paul wheaton wrote:Last night I watched DVD1 as one big chunk ....   it was missing a small piece.  

Unfortunately, I made two pages of notes of things that needed to be changed/mended and then we need to look again.  So it might still be another day or two.

I for one, am glad you are going through them again, as a finished presentation, looking for errors and omissions. We've waited this long, a little while longer to help ensure completeness and quality is well worth it!

4 DVDs vs. 8 DVDs.

A point of confusion may be whether there are only 4 DVDs, as appears to be the case, and was the original idea in the kickstarter. Thus the 8-DVD set would NOT include 8 different DVDs, but rather two sets of the same 4-DVD set.

Charles Deshler wrote:Really excited to get this coming. I just finished my new shop this week and i am ready for a Rocket stove to heat with. Does anybody know what the clearance is to walls/wood floor?

It depends upon how you design the build. Are you going to use heat shielding or not? I know Ernie and Erica discuss this in their new book, "The Rocket Mass Heater" so you may wish to get a copy of that. It is also discussed in various threads, which are free but require your time to be spent in greater amounts than reading their book. You may also look to your local building codes, and the ASTM standards for masonry heaters. A RMH is a hybrid of a masonry heater and a standard wood heater: the masonry standards apply to the thermal mass (bench, bell, etc); and the wood heater standards apply to the metal barrel.

Off the top of m head, which means I may be mistaken these are nominally a 3 to 4 foot clearance from the barrel, and half that distance if properly heat shielded, and as little as 4-inches for the mass, provided the proper thickness of mass is in place (a minimum of 8-inches as I recall). So if you are able to provide 4-foot clearance all the way around, I doubt you need to worry (this includes to the ceiling, which to my mind is certainly one of the more critical concerns: heat rises, after all). Anything less, and you need to take a careful look at your design parameters.

Not so much a specific answer, as it is a procedural answer ...which I think is best, not knowing what design you plan to build.

And always take extra precautions with the exit flue/chimney. Observe best building practices here, all the way around, paying extra attention to through-wall/ceiling and height of the chimney (insulated or multiple walled pipe is recommended in most cases, and may be required by your code - you won't know until you look at your local building codes). My opinion is that it is best to purchase pre-fab pieces for the through wall/ceiling and outdoor chimney - basically all the flue/chimney elements once they exit your RMH. And pay attention to the clearances.

Assuming you build the RMH as it is described in the literature found on this forum, on Donkey's forum, and in the few recommended books (again, Ernie and Erica's new book is probably the single best resource on that point now), the most dangerous elements are going to be how you exit the exhaust. I recommend being conservative in this area, whether you have local building codes or not, I would strongly recommend observing best building practices for your exhaust.

On a personal note, given how inexpensive temperature probes and thermometers are, I would plan to build several of these into your RMH at critical heat areas, so that you may observe these critical temperatures during operation.

Peter van den Berg wrote:

F Styles wrote:holy goodness 1800 degrees F? are you kidding me? For real are you really getting 1800 inside your system?

That level of heat isn't uncommon in rocket heaters. In my own batch box design the highest recorded temperature has been 1180 degrees Celsius which converts to 2160 Fahrenheit. At the time, I wasn't even sure whether or not I was recording the hottest spot. Exit temperatures at the top of the riser are routinely over 900 C or 1650 F. So Sky is right, he really need to keep down those freaky temperatures inside the core or the whole thing would corrode like mad in no time. Whether or not a gas analizer would be satisfied about the results since the temps aren't really up to the phisical limit remains an open question.

Yes, quite common temperatures. My prototype that I tested the last two winters (a basic nominal 6" j-style barrel RMH) commonly burned between 1400 (that's really quite "cold") to nearly 2000 *F (1960-1980 *F ranges, spot checked, not continuous monitoring) at the top of the entry point going into the burn tunnel from the feed tube. The hottest part should be found deeper in the burn tunnel, at the bottom of the fire riser as I recall, but I never measured that point.

But as far as testing heat output, why melt aluminum, just buy a digital thermometer and appropriate test probes. I bought a decent digital unit for $18 on Amazon and a variety of probes, ranging from 200 *C or so on the low end (for measuring heat penetration of the thermal mass and under the burn tunnel to verify I wasn't transferring too much heat to the floor) and up to something like 1000-1200 *C or so (I don't recall of hand) for measuring the hotter areas. That's not sufficient to get at the hottest areas, but it is usable for measuring the transition from feed tube to burn tunnel.

Another test option is to purchase a variety of test cones, as are used to test kilns.

I prefer the digital probes, but then again I'm really only doing it for my own testing, and these days you can buy very accurate (within a 1%-2% as I recall) devices. If one wanted to get all fancy, the digital thermometer could be rewired to run on electric a.c. plug-in (or simply hooked to a comparatively masses storage d.c. battery) and feed the read out into an old dedicated computer and log the data for the entire burning season. I doubt I'll do that, but it wouldn't be all that difficult. If anyone wants to go that route, there are also basic home weather stations that can be bought for a few hundred dollars and that could supply a decent number of environmental data points (and connected to various online weather station collection systems, if one also wishes to become a home weather station data point for others).

