rocket mass heater shippable core - wood box style

A lot of the shippable core designs were ending up far too expensive. So I came up with something that would work, but might need to be shipped freight instead of UPS.

I guess I was looking for some way to brute force a core where the materials cost would be less than $100. In this case, I think I ended up at a little over $100, but there is room for improvement.

This is not "the style", this is "a style".

Here is me getting started on what I called "version 0.2 alpha". A wood box with a perlite/clay/wool mix. Some firebrick and duraboard. And a metal wood feed.

Ernie finished it up (thanks Ernie!) and then put nine hours of fire to it. It seemed to be working fine until .....

So the design is generally sound. The trouble with the steel wood feed is that the steel really eats up the radiant heat. Eventually, it just gets too hot. I thought having lots of steel sticking up would act as a heat sink. I was wrong.

The bottom got burned out too.

So version 0.2 had half bricks of fire brick over an inch or so of perlite/clay/wool where the wood feed is. And the burn tunnel was an inch of duraboard over an inch (or so) of perlite/clay/wool.

Version 3.0 is the same, but with an extra slice of duraboard under the bricks and duraboard, plus an extra two inches of perlite/clay/wool mix. The steel wood feed is replaced with firebrick. A bigger box (made by Mr. Ettridge) is used.

The duraboard is very soft. When it is fired it gets hard - but brittle. I put a thick layer of perlite/clay/wool with extra clay over a double layer of duraboard on the top and a single layer of duraboard for the sides.

The beginnings of the manifold ...

We also put a piece of duct in to start building the collar.

Later we took the duct out and replaced it with a piece of duct about 8 inches long. One end we filled with screws that went into the clay/perlite/wool. The other end was crimped so duct could be connected.

When we built the bridge over the collar, we waited until the clay dried a bit and became firmer before adding more.

Then it took four people a lot of heaving to get it onto a trailer. We took it to the office and grunted it into position.



We finished smashing the clay/perlite/wool into place and then put the riser and barrel in place and connected the duct.

Hello Paul,

As far as your shippable cores. Is a material that will sustain 2180˚F adequate for use as a core material? If so, I have been working with geopolymers for the last three years. They are cheap and rather easy to use. If that is not high enough Magnesium Oxide can handle much higher temperatures, but the MgO we use is mostly from China, but there is an old mine that has reopened in New York.

Other cores were created with far more interesting materials. This one was made with perlite, clay and wool. The clay will tolerate 3000 degrees or more. The perlite will turn to glass at about 2300 degrees. And I am hoping that we will be pushing to 2800. I've been told the perlite will glass, but you end up with just a thin skin of glass and your inside becomes glassy.

In this case, I was using duraboard for the burn tunnel. I think it will hold well to 2700.

Paul Wheaton : Just to remove any confusion, a couple of links to Duraboard supply would be nice, google sends you to perforated peg board, you use hooks to hang your tools on !

Jason Learned :I will send you a P.M. so as not to steal this thread, I am interested in Geopolymers and several of your fellow members are playing with water glass ! Big Al

Please send a link or explain more on the perlite, clay and wool mix...And this duraboard thang.... The.

Tim is building the box !

Tony and I will be living with the rocket mass heater in a tipi on the Laboratory this winter and after some technical difficulties with our first core, we have built the Wooden Box style core to replace the other one and have some photos to share:

Duraboard burn tunnel is added and the beginning of the heat riser

shaping the heat riser and manifold with "cobbish" or clay, perlite and wool:

completing the manifold:

the first burn: performed well. It will take some more time to dry thoroughly.

the materials preparation is the most time-consuming task. Large amounts of clay are needed. We mixed it in a garbage can with a corded drill and paddle bit until it reached a whipped pudding state. The "cobbish" was mixed to different consistencies with perlite and wool for the shaping of the heat riser and manifold. z

Et Al : Put me down on the list, I want one for my yurt,portability will not be a problem it will be a plus! As this will be for my 12' yurt, I would like to experiment with a 4'' system.

Big AL !

I popped down and visited with Emily and Tony as they worked on this. I think it turned out mighty purty. I think that if we were to try to make another one of these, the box should be a little bigger.

This also makes me wonder if the riser in the office might be too short!

