation other than having developed a thin crusty surface. The developer of the 5 minute riser named Pinhead on Donkeys board has probably the longest run time with it and remember him saying that he has had no issues with it after many years.


 
Mark Dumont
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Thanks for the replies!

As far as how hot you burn, if the temperature isn't high enough, certain gasses will not combust to give a complete burn and you might as well just be building a regular wood stove that burns much cooler (and often dirtier).

My batch box is made from dense firebrick splits and adhered with fire cement. Half a season and are still intact and not shifting though I am very careful loading and am the only one who uses the stove so that can make a huge difference.



Gerry - What is behind your firebrick splits?  Are they backed with regular firebrick, or some kind of insulation?  
 
Mark Dumont
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And just to be clear, I believe that a super insulated batch box such as Matt Walker's, burns very hot and very efficiently, but a solid brick batch box with the correct proportions and dimensions would also burn very hot and efficient as well, correct?
 
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Hi Mark; Yes you are correct.
Using full size firebrick will make a fine batchbox.
Here is why they don't like them.  Until the bricks in the firebox gain full temperature, they are stealing heat from the combustion, so you stove is not burning fast hard and clean.
After they are up to temp your batch will be at top efficiency.  When you build with heavy brick the temp inside the box is radical, you will want a welding glove to insert wood.
Yes you can toss it in, but invariably one will need adjusting and your hand is not the tool to use. A regular fireplace poker would work.

Morgan super wool ceramic blanket is the best riser you can make other than true insulated firebricks.

Ceramic board is long lasting as well,  as long as it can not be abraded.
 
Mark Dumont
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Thank you Thomas, this explanation is really helpful.  This whole trade-off between insulation and durability is fascinating, I'm obsessed, and reconsidering full bricks vs. some combo of CFB and splits.  

I think I saw that you used Skomole bricks in your batch box, correct?  how do you like them?  How durable are they?  How long do you expect them to last?

I'm wondering just how hot bricks would need to be before they are hot enough to support a clean burn?  If I'm using my RMH daily and it's warmed up, would a full-brick unit reach operating temps fairly quickly?

Another consideration - I'm in Northern CA, not Montana.  With my relatively mild winters I 'm guessing I'll be stoking this just twice a day, not running it constantly, so I suppose the need for short, quick firings is another reason for good firebox insulation.
 
pollinator
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On the core build I'm working on, my compromise for the possible fragility of CFB is to just line the firebox with loose CFB that I can easily replace when needed. Since they're single pieces, no worries about them staying in place like fire bricks. The bottom piece locks the sides in. The only drawback is, you have to account for the extra space for replaceable CFB in your design measurements.
 
Mark Dumont
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On the core build I'm working on, my compromise for the possible fragility of CFB is to just line the firebox with loose CFB that I can easily replace when needed. Since they're single pieces, no worries about them staying in place like fire bricks. The bottom piece locks the sides in. The only drawback is, you have to account for the extra space for replaceable CFB in your design measurements.



Nice idea Matt.  how long have you been doing this?  How often do you find the CFB needs replacing?  Does it ever weld or stick to the layer behind it?
 
Matt Todd
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Mark DuPont wrote:Nice idea Matt.  how long have you been doing this?  How often do you find the CFB needs replacing?  Does it ever weld or stick to the layer behind it?



Lol, at the risk of sounding like I'm talking out of my butt... I've been STUDYING this for over a year and meticulously planning my build for the last couple months. I made a full size mockup out of 1" pink foam insulation to ensure all my pieces would fit, and I will be using those pieces as templates to cut the CFB I just received.

So I can't answer your questions from experience, just design. But as Matt Walker (champion of CFB) has repeated again and again, CFB holds up longer than you'd think and gets harder after the first firing. I have never heard of it sticking to the layer behind it, and can't imagine why it would. I say that because the nature of the material isn't such that it would want to, and it's so insulative that the back of the board wouldn't (shouldn't) get hot enough even if it was prone to sticking. Assuming 1 inch material, which is the minimum.    