Socrates Raramuri wrote:Sand?
I thought about this, as well, and watching one video it was mentioned that someone used sand as part of the mass but that this was inappropriate because sand insulates... So there ya go, i thought.
Would it be the best insulator? Probably not. But it's everywhere and free. And often 'the best' can be substituted by simply applying more of what might seem an inferior product.

Technically, it is the multitude of tiny air gaps between the sand that insulate

Sand mixed with clay, for example, is not at all insulative, but rather adds structural support to the clay, helps it not to crack so much, etc.

But if you fill a bench with sand, yes, that is going to insulate to a reasonable degree because of all the air pockets that are trapped between the particles of sand, and *air* is a very good insulator.

So I say go for it! Just use more of it.

Minor Update.

As I write this, it is July 2015, and obviously the winter heating season 2014/15 has ended. I never resumed the use of my flue damper. And I never noticed any trouble with just placing the fire bricks over the opening of my j-style rocket heater. I just kept closing the opening more and more, to 1.25 inch while the coals were dying down (the width of one fire brick split), and then just about an 1/8-inch or so open when I went to bed or to work. My thinking was this was enough to permit gases to escape, should any be present.

Other days, when the wind was from the "wrong" direction (I never got around to increasing the height of my chimney pipe, given this was a temporary and prototype build) I covered the feed tube opening first with a piece of heavy aluminum foil (still flexible, still for cooking use, just the heavy foil, not the very easily torn thin foil), then on top of the foil a couple of 12x12 inch floor tiles I had left over (which completely covered my feed tube opening), and then on top of the tiles, setting the three split fire bricks I used to moderate my feed tube opening when burning.

I cannot say that I ever detected a cold breeze flowing into the house using this set up. Thus, as of now, I am not convinced loss of heat is of great concern. Granted, I also did not simply allow a full 6-inch opening to remain fully open through my wall all winter either! But my opinion is that closing one end of a loop (feed tube, to barrel, to chimney pipe) is adequate to slow or halt the loss of indoor air to the atmosphere, to any appreciable degree.

I imagine a bottle, and trying to blow smoke into it, simulating a cold winter wind. How effectively does the smoke enter the bottle? How deeply? I am thinking, not so much. I'm not saying using a flue damper might not be a good idea; but my opinion is that it ought to be located in an obvious location, so that one remembers to *open* it before starting the new fire hehehe, having forgotten once or twice ...I can say the fire tends to draw much better when the damper is open

Now, bear in mind that I live in an older house and I *do* have old windows and doors, so I already have "heat leakage" in the house; this is *not* anything like a super-tight house.

If I understand your question, I'd say the answer is a qualified yes.

Whether there is a limit to how large the space between the fire riser (that is what the insulated vertical element of the j-tube is normally called) and the outer barrel (or welded up plate in your example), I do not know. I suspect not within reason; but what is "reasonable" is then the question. Think in terms of bell masonry heater design. So long as the containment area of the bell is less than that which will cause all the heat to be sucked out and vapor to condensate before rising out of the heater (through the chimney, into the atmosphere), it will work (assuming a properly designed and installed chimney is in place, and other factors that constitute a good draft, pulling the exhaust out of the house).

The next question, is why?

Why would you *not* wish to benefit by having thermal storage of the heat? It is this which moderates the heat released into the room. Without substantial thermal mass, the rocket heater works a lot like a wood burning stove: hot when burning and cold when not burning. The difference is the rocket heater burns through the wood much more quickly than a wood burning stove. For many reasons this is a very good thing; however, one reason this is *not* a good thing, is unless you have thermal mass to capture and slowly release all that fire energy (heat) it is less comfortable as a heat source, and requires being burned more often, and when burning requires more attention than a common wood burning stove (Peterburg's batch box eliminates that headache, but cannot alter the physics of high heat production without thermal mass to capture and then slowly release that heat: a high rate of energy production is only half the system; capturing and slowly releasing that heat is the other half of the system).

I burned a test rocket stove/heater this past winter. I was primarily concerned with testing the basic build and operation (how I feel living with my dragon, as it were). Since I also built this on my carpeted living room floor, above a basement which I did not add support joist under, I did *not* add thermal mass (too heavy). The result is the barrel got the place pretty darn warm, and even hot at times (roughly a 1350 sq.ft. upstairs and 1200 or so in the basement; it did not heat the basement at all, so far as I could tell: heat rises, after all). It was nice when burning, in terms of heat output. And the things in the living room warmed to some degree, I'm sure, but nothing like a proper thermal mass would. As a result, temperatures began to drop as soon as the fire stopped burning. So I would run the temps up to 80 degrees F or more, and then cover up under my electric blanket whilst temps fell back to 50 F or so, and then either go to bed or start the heat/cooling cycle over again.

