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rocket mass heater - exaust can be simpler than chimney

paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
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Somebody was asking me about the exhaust from a rocket mass heater and how it seems so much simpler than a traditional wood stove chimney.

So my best video was from a large rocket mass heater in a building called "myrtle".  And when the fire is running hard there, you wander outside and look at the exhaust.  There's this little bit of steam sorta dribbling out.  And it feels about room temp.

And you go back inside and look in the fire hole and there is this firestorm going on.

I'm told that what happens is that the combustion chamber reburns everything quite completely.  Matter and gas are converted to heat.  Not much is left afterward.  In the combustion chamber, you can kind of hear that "rocket roar" and you know there is a lot of material going through there, but the combustion chamber is designed to be freaky hot.  So what gas there is, is freaky expanded big.  And then the heat is transferred to the mass, so the gas shrinks. 

Interesting related tidbit: since nearly all of the rocket heaters there are made from salvaged stuff, they apparently ran out of 8 inch duct when making the mass heater for myrtle.  So they used 8 inch duct for the first half and 7 inch duct for the second half.  It's been running without a hassle for seven years.  Ianto (the primary instructor and the author of the book) made this decision saying that by the time the air reached the seven inch duct it has already shrunk so much, it would make no difference.  I think he mentioned that you could even use something smaller still.

The moral of the story is:  yes, the dryer duct style exhaust should be fine.  Keep in mind that what comes out of that is steam, CO2 and CO - so I wouldn't want to put it right next to a window or door.


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paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
One more thing.

If you had a conventional wood stove shooting the smoke out like a dryer vent, then your house would fill with smoke and the outside of your house would be blackened.  So conventional wisdom is to keep the chimney exit above your roof line.  That way, all of the pollutants/smoke will go away from you (and bother your neighbors?). 

To call the output of a rocket mass heater "smoke" just seems less than accurate.  "exhaust" seems more accurate.  And when you look at the little trickle that comes out ...  and the way it comes out, as a little trickle ...  and it seems heavier than the surrounding air ... then it seems like a bad idea to try to push all of that up to the roof line.  "raining" it onto the ground seems wiser.




                        


Joined: Apr 08, 2009
Posts: 9
I'm just curious, you kind of commented, but I thought that it was very important for the flue to be above the roof line so that the fire would draw properly. Also, is it important to be able to clean out the exhaust that goes under the bench? Thank you for your videos!

-Jeff
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
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If you have a conventional wood stove, then, yeah, you need to make sure the chimney is higher than the roof line.  There's a lot of reasons for that.  One of the biggest reasons is that you don't want all that smoke getting in your house!

But this stuff is different.  First, there is no smoke.  And by "no smoke" I mean that it is near zero.  So near zero that if you smell the exhaust it smells "smoky" like if you are out on a walk and somebody a few houses down might be burning a fire.  If you tried to smell a regular chimney the same way, you would be on the ground coughing up a lung for several minutes. 

Does this help?

As for cleaning out the exhaust - my understanding is that this is something one might do every seven years or so.

                        


Joined: Apr 08, 2009
Posts: 9
Yes it helps. What a very valuable technology!
Jocelyn Campbell
steward

Joined: Nov 09, 2008
Posts: 2669
Location: Missoula, MT
    
  71
Just to clarify JHi's question about the draw - you don't need a chimney for the draw in a rocket stove.  The draw happens in the combustion chamber, not from the exhaust pipe(s). So the combustion chamber pulls the fire and smoke from the burn chamber, and then pushes it out through the exhaust pipe. It's very different from how a chimney works.


Hands-on workshops in all shades of green - Cascadia & Seattle Eco Events Calendar | QuickBooks Consulting and Accounting Services - www.jocelyncampbell.com
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
Yeah!  What Jocelyn said!

Although I think of it more as a push. 

In a regular fire, the tall, tall chimney gets the air moving.  In the rocket stove, the combustion chamber gets the air moving.  And the combustion chamber is far stronger than a traditional house chimney.  So much stronger, that it can push the exhaust and make that "rocket-y" sound as it draws air from the wood feed hole.
                        


Joined: Apr 08, 2009
Posts: 9
Thanks Jocelyn and Paul, I am being educated. I think I will get the book as soon as I can to further digest this new information.

-Jeff
                                  


Joined: Apr 09, 2009
Posts: 6
I've been book studying the rocket stove mass heaters lately... the vid's, thanks Paul, seem to all show an exhaust flue about 5' to 6' tall on the outside of the house.

...has anyone seen an exhaust flue work simply coming out of the building, say, at knee height?

And if not, why not?  Why spend the extra money/effort on the height if the exhaust gases are being pushed out instead of drawn out.

Or, is it done, because the exhaust gases are being both pushed and pulled at various stages and the last bit of height adds a final pull?
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
Negligiblek wrote:

...has anyone seen an exhaust flue work simply coming out of the building, say, at knee height?



Have you seen the 12 rocket stove mass heaters video? 

There is one in there that is a little lower than knee height. 

I know that for the one we built at the workshop, it seems we could have made it go outside at knee height, but Ianto had us have it go out higher.  I think it was because he had us rig something up so that by fiddling with the pipe we could get inside to see and clean things if we needed to.  But I would think there would be other ways to do that. 

Hmmmm ....  I wonder why Ianto wanted us to do it that way ...
                                  


Joined: Apr 09, 2009
Posts: 6
Is it because it's easy to mount an outside thermometer on it for a early warning system like is recommended in the book?

Maybe we should ask?
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
Hi again,
Paul suggested I might like to comment on this thread.  Good questions. 

This is one of those issues where there isn't a single right answer - it depends on a lot of aspects of the situation.  (Even "why Ianto wanted us to do it that way" doesn't necessarily have a single right answer.  But you could ask him.)

The basic concepts you've understood: a rocket stove uses internal draft, which in some ways can substitute for a vertical/external "chimney."  So do you put a chimney on it as well, or not?