I'm hoping to build my core this week and you've inspired me to document the wear of CFB over time with photos because I think it's something a lot of people wonder about.  
 
Gerry Parent
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Mark DuPont wrote:Gerry - What is behind your firebrick splits?  Are they backed with regular firebrick, or some kind of insulation?  


With this build, I used rock wool insulation behind my splits followed by cob. It was tightly packed in there so it initially gave the bricks a fair amount of support.
However, I would not recommend rock wool to anyone as it is not a very high temperature material, but it was all I had at the moment.
I can imagine right now that it is loosing its integrity and crumbling apart, therefore is not providing much backing support any more.
This was an experiment from the start so I had little expectations of it lasting long.
I will be tearing it down at some point in time and replacing it with clay/perlite or insulated firebrick as a much firmer backing and higher heat tolerant material.
I will do an autopsy to show what happens when you don't use the right material for the job one day.



 
thomas rubino
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Hi Mark;
My Skamal bricks did not do well at all. After 5 days of test burns and they turned black and cracked apart!
They were only rated for 1750F.
I replaced them with heavy brick.  
If they had a higher rating they would have been perfect.   Tougher than ceramic board, but just as insulating.
 
Mark Dumont
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Thanks fort the responses folks!  After lots of obsessing and head scratching I've decided to try to the CFB lined with fire brick splits.  I'm working with Luke Parkhurst of rocketstovecores.com, who has been doing the research and development of the combustion chamber.  We'll be able to cob in the basic components (batch box, riser, manifold and bench fort a test run before we finalize the build.  I'll be sure to post photos when we get started!
 
pollinator
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In my Batch Box rocket firebox I've used board similar to this stuff (it's widely available in Europe)
Vermiculite board

In my experience it works well but does abrade after a while even when loading fuel carefully. I've also used it in several woodstoves as firebox lining and baffles and it does wear after prolonged exposure to high heat. It really needs to be treated as a consumable part.

Incidentally, My BB firebox uses the insulating board for the walls and ceiling and I use firebrick splits for the floor (backed up with superwool insulation). The splits cope fairly well with the logs being loaded into the firebox - they survive longer in that application than the insulating board.
 
Mark Dumont
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So Gerry, I've been mulling over your response:  

As far as how hot you burn, if the temperature isn't high enough, certain gasses will not combust to give a complete burn and you might as well just be building a regular wood stove that burns much cooler (and often dirtier).



While I understand what you're saying in the first half of the sentence, about the significance of the high temperature, I'm still questioning the second part, is a batch box built with full, uninsulated bricks just as inefficient as a regular wood stove? - my design started with s J-tube, and then evolved into a batch box.  the J-tube is already much more efficient than a regular wood stove due to the tall riser and the thermal mass.  By converting to a batch box I'm adding the improvements of a secondary air port and increased turbulence.  Are these added features non-functional in the absence insulation?  While the fire will take longer to heat up, and while that escaped heat may be "robbing" the fire, it is also heating the mass and my house, which is my prime objective (this is the point mason heaters are making to me).  As I've mentioned, I'm building this in my living room, and really wanting to avoid regular tear downs and rebuilds, so I'm wary of insulative materials such as CFB that appear to be much shorter lived than fire brick.  This trade-off is a fascinating question to me and I'm surprised I don't see it elucidated elsewhere on this forum, or maybe it is and I've missed it.  

So I guess I'm still on the fence about whether to use CFB in my design or not, I'm going to consult with a mason heater today and will report back my findings.  Meanwhile thanks for helping me puzzle through all this.
 
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As I understand it, if you just used firebrick with no backing insulation then what would happen is, the fire would keep heating the bricks until the heat saturated the brick, at that point they would not get any hotter because the brick is now loosing heat to the air.
If you use insulation then once the brick reaches saturation point the insulation stops it escaping so... the brick gets hotter?
 