I still saved a lot of propane. So it was worth the trouble, in terms of money saved. I may have even paid for the supplies I bought to build the prototype (I'd have to check my records to be certain of that; if I didn't pay for them outright, I paid for a lot of the materials in saved propane costs). But I also learned a number of other things. Namely, for *me* the batch box is a better design, because I'd rather read a book than get up every 5 or 10 minutes to tend the j-style feed. And I really, really do need to add thermal mass; for me that has always been the plan, and this years build will accommodate that; but had I been doubtful of the benefit of thermal mass to capture that heat and slowly release it into the room over a period of hours, instead of minutes, I'm convinced I'd consider the thermal mass to be a good investment, both in efficiency and comfort.

There are all kinds of ways of making thermal mass look nice. I would not give up on that easily (in fact, I would not give up on that at all). I certainly recommend planning on adding thermal mass, be that bells, benches, or a design that has the appearance of a more traditional masonry heater. There are many beautiful designs, so it is hard for me to imagine not finding something you would enjoy living with.

That's my 2-cents. Add a couple buck and you can buy a cup of coffee

I think I would look up building standards for masonry stoves. I don't recall the ASTM-E1602 specifically mentioning this. However, if you build according to that standard, I should expect you'll be fine. The long and short of that, in terms of what may be applicable, I would say is to make sure your stove thermal mass is a minimum of 8-inches from your other walls. Personally, I would also isolate your stove mass with insulation from the rest of your house, unless that is already isolated with insulation from the earth.

I've not used one.

It is a cook stove, not a heater for warmth. As a cook stove I suppose it might last as they claim. Cooking is of short duration, so maybe it'll hold up. But 10-years? That's a lot of cooking at 2000 F in the fire box.

I certainly would not expect it to hold up as a heater (which it is not being sold as, to be fair). 2,000+ F for such extended times, I would expect the metal to break down. But others who have actually used metal will be a better source than I am. I just know I've read of enough metal failures, I'm not going to waste my time even trying it for heat; I might for cooking, if I can get the metal cheap.

Antony Felton wrote:Hi all, thanks for the input, the burn tunnel has been shortened and works, the smaller this thing is the better for my situation. I will also lift the quartz slab (sorry still waiting to find out the specs.), with 4x2 plywood strips. As for the bell i going with the old school 200l drum, but a 50mm would or feels like it would be optimal. I will post an update on the reorganized design.
Ta ta for now Antz

If you are trying to lift the quartz to provide air flow (a very good idea in my opinion) I would consider using square steel tubing instead of a wood product (especially one which is impressed with chemicals you may not like off gassing into your living space). I've seen test lab builds online that have done this to protect their floors during test runs. I suspect it would work very well in your case too; better and safer than a flammable product in my opinion.

I would get a minimum of 1" square tubing. That should provide adequate air flow. More space = more peace of mind, perhaps, but costs increase too. I don't think anything more than 2-inch square tubing would be needed. On concrete I'd go with 1" and sleep well; on a wooden floor, trailer, etc, I may sleep better with the 2" air gap. Just my opinion.

The horizontal burning chamber looks too long to me. I'd suggest double checking your geometry.

Made me salivate! Must be doing something right!

I am assuming the main stack was a poured/packed refractory recipe of some kind, between two metal forms, and that the feed tube stove pipe is periodically replaced as it burns out.

Looks like a fast and easy way to do outdoor cooking in the hot summer months, with little investment in materials.

[edited typo]

Laura Emil wrote:

Erik Weaver wrote: I'd much rather read a book than tend a fire every few minutes.

the amount of info here (and elsewhere) is overwhelming to me, and I'm far from the point I'd be able to consider one of these. BUT in my outside world job, I asked one of my "energy committee" members to take a look at RMH as an alternative. (We're trying to help folks find affordable 'biomass' alternatives.) He was equally overwhelmed when I sent him to permies to reseach - and his one quick hesitation was "but how often would you need to feed this?" (He's leaning to pellet stoves.)

Now that it's part of my work assignment, I can take the time to echo that question:

How often does a RMH need to be tended? How does this work for the working guy who needs to leave the homefires 10 hours a day? Is this only 'supplemental' in that case?

thanks in advance!
~laura

Research the batch style. It is more like a traditional masonry heater, in terms of firing. Get it going and it should burn itself out in 45-60 minutes. Do that one in the morning, and then again in the evening (once or more, depending upon the weather). Seems weird if you're used to "wood heat" from wood burning stoves (which are very INefficient), but pretty normal mode of operation for a well designed masonry heater. So not all that bad in terms of managing them.