Here's some partial answers from Ernie's and my experience:

Reasons to put a chimney on a rocket stove:
Draft / Physical:
1) If your exhaust is coming out at 90 degrees Fahrenheit or hotter, a vertical exhaust will likely aid draft.  (This is the rule of thumb from Dale and the Masonry Heaters folks.)
2) Wind blowing across the top of a chimney generally enhances draft.  (Venturi effect?)
3) You want a reliable draft and avoid smoke-back into the house, even at the expense of losing a little extra heat to the outdoors.
4) Wind blowing directly into a low-down horizontal exhaust, or pressure differences such as can occur among buildings and landforms, can create draft problems.  A vertical chimney above the house avoids most of these localized wind problems via #2.
5) You don't use your rocket stove that often, or you like to start it when outside air may be warmer than the interior of the rocket stove.  A "primer hole" in an exterior chimney can help you get the draft started until the interior heat riser warms up to operating temperatures.
6) You want to place the masonry heat exchanger in the center of your house, the most effective placement to use its radiant warmth, without sending an exhaust pipe across a room to the exterior wall.
7) You have built your stove in place, and tested it both ways, and it works better with an external chimney.
Social / Biological:
You want the "smoke" or exhaust up out of people's way.  (There can be a small amount of smoke in initial lighting of any stove, and a larger amount in case of problems like overdue maintenance.  Invisible exhaust gases could include CO, and in a confined area even CO2 / the absence of oxygen could be dangerous.)
9) You have something that would not appreciate smoke/exhaust along that wall: smoke-sensitive plants, or a nearby fire that would choke on exhaust, or toddlers, monkeys, dogs, rats, or other curious critturs.
10) You want people to understand that the stove is hot, and having a chimney nearby seems like a good way to do this.
11) You want your stove to pass inspection as a "Masonry Heater," and you suspect that local code officials will be much more comfortable approving a factory-built chimney than a weird exhaust-pooper.
12 ) You want it that way.

Reasons not to put an exterior chimney on a rocket stove:
Physical / Draft:
1) Your thermal mass is sucking so much heat out of the exhaust that it exits too cold to rise (less than 90 degrees F, dependent on local climate variables).
2) You tend to burn the stove when it's cold and the weather is very hot, and the exhaust is comparatively too dense to rise.
3) You know the wind dynamics of your place, and have chosen to locate the exhaust on a side of your house that has a reliable natural draft or "vacuum" away from the building [under cold-season conditions when you'd be running the stove].
4) Your external chimney would need to be so long, and would tend to be so cold, that it would "plug up" with condensing, cold exhaust fog.  (But... a longer, hot chimney would draft better than a short one...)
5) You have built a rocket stove and tested it in your own location.  In practice, it tends to put out a dense fog rather than a light steam.  Removing the exterior chimney and letting the exhaust "poop" out makes this particular stove run better most days.
Social / Biological:
6) You are trying to pretend your rocket stove is a clothes dryer. 
7) "Temporary" installation: you want to test-fire the stove without putting a hole through your roof.  (We sent our exhaust out a window until we had gotten the right materials together to install the vertical chimney.  Three tons of cob is a strange idea of "temporary," but test-firing is a good idea.  You may be planning to build onto your house, or looking for more materials / saving up to buy new stovepipe.)
Your stove is not in a heavily-peopled area, and you want to do something interesting with the exhaust.  Like see if it will help you grow 'dinosaur plants.'
9) You have reason to suspect that the usual traffic at your location would mess with / bump into / not tolerate a stovepipe chimney, but wouldn't have a problem with a hole pooping out fog.  (Wheelbarrow path, or vertically-oriented monkeys, or something like that.) 
10) You want it that way.

Conclusions:
It can be hard to tell which of these considerations will be most relevant in a given situation.
  We've built stoves that didn't work as well with a horizontal exhaust, and worked great with a vertical chimney. 
  We've also built stoves that worked poorly with a vertical chimney, and worked somewhat better with a horizontal exhaust.
  Note that #5 "pro" and #2 "con" describe almost the same situation (cold stove, hot outdoors), and two different ways to deal with it.

So what to do?

  Some people put the exhaust exit at knee-height, and then use an external elbow or "T" to connect to a vertical chimney.  This creates the option of external draft, easy inspection and cleaning, yet you also have the option of removing the chimney and using a horizontal exhaust. 
Ianto's stove in the "Myrtle" is built like this.  It exhausts near a tree, where the pipe is protected from traffic bumping it.

Another option is to run the exhaust up next to the hot barrel of the combustion core.  This transfers a little heat to the exhaust, arguably wasting it.  But it does ensure an updraft out the chimney. 
The new stove in the office (from the 2009 Pyromania workshop) was built in this configuration.  Ernie and I use this trick on indoor rocket stoves to minimize smokeback, and sometimes on outdoor stoves to cue visitors' attention.  (Look, that barrel is right next to that smokestack!  It might be hot!)

Is this all armchair curiosity?
Or are folks seriously thinking about building one of these? 
Playing with prototypes is one of the best ways to get familiar with the design principles.
If anyone wanted to host a workshop,  Ernie and I have family in Washington and wouldn't mind traveling up there.  Let us know.  We're online at http://www.ErnieAndErica.info


Play with nature, make nifty stuff:
www.ErnieAndErica.info
                                  


Joined: Apr 09, 2009
Posts: 6
thanks for your experienced input...

..no, this is not arm chair curiosity.

I'm in NM and help out in MO. Rocket stoves both mass and cook stove types would save Natives here on the Reservations a great deal of fire wood collection.

But I have to get it right, just one failure out here on the Reservations would ruin the reputation of the Rocket Stove: hence all the questions.

I also have to convince OSGV that rocket stoves really work.  Again, I have to get it right the first time with this group too.


Kathleen Sanderson


Joined: Feb 28, 2009
Posts: 977
Location: Near Klamath Falls, Oregon
    
    1
My interest is more than armchair curiosity, too.  I live with my grandmother in her house, an all-electric manufactured home.  Even though she's lived with wood stoves all her life, she doesn't want one in this house.  *I* want a wood stove for, at the very least, back-up in power outages!  The only place I'll be able to put one is in the attached garage, but at least we'd have a space we could heat if necessary.  Cost is definitely a consideration, as I don't have a large income (just the opposite).  So far I haven't seen any better solution than the rocket stove.  I do have Ianto's book.  I'm hoping to have time to work on the stove this summer -- it's one of my priorities, but I've also enlarged the vegetable garden by 50% and need to put up as much food as I can, so we'll see how far I get with the stove!