Another thought: the core in the office has the duraboard and fancy riser stuff for the full burn tunnel and riser. Whereas the core for emily and tony is using a lot of perlite/clay/wool mix (cobbish). I wonder how the cobbish will perform after an eight hour burn. I have heard that the perlite will turn to glass, but stay in place - thus forming a very nice glassy layer. I am looking forward to seeing this! Plus, I am curious how it will do over a full winter.

Emily and Tony used SIX bags of perlite for this. At $16 per bag, that's damn near $100. We do have a source that could get that down to $80. And Emily and Tony think they might be able to do things that might reduce cobbish use by 25%.

There's over $100 in firebrick here. Probably $60 worth of duraboard. Maybe $50 in wood and screws.

I think we might be able to cut materials expenses to $60 (perlite) + $75 (firebrick) + $40 (duraboard) + $50 in wood and screws: $225. Ouch. I was hoping for under $100.

If a person were going for a non-portable solution, they could eliminate a lot of the perlite and duraboard. Plus they could eliminate all of the wood and screws. So I guess a person is paying an extra $100 or so to get a cob core in a box.

paul wheaton wrote:

There's over $100 in firebrick here. Probably $60 worth of duraboard. Maybe $50 in wood and screws.

Wow! how many bricks then? The going price is $1.5 a brick (whole or split) around here brand new from the building supplier. I didn't think I saw that many bricks.

If a person were going for a non-portable solution, they could eliminate a lot of the perlite and duraboard. Plus they could eliminate all of the wood and screws. So I guess a person is paying an extra $100 or so to get a cob core in a box.

A steel stove is $600 to $2000 as far as I can tell, but will never work as well (maybe 10%?). A masonry heater is closer to 10k + ( the + means multiply not add a few bucks) for similar performance. By the time the rest of the RMH is built I think even with new materials you are around the cost of steel wood burner, so that is not bad. The added plus, is that a transportable unit can be tested in a lab and may be able to used in permit built home... at some time down the road.

The original idea of the RMH, like wofati, is to build it for almost free plus labor. When you are building only one unit and have more time that money, you build slow and collect first. As the need it now part comes into play, it costs. Also when building any number of something for testing or to sell, it becomes better to trade a higher price for known supply.

The last batch of bricks I got for free. many are broken, but a lot are whole too.... I am not any where near ready to build, but I have to collect when the opportunity shows up.

There is no right or wrong way to obtain parts, buy or scrounge. I am glad to be able to watch the various ideas as they come out.

The biggest problem in our society is the "everything is a commodity" thinking. People get used to just buying the cheapest and pretty soon that is all that is available. choice is lost.

et al : Being generous with a figure for waste, and not a lot of time trying out templates, how many pieces of dura board bases can you make out of a sheet ?

So if I went in with some friends got two bases, or more, storing extra future bases in a box somewhere, say a nice sturdy wood one with wood and screws ?

There are commercially built 4'' systems so we Know it can be done, some use of 100 year old red brick to fill in corners, say 5 bags, we can get it down below
$200.

I bet that probably 40% of all the people who have built one RMH, have or will build more ! I was hoping for less cost, but right now I would settle for plans and
use these Forum / Threads for a helpline ! Big AL !

This pic came from Mr. Ettridge. This is the box from version 0.2 that you can see with the burns above.

I drew on the arrows and numbers.

1: note how the gap was very cooling.

2: this is where the firebrick met the duraboard. That particular gap was not cooling at all!

3: note how the duraboard was very insulative. The wood is still burned, but much less than the firebrick. And this is a point that is much hotter than the firebrick.


With the new version, the bottom seems to be staying quite cool while the back (side) will feel pretty warm after a three hour burn. I think the box should be about an inch longer in future versions and put a bit more perlite and clay behind the wood feed.

Duraboard.....duraboard. A lot of talk about duraboard. And other materials to build RMH's.

We need one place on permies where ALL materials to build RMH can be found. It should list Where to buy (online and offline), Who bought it and a review of the product, What material with Brand name & spec sheet, and the ONE place it needs to be used or multiple places it can be used in a RMH.
As well as any other pertinent information.

So I'll ask. Where can I buy duraboard? What's the brand name? How much will it cost me? What applications? Can I use it to keep high heat from burning my wall (studs, straw/clay) the RMH is next too?