The J-style is a different kind of critter. Maybe I don't know what I'm doing, but I had to keep fiddling with it every few minutes. Not major efforts mind you, moving the bricks to keep them reasonably snugged up to the quickly burning (and therefore reducing and disappearing) wood mostly. But that still means I have to get up and walk over there every 5 or 10 minutes. Maybe I'd get away with 15 or 20 minutes sometimes. Like I said, I'd rather be reading.

The counter argument might go something like this.... read by the J-style RMH, sitting right on the bench beside the fire feed point, so all you have to do is reach over and snug up the brick

I get that. I do. But I'm still building the batch style for next winter.

Glenn Herbert wrote:I think the best approach would be to ask Peter van den Berg for his opinion... he could probably give a guess at least.

Yes, I agree. I just thought I'd throw it out there and see if it catches anyone's eye. Peter's done the most careful analysis of which I'm aware. There aren't many of us experimental private builder's with a Testo meter!

I'm planning my re-build to move from J-style to batch, and I'm rolling around these kinds of questions. Undecided right now. Square is obviously easy. Round I think is very unlikely right now. I'm not going to buy the expensive refractory tubes, even if they were available locally. I'm not sure I want to mess around with casting refractory just yet either. So I'm looking at brick. Most likely. Unless I change my mind ;P

All of which begs the question... what is the difference between these shapes? How great is the loss in efficiency?

If a round riser is set to equal 100% efficient, is the octagon 80% efficient, and the square 50% 100:90:60%

I have no idea. Just wondering how great the difference really is.

Glenn Herbert wrote:So if you are going to make a square riser, make it 6" square. If you want to make a roundish riser, people have found that an octagon is pretty close to a circle in effect, and much easier than a 12-sided figure, as it only takes 45 degree angle cuts.

And if building a batch-style, vs. a J-style, PeterVDB has mentioned a number of times a preference for the octagon, instead of a square; and a circle CSA ideal. But again, if one is building out of brick, the octagon has been reported as good enough. People that go round seem to either cast the riser or use another round material (ceramic tube, or something - I forget what exactly).

So the preference would be (1) round riser, (2) octagon riser, and (3) square riser.

This is the DVD set I mentioned (which I have, and recommend). There are a number of purchase/viewing options. It is a pretty quick and easy way to get up to speed on many points concerning RMHs:

http://www.woodburningstoves2.com/

I would also recommend buying "the book" by Evans, Rocket Mass Heaters (2014 edition):

http://www.amazon.com/Rocket-Mass-Heaters-Ianto-Evans/dp/0966373847/ref=sr_1_1?s=books&ie=UTF8&qid=1434897353&sr=1-1&keywords=rocket+mass+heater

It's only 120 pages and a quick read. Page 28 offers a great summary/overview as I recall. Also, study the appendixes. This is the best way I know of to get up to speed on this material. If you don't care to spend $20 and an afternoon reading, then building your own RMH is most likely not for you conversely, if you devour this info and want more, a RMH may very well be right up your alley!

The one topic it does *not* cover is the batch style RMH as developed by Peter VDB. Fortunately, that is discussed quite thoroughly over at Donkey's forum:

http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions

Although, there are some threads on Peter's design here too:

https://permies.com/t/40007/rocket-stoves/Results-batch-box-thingy-Innovators

You'll also need to look into building on a wood framed floor. The Wisner's have at least one set of plans for doing that. Air movement to draw away the heat from the floor is key. Or, if you have more money, you can install a pier and / or foundation below the mass, as per a more conventional masonry heater, and build on top of that. Either way, you need to look into this and start drawing up some plans, and consider the pros and cons of the various options.

Also plan on a full chimney, meaning one that properly runs out of the roof and has at minimum a 2-foot height above anything within 10-foot radius of the pipe/chimney. Evans' book talks about routing the stove pipe out the wall, near floor level and exiting more like a drier vent than a wood burning chimney. Unfortunately that does *not* seem to work for many persons. Most likely you will need a pipe/chimney just like you would for any other wood burning stove.

I'd also pick up a copy of ASTM-E1602-03, which details safe clearances. This is one of the go-to references when designing masonry heaters.

Also, look up some building details on heat shielding. It'll be easy for your husband, given the back ground you mention. He'll just need a good reference so he can build it to safe specs. (It boils down to sheet metal and an air space behind the metal, and not direct route for heat transmission into the wall/ceiling.)

YouTube has a number of videos and clips done by or showing the Wisner's projects. I'd look at those too. Just be careful about what you see on YouTube, because not everyone there builds safely or reports their results after having used the system they built for at least one heating season (more longer, is more better, of course heheh). So take YouTube with a grain of salt, and give preference to those hosted by reputable persons experienced with RMH builds.

In any event, that ought to be enough to get you started. Browse around the above forums and see what you like.