I'm very pleased to see that expensive triple-wall or insulated flue pipe isn't necessary for a rocket stove!  And I'm also pleased to see that I might not have to make a hole in the garage roof in order to exhaust the smoke.  Clay soil we have, I can get sand (and straw for cob), and I'm sure I can scrounge a barrel or two, plus whatever else is needed for the stove. 

Thanks for all the information, everyone!

Kathleen
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
Erica,

As I read your post, I had another window open and typed in lots of questions.  By the time I got to the end of the post, you had answered all of my questions!

It's like you have .... powers ... or something ...

Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
Freeholder wrote:
I live with my grandmother in her house, an all-electric manufactured home.  Even though she's lived with wood stoves all her life, she doesn't want one in this house.  *I* want a wood stove for, at the very least, back-up in power outages!  The only place I'll be able to put one is in the attached garage...
...I've also enlarged the vegetable garden by 50% and need to put up as much food as I can...


Funny, the number of things my grandmother described from her childhood that I thought were cool, but she never wanted to repeat again .

In the permaculture spirit of "the problem is the solution," let's combine your food project and your rocket stove curiosity.
   How about building a temporary rocket cook-stove outdoors, and doing some of your canning or jam-boiling on it?  You can get a feel for how it works, test different proportions for the feed tube, burn tunnel, and heat riser.  You don't need to build the bench for cooking, just the stove core.  You can also put a barrel over it propped up on bricks to try out the covered version, good for simmering things. 
   By the end of summer, you'll have a pretty good idea how it works.  (If you can maintain canning temperatures by hand-feeding a woodstove, you've definitely mastered the design concepts involved). 

When it gets too hot to stand outside canning all day in the sun, you can start working on the "real" stove in the shady garage. 

(And I'm hearing my grandmother again, talking about a job where she was the last one left to finish a particular project, and how much she enjoyed being able to choose when to work in the shade or the sun.)
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
Negligiblek wrote:
thanks for your experienced input...

..no, this is not arm chair curiosity.

I'm in NM and help out in MO. Rocket stoves both mass and cook stove types would save Natives here on the Reservations a great deal of fire wood collection.

But I have to get it right, just one failure out here on the Reservations would ruin the reputation of the Rocket Stove: hence all the questions.

I also have to convince OSGV that rocket stoves really work.  Again, I have to get it right the first time with this group too.



No offense meant with the "armchair curiosity" question.  Some folks come at this from a "science playground" perspective, others have very practical needs, but the initial questions can sound the same.

I can appreciate the need for success.  We're trying to do the same thing getting a stove permitted here in Portland, Oregon.  I'm told that persistence is more important than being right the first time.  If they keep raising concerns and you keep coming back, either you annoy them into submission, or you work out an improved solution that is a win-win.

There have been a lot of experiments on the reservations, and disappointment has been all too common. 
  In India, the villagers tend to love the rocket stove idea: It's a vast improvement over 3 bricks on a hearth.   People on American reservations have a wider variety of existing options.  They may want "all mod cons" or all traditional, or something in between.  A rocket stove is none of the above.  There are also cultural needs & barriers, and some well-founded mistrust of outside suggestions.  We've made some SNAFU's ourselves, like bringing cookies to a meeting with a lot of diabetics.  ops:

   You know to contact elders first, and listen, right?  I've heard about activists being turned away in anger by functioning tribes, because they contacted a young rebel first instead of the appropriate elders. 
  There may be hidden aspects to the situation.  Collecting and splitting firewood may be an important part of growing up. 
  My uncle got hired by an Alaskan tribe to help them with youth programs - one question was whether to install a basketball court.  He asked, "What did you do when you were young?  You didn't play basketball, did you?  So why a basketball court?" 
   They said, "We went to get water every day.  We collected fuel for the fire and the lights.  Now the water turns on at the faucet, the lights turn on with a switch.  They sit around with nothing to do.  We think a basketball court might help."
  There may be a more pressing problem - like the people who should be taking care of the firewood, are moving away.  Or the homes are poorly insulated / stick-frame instead of masonry, and traditional fireplaces don't work in a paper-thin house.  Rocket stoves might not be the whole answer, or even the most important one.  You might have to haul a few cords of firewood for someone in need, or build a stove in your own home, before anyone would listen to your ideas about their situation.

  If you already have a good relationship with the tribe in question, and know that this is something they want, good!  Try rocket stoves.  If they don't work, you'll find something else.  Keep trying, take your time.

  Ways to small: maybe build yourself a Pocket Rocket and try it out as a warming fire for a backyard get-together.  See what people think. 
Or an outdoor cookstove like I was suggesting to Kathleen, or a smoker.   
  Invite people to be part of the experiment.  See if there's a local adobe-maker, plasterer, oven-maker, or fire-keeper who might be interested in the project.  There might be a modified version that's better suited for local traditions.

  If someone can attend a workshop, check www.naturalbuildingnetwork.org for the closest ones.  Maybe in California or Mexico.  Cob Cottage Company will be in Tlaxcala in August and December - you could inquire whether the instructors will be covering fuel-efficient stoves.  http://www.cobcottage.com/workshops

There's also the Natural Builders' Colloquium, or the Village Building Convergence.  And most instructors are willing to travel if you can cover the travel costs.

Here's an example of cob building by a young Hopi woman, who wanted a masonry home instead of government housing.     http://indiancountrynews.net/index.php?option=com_content&task=view&id=1678&Itemid=33

Approvecho has done some work on fuel-saving cooking rockets for arid regions.  They don't self-feed like the J-tube variety, but they're working with people who are used to hand-feeding fuel as they cook.   Their L-tube with pot-sleeve is quite fuel-efficient.  http://www.aprovecho.org/web-content/media/rocket/rocket.htm,

As seen in India:
commercial/metal versions:    http://householdmatters-india.blogspot.com/2009/02/brilliant-rocket-stove-was-developed-by.html

home-built/adobe versions: http://e-goodstove.blogspot.com/

Please note that in India, these replace a 3-brick open-hearth "stove" which is smoky and can cause severe accidental burns.   In the USA, people's standards may be higher, and they may be harder to please. 