Gerry Parent
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HI Mark,    The context I was referring to was when you said "do I really need 2000 F in my stove anyway?". In my response, I just wanted to make sure that we were on the same page together in that burning HOT is where its at with rocket stoves of all sorts. I see by your response that we are.

Using un-insulated bricks by themselves doesn't mean it will burn like a regular wood stove, it just means that it will take longer to get up to these high temps that insulated ones would do very quickly as you mentioned. There is a trade-off in abrasion resistance which is why the hybrids have come into play.  

The J tube has had a lot of refinements made to it over the years by all sorts of people. Peters modified design now sold by Dragon Heaters has about all the extra bells and whistles that would ever make it better - Tripwire, P-channel, back-sweep etc.  Adding a separate secondary air channel anywhere in the core from my understanding from following the development threads has never improved its performance.
When it comes to batch boxes however, without the addition of secondary air, performance is very poor. A different beast altogether.

As Matt Walker keeps pointing out is that you have to look at your goals and match your stove to those. There is no perfect system for everyone and one is not making a horrible mistake by using one or the other. A masonry stove person is definitely going to emphasize high mass and that makes sense. Masonry stoves have a long history of being made with these heavy materials all the way throughout the builds over the centuries. It is so well ingrained that some of it has even become a tradition not to be messed with even if newer high heat materials have come available.
Researching all of these different views (which all sound great) can bring up a lot of confusion as to which way to go and I think is the source of most of your questions. This is why the forums are so helpful. Its a great place to chat and get many angles that you may not have considered.

Bottom line, from all of your descriptions and questions, I would say you would most benefit from a hybrid system.  
 
Gerry Parent
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Scots John wrote:As I understand it, if you just used firebrick with no backing insulation then what would happen is, the fire would keep heating the bricks until the heat saturated the brick, at that point they would not get any hotter because the brick is now loosing heat to the air.
If you use insulation then once the brick reaches saturation point the insulation stops it escaping so... the brick gets hotter?



All rocket stoves (J tube, batch boxes or other variants) HAVE to be insulated on the outside of the core. Whether a person goes with a dense liner of heavy bricks, uses something like ceramic fiber board to make their entire firebox out of or a combination of the two is a matter of choice depending upon what you consider to be most important for your needs.
 
Scots John
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Is there an official recognition for the term rocket stove?
I just ask as there are thousands and thousands of some type of rocket stove that have no insulation?
I think there are some well known builders like Yasmin and others who don’t like or use ceramic fibre especially if there in an in line black oven, even insulating bricks have issues with breaking down and polluting particles .
I also see a few of the batch box variants without insulated  fire boxes but with insulated afterburners.
I know Matt Walker is a huge fan of ceramic fibre but there are other builders who are not so smitten.
Of course I don’t actually know anything through experience just from reading so I could easily be wrong!
 
Gerry Parent
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Its a long time debate over what the exact definition of a rocket stove is and has been discussed a lot on Permies.  
Just like any cheap Chinese take-off of a brand name item, rocket stoves are not exempt from this same type of...... lets say demotion.
I won't get into them all, only the part that we are currently talking about and that is insulating the firebox.

Without high enough temperatures, the wood gases that are released from the fuel will not combust and end up going out the chimney as a pollutant.
Some of these gases are not visible as smoke and therefore is assumed that if there is no smoke, combustion must be clean.
So lets say that when one of these thousands of metal rocket stove are made and the firebox is left uninsulated, the heat is shed immediately away and therefore not able to contribute to getting these unburnt gasses any closer to stoichiometric or complete combustion.
Now there is nothing wrong with these people modifying their stoves in this way as it is obviously meeting their needs, but as to where it crosses the boundary to not being classified as a rocket stove anymore is sketchy.
Perhaps you can find some of these threads that get into the definition a lot further. I know Erica and Ernie Wisner have done some good write-ups.

In Stove Chat 10, Matt covers the history of rocket stoves and its development.
Also, Larry Winiarski lists the : ten-commandments-of-combustion

 
Mark Dumont
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Thanks for the replies, and for your explanations Gerry, very helpful!!
 
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