"Boom Squish" is the popular phrase describing this. Boom goes the steam heating, squish go the people, and house. There is a tremendous amount of energy available in steam, as well as expansion rates. If you are not a trained boilermaker, I would not do this. There is a level of complexity and safety concerns that I do not think you are likely to manage in a few short months. If you really want to pursue this, plan on taking your time and research it thoroughly. Ernie Wisner has written about it here, and he speaks to it in the first 4 DVD series; in fact one of the DVDs is devoted to that topic, so that would be a very good place to start.

Instead, I would recommend building a standard rocket mass heater, be that the J-style or batch. Safer, by far, and easier to build. That is a project you may reasonably expect to have in place before winter hits.

Hi Ernie,

Ditto on this getting so few replies. Perhaps this round of remarks will renew the spark

The points you mentioned are all important. What I think is of great importance in a hands on work shop, is getting one's hands in the mix! It's one thing to watch videos and listen to descriptions of the cob and perlite/clay mixes. But quite another thing to FEEL it in your paws! That would be the big "extra" point I'd want to out of a live work shop. I want to develop a feel for the goop! And of each major type (insulating cob, base cob, finishing cob, and also the perlite clay mix for those who want to insulate their fire riser).

Yes, you can do it, but I too highly recommend building it on the floor of the main living space.

But you must research the weight, the distribution of weight & supports (most likely a modified post and beam under the floor in the basement), and the means of safely burning a 2,000 F firebox above a wooden floor (yes, it can be done, I just ran my first test build, on a carpeted wooden floor this past winter - look up Ernie and Erica Wisner's posts for among the most thorough explanations of this technique). Also research the proper chimney design. I suspect few really work well without a good chimney, just like any wood burning stove would need.

Glenn Herbert wrote:I think that the critical distinguishing feature between a masonry heater and a RMH is the firebox. If it has a J-tube or batch box with a verticalish heat riser, it qualifies as a RMH. It's the combustion zone that is the unique factor. The way the heat gets into the mass is interchangeable. And some RMHs could be considered as a subset of masonry heaters (ones built largely of cob with horizontal bench mass might be disavowed by the masonry heater trade.)

Hey Glenn,

You may or may not be correct in these definitions, however if speaking to an insurance company or a permitting/inspection agency for the government, keeping as close to the age-old technology is the safest bet, and I feel confident the better chance of obtaining the results one wishes (insurance, getting permitted/passing county inspection, etc). That was my main point.

I suggest breaking it down into simpler to digest parts:

1. Permitting and/or county inspection is one hurtle.

2. Getting insured in another.

3. Building it is another.

Start with a safe design, sold as a type of masonry heater. Don't use new vocabulary. The thing has tons of mass, it burns wood, and with minimal modification, I cannot imagine why it cannot be a type of masonry heater. After all, isn't a batch-style RMH really just a subset of masonry heater? If it is not, why not?

Perhaps because we tend not to like outside fresh air intake? Then build one of those into the system. What else? Because many prefer cob over brick? Then build the core in brick and just use the cob as a heat/fire-safe exterior finish treatment. What else?

I can't speak from experience, so I'll be very interested to hear how this turns out.

My best guess is you will need to subcontract a master mason ( vetted, bonded, etc ) to do the firebox-critical work. Sell it to the insurance company as a Masonry Heater ( which in my opinion it is, provided you are making a batch-style, as per Peter's design ) and start researching them, perhaps beginning with ASTM-E1602-03 (reapproved 2010; Standard Guide for Construction of Solid Fuel Burning Masonry Heaters) a document with which I believe we should all be familiar anyway.

Then with drawings in hand, you may be able to find an architect or engineer who will sign off on a more suitable footing design for a modification based upon more typical RMH arrangements, which as we know have a much larger footprint than most masonry heaters ( less weight per square foot; smaller footer to be poured )

You can then either do the non-firebox work yourself or subcontract that too.

Bottom line as I expect it to boil down to is who/how is the exposed neck of the insurance company going to be covered? Stamped plans, based upon ASTM standards and safe building practices of the masonry heater trade will be the first step. Then the build being done by a professional mason with lots of experience is the second step. If you are lucky, you may be able to get away with passing a fire-safely inspection, if your county has such a thing, and is any good at doing them.

This area of county inspection and getting insured is a sticking point right now. Not many have done it. Do look up the thread(s) in which the Wisner's discuss their efforts at getting a permitted build done in WA (Seattle? Portland? I forget where). And please post back if you clear these hurtles successfully!

I too encourage you to continue reading and studying other successful designs, with more weight given to those running for two or more heating seasons. If I properly understand what you describe, I think you'll be able to add a significant amount of comfort to your living conditions. By the way, I also suggest you read up on masonry heaters, there's a lot you can pick up in that field as well. And really, despite some differences in vocabulary / terms, a well designed RMH strikes me as a variation of masonry heater ( especially the batch-style systems )

So study up this summer, and take your time to lay out your plans. Kick around ideas. Find out what resources you have near you and start working out pricing options.