Whether these are an improvement over local designs, I don't know.  Local stoves may be well-adapted to local conditions in ways that are not immediately obvious. 

For example, one friend taught an "earthen oven" workshop in South America.  The idea was, every family could bake their bread for free, in an insulated oven, instead of hiring the local baker. 
   Returning some time later, he found that the village was using more fuel and had lost some of its social stability.  The baker's expertise was no longer valued, and every household was competing for fuel.  More fuel was being used to bake less bread, because each oven was being heated up and then only used for one or two batches.  (I bet the bread wasn't as good, either.)  The 'improvement' turned out to be an unwarranted displacement of an adaptive, local solution.

  So now I've probably insulted you again, by offering unsolicited advice about how to do things with tribes instead of to them.  At least I'm consistent.   

What's OSGV? 

-Erica
http://www.ErnieAndErica.info
                        


Joined: Apr 08, 2009
Posts: 9
So Erica, you are a wood fuel savant! I have a bit of scientific curiosity, though my hands are way in the dirt. I live in the high desert areas of Idaho and working on permaculture studies. It gets really cold here. I'm trying to understand how the long exhaust works, the cob must obviously take on the heat of the vent. (hence the butt warmer effect) There is straw in cob, does the straw kind of burn out leaving a sort of insulated brick? It seems like you wouldn't want an insulator like material at that point, but a material that soaks up and transfers as much heat as possible, and as fast as possible. If that premise is correct would it be possible to pour sand around the pipe or even use concrete? I am curious because I don't know of any clay beds around here, though the options I mentioned are not so earth friendly. I hope you are not getting tired of hammering simple info into brains like mine, I think that as a society many of us are just so far removed from natural processes and looking back longingly, and hoping we can return.

-Jeff
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
JHi wrote:
So Erica, you are a wood fuel savant! I have a bit of scientific curiosity, though my hands are way in the dirt. I live in the high desert areas of Idaho and working on permaculture studies. It gets really cold here. I'm trying to understand how the long exhaust works, the cob must obviously take on the heat of the vent. (hence the butt warmer effect) There is straw in cob, does the straw kind of burn out leaving a sort of insulated brick? It seems like you wouldn't want an insulator like material at that point, but a material that soaks up and transfers as much heat as possible, and as fast as possible. If that premise is correct would it be possible to pour sand around the pipe or even use concrete? I am curious because I don't know of any clay beds around here, though the options I mentioned are not so earth friendly. I hope you are not getting tired of hammering simple info into brains like mine, I think that as a society many of us are just so far removed from natural processes and looking back longingly, and hoping we can return.

-Jeff

If by "savant" you mean "person who likes big words," I'll accept the compliment.
I like explaining things.  Your main danger is that I especially like explaining things late at night, with variable results.

That's a very perceptive question you asked there. 

The cob does soak up heat from the exhaust pipes; we sometimes refer to this "butt-warmer" section as the "heat-exchanger" or "thermal battery."

Straw as insulation:  Yes.
That's exactly what happens with wood-fired earthen ovens, where the insulation helps retain heat inside the oven.  There's usually a dense plaster layer inside, then a straw-containing insulative layer outside.  Some of the straw burns out, leaving insulative air holes. 
You can create insulative ceramic foam by mixing straw or sawdust with clay, drying and firing it.  Or you can get perlite or vermiculite, and stabilize them with a little clay slip.  (A slight sacrifice of insulative value, but it avoids the perpetual risk of the "hole-in-the-bean-bag" effect.)

With a rocket stove, you want insulation in the areas where you don't want heat transfer.  E.g: right around the burn tunnel and heat riser, to maintain a hot internal burn temperature; or between the stove and an exterior wall.

Take a look at our rocket stove pictures at http://picasaweb.google.com/eawisner .
You'll see that the internal layers (right next to the flue-gas ducts) are straw-free.  We call this "thermal cob." It has more thermal mass than sand.  It's probably a similar density to brick.  Concrete is more dense, but also much more permanent - a disavantage if it's your first stove and you might want to correct an error or install an extra cleanout down the road.  Cob's advantage here is that you can re-wet it, remodel it, and re-mix it, and re-use it if you need to change anything on the stove.  You can incorpoate dense chunks of concrete rubble, brick, or local stone into the thermal mass too (and we've done so).

We use a thin layer of straw-containing cob on the surface for structural reasons: it helps avoid chipping off the edges of the bench.  In walls or structures where there's tension (like when someone sits on the bench with their weight toward the edge, "pulling" the edge away from the main mass), you need tensile strength.  This is provided by the straw or something to replace it.  (timber, fabric, wire, etc).

We used perlite insulation between our stick-framed walls and the stove's thermal mass. 

"Burning out:" Around the heat riser, straw or sawdust will burn out.  Further down the heat-exchange ducting, it's not likely to get hot enough to burn off.  But we still avoid it for the thermal mass reason.

Side note:
Ernie's favorite building solutions for extreme climates (where the average seasonal temperature is uncomfortably hot or cold):  Insulate the building's exterior, then use thermal mass on the interior.  Ernie likes "bale-cob" (straw-bale insulative walls with a cob thermal mass layer on the inside). 
Other exterior insulation options include light straw-clay, conventional insulation, fiber-fill, fancy foams, or other alternatives. 
Thermal mass options include cob, (insulated) concrete slabs, earthen floors, or even black-painted water barrels.  A sealed water barrel can get surprisingly hot in the sun.

Any of these, located conveniently inside your insulation envelope, will soak up solar energy if they're exposed, and re-radiate at a lower temperature for hours.  Thermal mass outside the insulation might keep the wind out, and give you sturdier walls, but there's much less benefit from the thermal storage.   

Heat Capacity:
If you really want to get fancy on an unlimited budget, soapstone is the "hot," "new"  masonry material with the highest heat capacity.  It's almost as dense as water, and about twice as dense as brick or concrete.  And it's G-O-R-G-E-O-U-S!  But the cost and embodied energy for cutting and transport are substantial.