Living in a small town, many miles from a big town, it's nice to know what, where, how much, etc. before you drive miles to buy/get it.

how about this... build a mold for your core and ship that(along w/the formula for ericas mixture). i'm a ups person and can tell you about the process of shipping up to 150lbs if you'd like.

Pack McKibben wrote:Duraboard.....duraboard. A lot of talk about duraboard. And other materials to build RMH's.

We need one place on permies where ALL materials to build RMH can be found. It should list Where to buy (online and offline), Who bought it and a review of the product, What material with Brand name & spec sheet, and the ONE place it needs to be used or multiple places it can be used in a RMH.
As well as any other pertinent information.

So I'll ask. Where can I buy duraboard? What's the brand name? How much will it cost me? What applications? Can I use it to keep high heat from burning my wall (studs, straw/clay) the RMH is next too?

Living in a small town, many miles from a big town, it's nice to know what, where, how much, etc. before you drive miles to buy/get it.

Pack,
This is a product from Johns Mansville. It is used in construction as an underlayment for roofing product and other applications such as sound (acoustic) attenuation. Size wise, they are 2'x 4'. Here is the spec sheet
http://www.specjm.com/products/roofing/duraboard.asp. You can get them online but the shipping nearly doubles the price. You can special order them from Home Depot or Lowes and avoid the shipping costs. You won't find something as specialized as this in stock. They may be found at a roofing wholesaler or insulation or acoustic supplier. (Depends how far away from 'civilization' you are.) I'm seeing prices of about $50 for 6 sheet bundle or they can be purchased in single sheet quantities. In your situation, your phone is your best friend and call around to see who has heard of it and can get it on special order, if they don't carry it in stock.


Duraboard is only a trade name. Roxul makes a similar board Rockboard 40,60, or 80 (different thicknesses), Owens Corning makes a board called #705 (not very sexy name), Certainteed makes a board called 'Symphony'. and the are several other companies. Just Google "vermiculite board"

Another words. Don't get wrapped up in searching for only one particular brand that may not be carried in your area when other manufacturers are probably supplying the need with their own brand of the same generic product.
Good Luck,

Jim: thanks for that info. I have Roxul's rockboard 60 on my list. And you're right about different names, etc. I went to Lowe's today to place an order. Thanks again for your help.

Pack McKibben wrote:Jim: thanks for that info. I have Roxul's rockboard 60 on my list. And you're right about different names, etc. I went to Lowe's today to place an order. Thanks again for your help.

Just to finish the thought for others. Here is a link from the JM site to find a retailer in your area. http://www.specjm.com/locator/default.asp?tab=roofing&type=&menu=contractors
Your Welcome.

Is there anything that can explain the performance, or practical use of the dual exit flue area? Does this dual exit flue have beneficial/practical use because of variable outdoor wind, or during cold start-ups?... or maybe this is just the way your lab testing apparatus is setup.

james beam

James,

Response in this thread.

Is the wool in the "cobbish" rockwool?

William Bronson wrote: Is the wool in the "cobbish" rockwool?

Sheep's wool. It can take temps up to 1100 degrees.

Whoa! I would never have thought that...

Paul,
re: The Shippable Core

I know this is an evolving topic as we speak but....

I was wondering if you could expound on the criteria you and Eric would go by, to 'BLESS" a RMH core, to feel like a stove or idea or plan was an endorsible idea if we were to either present to you as a finished project, a plan or a goal to shoot for. REALLY, what do you see is a truly realistic, marketable stove concept?
1. How much would a core retail for? (2x+ the materials and manufacturing costs.) vs. how much would people be willing to pay? (It would be great to get other feed back on that from the rest of the audience.) (Yes the 'everything for nothing down' argument will always be there on the sidelines.)
From your own personal experience you can see that JUST the materials alone could easily hit $100+. Then who works for free? What's your time worth? Other business related expenses like P and O.
2. Since we really can't measure BTU's, how about some target temps at the feedtube, burn tunnel, bottom of the riser, midpoint of the riser, top of the riser.
3. How much time and effort would be required by the end consumer? Is this going to be plug and play? Are you considering maybe a wooden burnout model where the end user provided their own cob mix? Or the consumer tracks down their own barrel or perlite.
4. I personally can't see how shipping a cob product could make any sense. I'm imagining that the breakage factor alone, due to poor handling by third parties could eat up ANY possible profits when claims are brought back to the manufacturer for shipment damage.