I built a prototype last winter, without any thermal mass, just to test the j-style operationally. For myself, I am better suited to a batch style. I'd much rather read a book than tend a fire every few minutes. Thus, I learned that the basic system works well in my location, although a side vented chimney does NOT, and I now know I prefer the batch style ( more similar to a traditional masonry heater design ) and also that I can build a safe j-style ( I very closely monitored temperatures at a number of points in the system, so I have a good overall idea of the temperatures I generated for the weight of wood burned, and I was able to see how the materials I used help up, etc) even on top of a carpeted wood floor ( although I would never build a thermal mass on top of my floor without a great deal of bracing in the basement - the temporary prototype I built ( j-style with 55-gallon barrel ) I calculated to weigh in at under 40-pounds per square foot; and I would exercise extreme caution if doing something like I chose to do, researching methods of keeping the heat from the floor ( which mine controlled extremely well ), and very, very closely monitoring the actual temperatures generated throughout your system )

I am of the opinion that a well-research, well-built, well-tested RMH may be of great value; and I am of the opinion that cutting corners may result in building a death trap. Caveat emptor.

In that kind of design, I would also strongly consider making the top of the barrel removable (Ernie and Erica Wisner have at least one design that shows this option) so that if needed you can just pop the top open for inspection, and cleaning.

A great place to start is the book by Evans, "Rocket Mass Heaters." And read everything Ernie and Erica Wisner have posted here (they are soon to release a book too, and I expect that will be well worth reading too).

Your question suggests you have a great deal of reading ahead of you this summer. You have a couple good months in the summer to read everything you can find.

And I highly recommend beginning with an outdoor build, just like you are planning. We never want the rocket stove to burn itself down, but if that's what we're going to do, far better for that to be outside in the garden!

What you describe will not work. I suspect the fire riser would burn itself out, and then the heat circulation would fail. Then the fire would either become smothered, or just burn like a small fire built in the feed tube. Not terrible if outside. At best super nasty/smokey if inside, and potentially far more dangerous. Always test outside first, and be prepared to put out the fire if it gets out of control.

I have heard of people mixing in saw dust, which ends up getting burnt out by the high heat, but the idea is it holds it's place long enough for the clay to set up around it before the saw dust burns up, leaving behind a tiny spot of air space. And air is a really good insulator.

If heat accumulation is a concern, I would suggest adding air channels to allow cool air to enter at the bottom and flow up, as it heats. See some of the Wisner's builds for examples. I did something like this last winter and it worked very well for me. I was very pleased with the results.

And obviously, for the insulated cob surrounding the insulation used around the firebox, straw is a very common, popular choice that has met with great success.

A couple general observations...

Debris area design so that you are OK with the bits of wood bark and ashes that will be a tad messy and a good route for carrying wood. While you're at it, a integral wood storage/chopping area might be something to consider.

Type of rocket, J-style or batch? Do you want to play / fiddle with the fire every 5 or 10 minutes whilst it is burning or do you want to start it and just let it burn? J-style for the former, and batch style for the later preference.

It sounds like you are covering this part anyway, but people seem most pleased when the RHM is built into an active area of the home. NOt so good being stuffed away in the basement unless you live there right near it. These are *not* forced air heating systems so they work best when you are close to the masonry heater.

Oooh, very interesting! I look forward to hearing / seeing more

Nick DePuy wrote:Awesome! One last question, has anyone burned 'Paper Bricks' in their RMH? In case you are not familiar, you soak the paper in water, blend (if you want-not necessary) and compress into 'bricks' using whatever form you want. (A friend of mine used a pvc pipe with holes drilled in it, a ram and a couple bowling balls.) I work in an office that generates HUGE amounts of shredded paper so I thought this might be a free source of fuel. Thanks!

I've not tried it myself, but I've seen a few YouTube videos on it. Time seemed to me as the biggest negative factor, and for me time has more value than buying a cord of wood. Even if I have to split the wood, it seems faster and easier to me. But you may feel differently about that, so yes, try it and let us know what you think! If you could rig up some kind of ramming system that might speed the process.

In terms of paper, the question I would have is what chemicals are in the paper? Black and white paper may be OK, whereas four color printed paper maybe not so good. You may want to look into that, and find out what the chemicals break down into when burning at various temperatures.

For fire brick, go to a local brick seller. They can get you pointed in the right direction. I'm in Missouri and pay about $1.75 for a half brick 9 x 4.5 x 1.25 inches, and about $2.5 for a full fire brick, measuring about 9 x 4.5 x 2.5 inches. Rated at 2,500 F as I recall; maybe 2400 F, maybe 2600 F; something like that (higher than 2,200 F, but not as high as 3,000 F).