So an "ideal" rocket stove from a permanent, expensive design perspective:
- a reinforced, insulated concrete footing slab, to meet or exceed building codes;
- insulated burn chamber with refractory materials, but keep the firebrick liner for optimal turbulence;
- a thick replaceable barrel or custom radiant heat container, which is big enough and cools quickly enough to give you the necessary drop in pressure;
- and then a bench made of soapstone.  Either square masonry channels, or stainless-steel ducting mortared in with a high-density (soapstone-grit?) mortar.
-Get your cleanouts and proportions right the first time, because this baby isn't going anywhere.

I don't know if it would be as comfortable as a cob bench, though.  Hard to imagine that sleek, stone slab feeling like a sofa; it might feel more like a tomb.  The European masonry heater folks who are exited about soapstone tend to be building vertical columns, not furniture.  They sometimes do tiled "sleeping shelves," where you have to climb up beside the chimney to enjoy the contact heat. 

Maybe you could design a rocket masonry heater as a radiant-heating wall or sculptural "mantlepiece," and just let people lean nonchalantly against it when nobody is looking.

Another Wee Hours Post By:
-Erica Wisner
                        


Joined: Apr 08, 2009
Posts: 9
Hang on my head just exploded, too much info, m-u-s-t  s-t-a-r-t  b-u-i-l-d-i-n-g. OK. I'm getting most of it, I built a pocket rocket and cooked some food with the kids, and I really want to heat with thermal mass. (By the way the pocket rocket kicked butt, its my new camp stove!) I may not be able to use a rocket thermal mass in my current house but I think as soon as I can I'm going to start testing in the garage. One problem about that is the gas water heater. Pretty sure I'll need to take that sucker off line before I have open flame out there! This is really important stuff, thanks for sharing your experience and wisdom, and middle of the night lucubration. Wait a minute I'm doing the lucubrating! Narf.

-Jeff
Erica Wisner
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Eeew exploding head bits.  Now you know why all those West Coast hippies have the giant mushroom hats. 

You might be able to find some specs on your gas heater for how far away it has to be from open flame. 
(The Rocket Stove is arguably more like a semi-enclosed flame, but arguing has surprisingly little effect on combustibles.  You'd have open flame when you light it, anyway.)

Don't know if it's worth putting in a dividing wall to keep your furnace isolated, or just move the rocket stove experiment to an outdoor courtyard if you can.
                        


Joined: Apr 08, 2009
Posts: 9
Good idea, I'll check the specs. Btw I'm on a bar soap budget and not a soapstone! 
Laura Sweany


Joined: Aug 08, 2009
Posts: 218
Location: Seattle, WA
Hi, all - Erica, thanks so much for the photos on Picasa!  I love seeing the process from start to finish. 
My curiosity is centering around the exhaust; I'm thinking that the warm steam/smoke-ish yield would be great to pipe into an attached or close-in greenhouse.  It's extra heat in the winter (when us Pacific Northwesterners get a little on the cool side), it might have a little extra nutrition from the smoke, and the water vapor might be convinced to condense on some porcelain additions to the raised beds (perhaps a few old toilet tank covers planted vertically in the beds between the plants?) for a passive watering system.  Am I following a fruitful line of reasoning, or are there major holes that I'm not recognizing?


"It is, of course, one of the miracles of science that the germs that used to be in our food have been replaced by poisons." - Wendell Berry
Erica Wisner
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lauraflora wrote:
Hi, all - Erica, thanks so much for the photos on Picasa!  I love seeing the process from start to finish. 
My curiosity is centering around the exhaust; I'm thinking that the warm steam/smoke-ish yield would be great to pipe into an attached or close-in greenhouse.  It's extra heat in the winter (when us Pacific Northwesterners get a little on the cool side), it might have a little extra nutrition from the smoke, and the water vapor might be convinced to condense on some porcelain additions to the raised beds (perhaps a few old toilet tank covers planted vertically in the beds between the plants?) for a passive watering system.  Am I following a fruitful line of reasoning, or are there major holes that I'm not recognizing?

Glad you liked the photos.

Exhaust-fueled greenhouse:
It's a line of reasoning I've been playing with myself, so I'd like to think it will be fruitful someday....
Things to keep in mind:
- Smoke & exhaust products are not good to breath.  Some plants (like tomatoes) can't handle smoke either.
- Plants use oxygen at night, and release a little CO2; this is the opposite of what they do in the daytime.  So you'd still want an air-rich mixture in your greenhouse, not just the exhaust.
- Greenhouses get steamy anyway, and condensation seems to be mostly onto the glass itself (the walls are the only part that gets cold once the greenhouse is warm and functioning).

The version I've been fantasizing about involves smoke-tolerant plants, possibly specifically selected for being evolved in the Dinosaur Age when there was more atmospheric carbon (I'd like to see how they grow in their favorite conditions).

And for safety, it's not possible for a person to be enclosed in the greenhouse.  Either it's a tent-like "cloche" that you pick up and set aside when you want to tend the plants, or it's got a big garage-door or removable wall as the only entrance, forcing you to let the air circulate as you enter.

If anybody happening upon this thread knows of a good list showing smoke-tolerant and smoke-sensitive plants, I'd love to find that info.

You can also build a rocket-warmed potting bench into a greenhouse, and give it its own dedicated firing cycle to control temperatures more carefully.
paul wheaton
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Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
I think it would be wise to route to the exhaust through a greenhouse, but not to a greenhouse. 

Beyond the issues of trace amounts of smoke, I think the biggest issues would be the invisible gasses that have no odor.  People would think they are just fine until they are seriously ill or dead. 

I think fooling with this rates right up there with trying to heat water - fiddle with this at great risk.  Even if the person that chooses to travel this path is certain that they have  the discipline such that it will never be a problem, it seems only a matter of time until a fool or a child wanders in ...