I can't think of many other parameters to set up but a discussion would help to clarify the discussion.

Jim LaFrom wrote:Paul,
re: The Shippable Core

I know this is an evolving topic as we speak but....

I was wondering if you could expound on the criteria you and Eric would go by, to 'BLESS" a RMH core, to feel like a stove or idea or plan was an endorsible idea if we were to either present to you as a finished project, a plan or a goal to shoot for.

I can't answer directly, but I do know there has been work on making a "permit-able" design. In the end any shippable core will require a lot of extra work to install anyway, so why bother? Unless this core allows something more. Having a wood heater that works with a permit normally requires:

1) A small enough unit it can be sent to a lab for independent testing like all those metal boxes.

2) A certified masonry stove builder to build it for you and a finished weight of over 1800lbs. (these start at $10k and go up to forever)

3) a unit that is the only method of cooking in the building. (in some places)

A shippable core can be standardized and tested. It can be assembled exactly the same way every time and behave as tested. The bench is part of the flue pipe and only has to meet normal safety rules. It can be used or not. In fact the core can be sold as a rather efficient stove all on it's own even if the exhaust goes direct to a vertical flue (not as good as with a bench sucking more heat out). But before a lot of people can use it, there has to be that little epa stamp on it. (or whatever the local county/district/whatever wants). The heater could be rated for a number of feet allowed to be horizontal before attaching to the vertical flue.

Anyway, I don't know what exactly is possible is a certified core, but that is what would allow wide adoption of the RMH.

I think this thread is for talking about how to make this wood box core. Is that what you want to put together?

If somebody wanted to make and sell this core, I could see giving my blessing. As for how much people would spend: I think that right now somebody would spend $650 to have this core at their house. It would probably be about $300 for freight and $250 for materials, so it would take some work to get the materials cost down and possibly the freight cost down.

I like to think that there could be a half dozen other designs that will come to market in the next year that will have lower materials cost and possibly lower shipping costs. But there is a lot to be said for a core that is available TODAY.

connie me wrote:how about this... build a mold for your core and ship that(along w/the formula for ericas mixture).

I am a newbie.
Nonetheless, I am intrigued that - as far as I can see - this idea of shipping just the mold only has not been explored further.

Such a mold could be sacrificial or re-usable i.e. pass on to next person wanting to build, and very likely lighter than 50lbs.

The only other thing needed is the recipe for the cob mix - is it to have straw/ wool/ perlite or what added to it?

Since good-enough clay is usually available for free in most places in the world, sourcing clay itself should be no problem locally and so would save nearly all shipping cost.

As for recognising suitable clay - refer Build Your Own Earth Oven: A Low-cost Wood-fired Mud Oven; Simple Sourdough Bread; Perfect Loaves

Gavin Phillips wrote:
Nonetheless, I am intrigued that - as far as I can see - this idea of shipping just the mold only has not been explored further.

Such a mold could be sacrificial or re-usable i.e. pass on to next person wanting to build, and very likely lighter than 50lbs.

The only other thing needed is the recipe for the cob mix - is it to have straw/ wool/ perlite or what added to it?

More on this!!!

Gavin Phillips wrote:
Such a mold could be sacrificial or re-usable i.e. pass on to next person wanting to build, and very likely lighter than 50lbs.

Sacrificial would need to be ultra cheap, think blister-pack packaging. Which would not work well for this application... A nice 3/16" thick reusable polyethylene mold would certainly be well under 50 Lbs and UPS shippable. Tooling costs for injection or blow molding would be prohibitive ($100K) However rotomolding tooling for something this size could very well be under $10K for a professionally built cast aluminum mold. Possibly half that for fabricated (welded) tooling and less still for a crude prototype.

Production costs are higher for rotomolding but every thing is a trade off.

Oh, hi I'm new here.

connie me wrote:how about this... build a mold for your core and ship that(along w/the formula for ericas mixture). i'm a ups person and can tell you about the process of shipping up to 150lbs if you'd like.

Brilliant solution to the problem, while injection molding and roto are still very expensive, there is no reason some ingenuity and engineering couldn't be put to good use in coming up with alternative cheap and easy solutions sourced locally or provided in "disposable" forms.

Definitely should be explored more.