Internet sources are widely variable in quality of information. Be careful. The most recommended book is the one by Evans, Rocket Mass Heaters (3rd edition), which you can find at Amazon for about $20 USD. Get that and start there. That will give you a good working knowledge, and you can grow from there, and pick up more recent innovations here and at Donkey's forum too.

Terminology takes a while to get used to, and many items go by different names in different parts of the country and world, sometimes using the same words to describe different things, or different specifications. So it takes a while to get used to all that. As I say, start with the Evans book. I would also suggest watching everything on YouTube you can find that features Ernie and Erica Wisner, they are a quality source of information too, and have built hundreds of these, so they have a lot of practical experience to share.

Also, pocket rockets get super hot! Perhaps start with a demonstration of those (outdoors?) but I'd NOT want a red hot bucket around a troop of kids for very long.

I second the idea of the full RMH as a wonderful final stage of the series of projects!

I'd consider a progression, pocket rocket demo, followed by rocket stove demo (complete with hot food to be eaten!) and moving on in logical stages until they are able to build the full rock mass heater. Tons of cool stuff to learn! A great idea

I'm cautious on this point. I too recommend an air gap under the fire box. It is what I have done.

At what temperature does a wood floor burn? There is a case cited of a hot water pipe causing a wall fire. Different circumstances, but that temperature was about 170 F. That's the lowest figure of which I am aware that actually caused a known fire in a home. Ernie Wisner recommends 125 F as the upper limit against wood structures, which I think allows a nice safety margin. For sure boiling temperature is going to be a real cause for concern (212 F/100 C), because as temperature approaches that level, moisture is driven out of the wood. Over time as the wood dries there is concern for lower burning temperatures.

As far as my mind is concern, there are two critical factors: 1) Temperature, and 2) Air Movement (or the lack thereof).

Insulation laid against the floor has no where for the heat to go, so it saturates the insulation. If the heat is not removed, the heat increases. Insulation does not stop the transfer of heat, it only slows it down. This is why walls/floors/etc may reach unsafe temperatures even with a layer of insulation between them and the heat source.

Air which is free to move (meaning a minimum of one-inch gap to my mind, although I do read of a 3/4-inch air gap being used in some cases), is able to carry the heat away, and that is what keeps the temperature from building up. Just like in heat shielding used to protect walls and ceilings. Look up examples of how those are built to get a good idea of the mechanics. They typically amount to a 22-gauge (or thicker) piece of metal with a one-inch air gap behind them, which allows the heat to rise behind the metal (therefore there are openings at the top and bottom to encourage this flow of air, and the mounting screws do *NOT* directly reach from the outer surface to the wall itself, because they would act as heat bridges, directly heating the wall through the screw).

I tried two prototype builds this winter. Both were built above a wooden floor. I built a "deck" out of 2x6's, to which I attached a perimeter "lip" made from 2x4's, and under the 2x6 deck I spaced it above the floor with a 2x4 on it's flat side every foot or so, providing about 1.5-inches of air gap under the deck and above the floor. The whole thing was built upon another 2x6 deck, which I laid directly on the floor; this bottom deck was there to protect the floor and distribute the weight across a larger surface area (support is another critical aspect of building upon a wooden floor - you have to know it can take the load, and really you should design for it to take at least twice the maximum load you even think will get piled up there: bricks, wood, people, etc).

I then filled the area of the upper deck which was enclosed by the 2x4 "lip" with a mixture of perlite and fire clay. This was my basic insulation. Obviously, it is 3.5 inches thick, the height of a 2x4 standing on edge, on top of the 2x6 deck. On top of that I have a fire brick floor made of full fire brick on their face, so it is 2.25-inches thick (or 2.5-inches, however thick my fire brick is).

For the first build, I used that fire brick as my firebox floor, and built a 6-inch J-style rocket heater on top of that. Heat immediately below that, runs up over 500 F, and at times if I recall correctly, I measured just below 600 F (I'm not looking at my notes right now, this is from memory). A temperature probe reaching down about 3-inches into the perlite-clay inslulation, below where the coals build up, and measuring approximately 1/2-inch above the 2x6 deck (which was lined with aluminum foil) ran up to 150 F to 170 F pretty quickly. And it would not always come all the way back to room temperature overnight (50 F is where I set the propane heat to kick on at, so 48 to 50 F is my indoor lowest low's) before I was ready to build the next fire; so I was concerned the temperature would steadily increase over time.

My concern was that the temperature against the 2x6 upper deck would reach or surpass 200 F, driving out the moisture and lowering the temperature at which the deck may burn. To be clear, the lower wooden 2x6 deck, which had the 3.5 1.5 air gap above it never got very hot, so there was never a concern with the floor burning (assuming the upper deck itself would not catch fire, heheh).