Laura Sweany


Joined: Aug 08, 2009
Posts: 218
Location: Seattle, WA
Yeah, good points.  I have no desire to suffocate my kids, friends or random critters that might wander in.  Are there any general guidelines about how much run you can get out of a system; i.e. how long you can make your tubing before the exhaust is exhausted, as it were.  I'm sure it has to do with how frequently the system gets recharged, how wide the vent pipe is, and other variables; but running warm-ish exhaust under a raised greenhouse bed after it exits, say, 25' of interior run still seems like it would be better than nothing, no?
Erica Wisner
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Location: Okanogan Highlands, Washington
    
  89
lauraflora wrote:
Yeah, good points.  I have no desire to suffocate my kids, friends or random critters that might wander in.  Are there any general guidelines about how much run you can get out of a system; i.e. how long you can make your tubing before the exhaust is exhausted, as it were.  I'm sure it has to do with how frequently the system gets recharged, how wide the vent pipe is, and other variables; but running warm-ish exhaust under a raised greenhouse bed after it exits, say, 25' of interior run still seems like it would be better than nothing, no?


Especially if you had a relatively big system, you could get another 20' of productive warmth (lower temperature) for garden beds warming. 
  If you had a little exhaust port between the greenhouse and main house, both for cleaning and for an alternate exhaust, that would be almost perfect. 
  In hotter weather, the plants don't need hot feet, and stove draft dynamics are less powerful compared to outside air.  (You need the chimney or heat riser hot, and the outside air cold and dense, for optimal draft.) 
"Pushing" another 20 feet would be extra work for your stove, but opening the middle port to let the exhaust out "early" into open air, works for both stove and plants.

-Erica Wisner
http://www.ErnieAndErica.info
Bill Kearns


Joined: Feb 13, 2009
Posts: 154
Location: E Washington steppe
    
    2
So, I've read and read but haven't solved the problem in my brain ... perhaps it's here somewhere and I've just missed it  (I have a hard time with the videos on my very slow internet connection, so if it's there ...  )

How exactly do you make the transition from "under the 55 gallon drum" to the ducting embedded within the "mass"?  Is this a channel cast into the cob?  This detail has been bothering me while everything else seems pretty straightforward.

Any insight to offer?

Thanks!
Bill


Permaculture is a gestalt ... a study of the whole. Not just how to produce more and better food, but how human life on the planet affects and is affected by the surrounding environment.
Bill Kearns http://columbiabasinpermaculture.com
paul wheaton
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Posts: 15229
Location: missoula, montana (zone 4)
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It sounds like there are several ways, but both times I have seen it done, it was done by ernie and he just made it out of cob.

Erica Wisner
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Location: Okanogan Highlands, Washington
    
  89
9anda1f wrote:
So, I've read and read but haven't solved the problem in my brain ... perhaps it's here somewhere and I've just missed it  (I have a hard time with the videos on my very slow internet connection, so if it's there ...  )

How exactly do you make the transition from "under the 55 gallon drum" to the ducting embedded within the "mass"?  Is this a channel cast into the cob?  This detail has been bothering me while everything else seems pretty straightforward.

Any insight to offer?

Thanks!
Bill


We call this area the "manifold," and there are several ways to do it.
They all bug your brain; it's usually easier to do it, or see it done, than to explain it or understand from reading.

Picture a "manifold" for a car, sewer, etc.  It's where a bunch of pipes come together, and it is whatever shape it needs to be to join all those pipes from all those different directions.

Here are three versions of the Rocket Mass Heater manifold that we've used:

The simplest to understand: A piece of ducting.
We found a recycled ducting part that went from 3x10 rectangle to 6" diameter round.  We tucked the rectangular aspect under the lip of the barrel, and joined our round ducting to the round end.  We supported the ducting with brick & mortar, and used more mortar and then cob to completely seal the joints including the rest of the lip of the barrel.  As long as the opening is at least as big as the ducting, and can't clog up with ash, this works.
Drawbacks: Not a lot of room for ash to collect, so we clean it out (through a capped T-joint) about every 6 months.

The most common/practical:  A masonry box / "hole."
  Leave an ash pit as deep as you conveniently can, between the barrel and the beginning of the ducting.  Build a box or channel around the pit with masonry.  (Brick or cob.)  Support the barrel on at least 3 sides with brick, but leave as big a gap as you can for the gas to flow into the box.  Build up the walls of the box, sealing the ducting and barrel with mortar/cob as you reach them.  Use corbeled brick, hardware cloth, scrap metal, tile, or any convenient material to cap the opening, and cover the entire thing in cob several inches thick.  Make sure the capping slab / cob is thick enough to support a person's weight in case they sit or stand on that spot. 
  Leave a cleanout to access the ash pit - depending on its size, you may only have to clean it out every 5 years, or when  rodents nest in it.

The most artistic: A "beehive" or tower.
Build a brick "beehive" or freestanding cob structure to support the barrel, leaving an opening for ash pit, and routing the ducting in through one side.  Bury the whole thing in cob, and make sure you leave access for a cleanout nearby or you'll have to pull the barrel to get at it.

You can build this manifold in almost any shape, to suit the space, cleanout methods, and parts available.  It just has to be big enough for unrestricted gas flow (same cross-section as your ducting or bigger), and it helps if there's a place for ash to settle out without clogging the system.

If you want to "cast" it in the cob, you can use hardware cloth, scrap metal, or whatever is handy to be the "form" for the cob.  I think of it as sculpting more than casting; the bricks and metal are my armature, and the cob is the actual structure that people see and sit on.

Hope that helps,
Erica Wisner
http://www.ErnieAndErica.info
Bill Kearns


Joined: Feb 13, 2009
Posts: 154
Location: E Washington steppe
    
    2
Thank you Erica for the excellent reply.  You've answered my questions and I can visualize this transition now!  Planning on an experimental heater project this fall/winter. 
Erica Wisner
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9anda1f wrote:
Thank you Erica for the excellent reply.  You've answered my questions and I can visualize this transition now!  Planning on an experimental heater project this fall/winter.   


Glad you liked it, and good luck with your project.

-Erica Wisner
http://www.ErnieAndErica.info
paul wheaton
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Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
9anda1f wrote:
Thank you Erica for the excellent reply.  You've answered my questions and I can visualize this transition now!  Planning on an experimental heater project this fall/winter.   


When you do this, are you gonna do a workshop?

Bill Kearns


Joined: Feb 13, 2009
Posts: 154
Location: E Washington steppe
    
    2
Out here?  LOL, the only guests I would have would be my cats!
paul wheaton
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Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
That's why people have tents!