So I tore this down to the fire brick that made the floor (the full set of fire brick laying on top of 3.5 inches of perlite clay insulation). Then I built the second prototype on top of that, making an air gap space, open at three places, under the new fire brick floor of the fire box (the new fire brick floor was made of half fire brick / split fire brick). As expected, temperatures in the approximate 2-inch gap under the new fire box floor commonly rose higher than 500 F, and maybe into the 600's F (I never recorded a highest high into the 700's F in that air gap under the split brick floor of the fire box).

I believe the highest temperature I measured right in the opening of the fire chamber, where the feed tube enters the fire chamber, was 1925 F. So it was getting plenty hot in the burn chamber. However, the temperatures measured by that probe reaching 3.5 inches into the perlite fire clay insulation always remained low enough I never was concerned, never reaching over 95 F. These were spot measurements taken by hand, but they were taken regularly during the month of January. Early burns I took measurements every 15-minutes and later burns every 30- to 45-minutes. But the insulation tends to slow the heat penetration, so I'm sure they are representative temperatures with reasonable accuracy as well as precision.

I am a firm believer in providing an air gap under the fire box.

Edit: Corrected a typo, and added a couple words for better clarity (additions in bold, deletions struck through).

I would prefer to go with the weight. You can measure that as you load the stove if you wish, and if you buy a moisture probe you can also take spot readings to estimate the moisture content. Water in the wood takes up available heat, and that lowers the total heat efficiency, which is fairly obvious, this is why burning green (wet) wood doesn't work as well as burning properly seasoned (dried) wood. Efficiency of the stove, and how much heat escapes up the chimney should also be estimated.

Below are three web sites that discuss these points, and offer some starting values, as well as explanations of the calculations:

http://worldforestindustries.com/forest-biofuel/firewood/firewood-btu-ratings/

http://mb-soft.com/juca/print/311.html

http://www.chimneysweeponline.com/howoodbtu.htm

Bottom line? Depends on the species of wood and how dry it is. And how efficient your stove system is in using the heat produced. Off hand, there appears to rough agreement that somewhere around 6200 or 6400 BTU per pound, and about half that if it is wet or green wood, is fairly typical. Better estimates will result is you know the species of wood you are burning, and the moisture content.

Off hand, I'd say smaller opening to the feed tube and make it taller. Also finish the chimney (to two-feet above any part of the roof or structure, etc, within a ten foot radius). And if possible increase the fire riser height, as has already been suggested.

I have only used clay and water for my mortar and I had no trouble at all. The trick is that it must be very thin. Start with the dry fire clay, add just enough water to make it like pudding, then wipe that onto the surfaces of the fire brick that will be against one another, and I moved or grinded them together, whatever the right word is, until I actually felt the surfaces of the two faces touching. It is that thin of a "mortar joint." No thicker than 3-mm (roughly 1/8") and from what I've read, 1-mm is very common, and actually rubbing the faces together is not uncommon (as I chose to do).

I too am surprised that your floor did not get too hot. I assume you had a temperature robe down there? I've measured well over 500 F under the floor of my 6" j-style quite often.

Your thoughts on the modified feed tube have been tried, and I do not recall anyone reporting they liked the results. One caution if you try it, flame-ups have been reported when you lift that lid; folks report that if excess smoke puffs up while there is a sufficient flame present, it may release a large tongue of flame, potentially into one's face.

Heiss! Hot! heheh, that was cute. Glad you got the first test burning well. That is always a good day!

I concur with the batch box design instead of the J-style feed. Unless you plan to sit with the firebox and fiddle with the wood every 5 or 10 minutes while it is burning. If you like that, then go with the J-style feed.

Another factor may be the chimney. More is better. But I suspect you'll need to address the geometry issues first and then consider the chimney height.

Sounds to me like the geometry is wrong. Too tall of a feed tube for the riser, and not enough open space in the larger barrel. I these are the most obvious problem areas to look at. Read Evan's book for the proper geometry. The summary is on page 28, if I recall correctly.

Filling with concrete is not the best idea either, although that may not become an issue until it has been used for a while and the concrete begins to fail under the high heat. Basically as the temperature approaches the boiling point of water, the water inside the concrete must be able to expand and escape, and that usually means breaking the concrete.

EDIT (2nd edit for typo):

Just caught that the "cement" poured in at the bottom of the barrel is also refractory. As already noted, there are no overheating concerns in that case. A small point, would be to observe that Peter VDB has mentioned this manifold/transition area works more efficiently is there is a small height difference between the bottom of the barrel and the outlet; I'm thinking on the order of two inches, but I expect that is a ratio of something.

To know of if the area surround the fire riser is sufficient, you need to calculate the CSA. There must be as a minimum the same CSA of the fire riser as is available to flow down the sides of the barrel, and given there is more surface area, and therefore more friction against those sides, somewhat more would seem to be logical. The same is also true of the area immediately above the top of the fire riser and the top of the barrel.

Which makes sense. If either of these areas are smaller in CSA than the fire riser, there will be a constriction or bottleneck of the air flow.