                                


Joined: Sep 06, 2009
Posts: 16
Location: Two Rivers, WI
Good info, everyone. Does anyone know how to modify the mass stove for use in a basement of 2-story house? I am in NE Wisconsin, where winter can be terminal, and currently have no heating system so am seriously considering building one of these.  Do they really weigh 6 tons? And if in basement, will the heat rise sufficiently well through 2-inch wood floors to get to rest of house? 
I suppose it would have to have at least some vertical exhaust venting because horizontal would end up underground.  There is an existing brick chimney that could be used, but is that too much vertical? 
And does one have to feed it often? For coldest days here I need about 60,000 BTUs steady output. 
Also, can they burn stuff besides wood--ie, pellets, cherry pits, junk mail, and other convenient combustibles?
Have any of you had trouble getting homeowners insurance as a result of having one of these?
And less relevant, anyone know what the actual combustion temps are in the hottest part of the J? Am wondering if can adapt design for pottery kiln--it would be especially nice to be able to do so and heat the house at same time.
Thanks for any suggestions!
paul wheaton
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Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
The heat will rise, but .... my feeble understanding around all of this suggests that it might not be enough unless you route the exhaust upstairs before it goes outside. 

I think that you will have it plenty warm in the basement and that will make the upstairs maybe 10 to 15 degrees warmer - which probably won't be enough all by itself. 

But .... maybe you can include some pics and maybe a drawing so we have a better idea of what your situation is.
Erica Wisner
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Posts: 747
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  89
keralee wrote:
Good info, everyone. Does anyone know how to modify the mass stove for use in a basement of 2-story house? I am in NE Wisconsin, where winter can be terminal, and currently have no heating system so am seriously considering building one of these.  Do they really weigh 6 tons? And if in basement, will the heat rise sufficiently well through 2-inch wood floors to get to rest of house? 
I suppose it would have to have at least some vertical exhaust venting because horizontal would end up underground.  There is an existing brick chimney that could be used, but is that too much vertical? 
And does one have to feed it often? For coldest days here I need about 60,000 BTUs steady output. 
Also, can they burn stuff besides wood--ie, pellets, cherry pits, junk mail, and other convenient combustibles?
Have any of you had trouble getting homeowners insurance as a result of having one of these?
And less relevant, anyone know what the actual combustion temps are in the hottest part of the J? Am wondering if can adapt design for pottery kiln--it would be especially nice to be able to do so and heat the house at same time.
Thanks for any suggestions!


Winter can be terminal, it's September, and you currently have no heating system?
Sounds like a major appliance story from hell.  Hope you have some friendly neighbors for back-up.

On the plus side, wood-burning devices get exempted from all kinds of requirements if they're the only heating option to keep you alive.

   One of the case studies in the Rocket Mass Heaters book is about Jay Naydiuk, who lives in Canada.  He built a super-tall rocket mass heater to use for a wood-drying kiln.  There was some welding involved - he built his own steel combustion chamber, and joined two barrels for extra height.  But it's still an 8" diameter system, within the normal design parameters.
  You can probably google him by name - he has a business milling wood.  Or try www.rocketstoves.com for pictures of his stove.
Where he's at, temperatures get down to -20 or so.  If I interpret the book right, it says he gets his wood-kiln up to about 130 F.

If this is your first stove, you'll be on a steep learning curve, and trying to make it double as a home heater AND a ceramic kiln is not recommended.  You can do both things, but maybe not with the same system.  And you'd want to do ceramics in summer too, right?  You don't want to heat your house in summer.

So yes, it can be done. 
2-story house?  What age is it?
Whether it would work depends on a lot of things, including insulation, and whether the house is designed to move heat easily from basement to upper floors.  (vents, or a central staircase can help get a flow going quite nicely.  But then the upward movement of air can also create negative pressure around the stove, meaning you'd want to provide outside air to the room the stove is in for smoke safety).   As far as I know, nobody has tried to calculate BTU's of rocket stoves: it's too variable with design, fuel, and heating cycle.  Do use robust materials if you anticipate burning it constantly.

You can build a mass heater to draft up a chimney - it's a little bit technical, but with outside air coming in and a short heat-exchange run, it is very do-able.  Masonry stoves draft vertically by using <20 feet of heat-exchange, and burn more wood fast instead of bit-by-bit.

  If you need to keep your insurance, talk to your insurer to see what the rules are.  Get a copy of the ASTM specs for masonry heaters - http://www.astm.org/Standards/E1602.htm - and some pictures of Rocket Mass Heaters.  Explain that they are exempt from EPA due to their weight (a system for your house could be easily 5 tons or more).  This is because, as a type of masonry heater, they are remarkably clean-burning; and they're site-built and unique; EPA doesn't feel the need to test each one.
Plus, it's difficult for anyone to injure themselves on it, since the fire is contained entirely within sealed masonry.  And that it's something you can afford to build by winter, which may be necessary to preserve the value of your house.

  The insurer may not be able to make the call.  They often simply need it to be code-approved.  The ASTM spec drives the cost up a bit, but it's mostly commonsense stuff like clearances to combustibles.  Might restrict your options for innovation; and might require you to use ceramic/refractory masonry instead of earthen.  If they can't let you use the metal-lined earthen masonry method, then I wouldn't want to call it a rocket mass heater. 
At that point, you're building a masonry heater, and it's best to use known designs that work with those materials.  Pre-fab cores can be purchased; and installation or supervision by an experienced mason is recommended/required.

  If you can find a licensed engineer or architect to stamp your plans, you have an ace in your pocket: you can play the card "any changes to these plans, for which my architect has assumed professional responsibility, would mean you are accepting the responsibility [liability] instead."
 
 You might need to go through a special appeal to show that it's equivalent or safer than code.  It depends entirely on your local building officials, and insurers, whether this will be a simple, reasonable conversation, or a multi-year legal process.

There's a more technical forum here, you have to register but it's free, and some of the folks working on development hang out here and talk about refractory ceramic castings.  They might be able to do a ballpark for BTU's... http://donkey32.proboards.com/index.cgi

Hope that helps. 

In the race for winter, if you absolutely need homeowner's insurance more than you need to heat the whole house, you might be better off getting the best EPA-certified woodstove or fireplace insert that you can afford (even if it heats only one room) for this year.  You'd be buying yourself a safety margin, and the time to design and get approval for a better system next year.  The Chimney Sweep Online, or Wood Heat.org, are two nice websites where you can get the inside scoop on which stoves people have been satisfied with in the past.

Hope you also have a supply of dry firewood.  Burning small amounts of the alternative fuels you mention is something you may be able to get away with, but I wouldn't expect them to work as the sole source of fuel in a rocket heater. 

Ianto Evans and Linda Smiley did heat their (cob) office for a whole year using nothing but junk mail and a Pocket Rocket.

There's an idea:  A Pocket Rocket is a relatively clean-burning emergency heater, made from junk metal parts.  If you want a low-cost experiment while you're considering other options, you could make one now for an outdoor warming fire.  Use it in a nice deep sand-bed or fire-pit; the metal gets hot enough to char whatever it's standing on.  You'll get a sense of how rockets work (thermosiphon and fuel handling).
  I would not use one of these to heat a large house; the surface temperature would be nasty hot, and the materials could break down under constant use.  Like any "barrel stove," it can be hard to tell it's breaking until it's broken, and spilling hot ashes and gas into your room.  But a small one, carefully inspected and maintained, and set in a fireproof masonry bed /heat shield, can make the difference in keeping a person alive and warm. 
  We know of at least one Midwestern traveller who keeps a Pocket Rocket in her car (the parts break down to fit in a bucket with some kindling), in case she gets stranded on the road in winter.

Hope that helps with your decisions.  Best advice: keep it simple.

Good luck,

- Erica Wisner
http://www.ErnieAndErica.info
 
paul wheaton
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Joined: Apr 01, 2005
Posts: 15229
Location: missoula, montana (zone 4)
    ∞
Excellent post Erica!  Loaded to the gills with great stuff!

I'm currently staying at a friend's house and in all of my enthusiasm over rocket mass heaters, I have this idea ....

He has a brick fireplace level with the floor.  Just outside of the fireplace is a bunch of stone flooring. 

I know my friend is curious, but at the same time doesn't want anything too permanent.   But something temporary and appliance-ish could be cool.

I was thinking .....  what if I made a big bench out of 2x4's  ... something seven feet wide, three feet deep and a foot and a half high.  The bottom would be 2x4's.  Then do a six inch system in this with the barrel and fuel feed at one end.  Then routed the exhaust through this bench three times and then under the chimney part once or twice - then up the chimney.  Then sort of plug the chimney around the exhaust.    I would then fill the box with rocks and sand. 

So the overall weight would not be too much.

A lot of the heat would be left in the room.

Dismantling would be fairly easy (carry out the rocks, rebag most of the sand, and shop-vac the rest of the sand).

Right now, this house functions on pure electric heat and is freaky expensive.   If this cuts the bill even 30% it would be a huge help.  The fireplace itself certainly isn't a big help.

I know I'm the source of a lot of foolish ideas.   But maybe one idea out of 20 could work out to be worthwhile.  Perhaps this is one idea where the way it is presented is unworkable, but with a little polish it could become workable?

Erica Wisner
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Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
paul wheaton wrote:
Excellent post Erica!  Loaded to the gills with great stuff!

I'm currently staying at a friend's house and in all of my enthusiasm over rocket mass heaters, I have this idea ....

He has a brick fireplace level with the floor.  Just outside of the fireplace is a bunch of stone flooring. 

I know my friend is curious, but at the same time doesn't want anything too permanent.   But something temporary and appliance-ish could be cool.

I was thinking .....  what if I made a big bench out of 2x4's  ... something seven feet wide, three feet deep and a foot and a half high.  The bottom would be 2x4's.  Then do a six inch system in this with the barrel and fuel feed at one end.  Then routed the exhaust through this bench three times and then under the chimney part once or twice - then up the chimney.  Then sort of plug the chimney around the exhaust.    I would then fill the box with rocks and sand. 

So the overall weight would not be too much.

A lot of the heat would be left in the room.

Dismantling would be fairly easy (carry out the rocks, rebag most of the sand, and shop-vac the rest of the sand).

... with a little polish it could become workable?



I think it's been done; some of the researchers used sand-beds when they wanted to be able to tweak the heat-exchange ducting and try different configurations.

Opinions differ on whether it was successful:
- Ernie feels sand is not efficient enough at heat transfer & storage (too much trapped air). 
- Another researcher, Donkey, uses wet sand for prototyping; damp, he feels it's an adequate model of cob.

Definitely worth a try, in my opinion - especially if you can see daylight and it looks very do-able to line the chimney. 
You might actually benefit from the inefficient heat transfer because your hotter exhaust would draft better up the big vertical [cold] chimney.

Ernie also thinks that due to the inefficiencies, you'd want to go with an 8" system (Erica interpolates: 8" is closer to the wood-eating capacity of a small woodstove, instead of a 6" wood feed tube which is really more like a torch's worth of fuel.)

First steps:
See if you can find out what weight the floor itself is rated for.
If it's just household, it might only be 50 lbs/sf, but if it's rated for storage (as some basements are) or for a water-bed, you have more capacity to work with (up to 125 lbs/sf).  If the hearth slab is part of the chimney, and you can stay on top of it, you may have a marvelously overbuilt foundation and very high weight options.

  Then make a foot or two box o'rocks and sand, and see what you are looking at for weight per square foot.
  If you can stay within tolerances, I'd be very tempted to try different mixes for best packing, or even try a natural (non-cement) mortar mix like lime or clay, for heat transfer around the pipes.  You could stack it out of bricks / various sizes of rocks, self-supporting, and get better heat capacity.

Come to think of it, I'd feel safer thinking in terms of a brick box, with terra-cotta tile or something pretty like that on top, so we're not looking at flammable wood supporting something hot.  Or using some kind of metal trough or something.

Ernie thinks wood for the box is fine, as long as there's enough insulation anywhere the exhaust comes near the wood.  Just gotta make sure the wood box can't get hot.

How's that for initial thoughts?

-Erica


 
 
subject: rocket mass heater - exaust can be simpler than chimney
 
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