Building rocket stove out of soapstone slabs

Greetings,

I am a newbie who is looking to build my first rocket stove. My goal is to heat my home this winter and maybe heat my water. I have managed to acquire some soapstone slabs. All of them are 2.5 inches thick. Some of them are quite big (22x28 x2.5) I am interested in building the entire rmh out of it incuding the burn chamber, the heat riser and even the barrel. The barrel would be a box shape instead of a barrel shape. The reasons are:
1. Soapstone does not burn
2. It has incredible thermal properties
3. I can shape it relatively easily
4. I dont have experience with working with other materials and dont have the money to go out and get them currently.
5. I have some slabs of it that are ready to go (dont ask how - too long a story!)

My questions are as follows:
1. What do you think of this idea? What are the downsides? Upsides?
2. Do I need to chop it up into bricks or can I just use the slabs in the design, provided I used commonly accepted rmh dimensions?
3. Is an RMH the way to go to accomplish my objectives?


Many thanks for your assistance,

Carlos

Carlos Rodriguez : Welcome to Permies.com, our sister site Richsoil.com, and a BIG Welcome to our Rocket and Wood Stoves Forum Threads ! With ~35,000~

Fellow Members World-wide, You can come here 24 / 7 and talk to someone who wants to talk about what you want to talk about ! Here are some links to make

finding things here easier :


https://permies.com/t/43625/introductions/Universal

https://permies.com/t/34193/tnk/permies-works-links-threads


Congratulations on your Soapstone score ! Elsewhere in the Forum you mentioned building Your rocket outside, This will make your soapstone that much more

Useful. Initially we want Your Rockets Combustion core ( Burn Tunnel and Heat Riser ) to be extremely well insulated. Generally The best material for that job

is Firebrick, *also the soapstone would need to be cut down to the same size as 1/2 thickness firebrick to be able to stand the Thermal Shock of its uneven heating.

The best place for your Soapstone is probably as the shell of your Rockets Thermal Mass, this will also allow its unique beauty will add to your RMHs build !


Also remember that Cob, mainly clay and sand 1) will not burn, 2) Will not Rot, 3) Termites won't eat it 4) shapes well with homemade tools 5) is Dirt Cheap

For the Good of the Crafts ! Big AL

* I Strongly suspect this to be true, I have not had Your opportunity to try it ! A.L.

I agree my reservation would be using it in the burn chamber and the barrel, where it will heat unevenly.

But boy do I envy that group of slabs! Nice! I want to stick one on my lathe!

Thank you both for you comments. Allen, when you say "also the soapstone would need to be cut down to the same size as 1/2 thickness firebrick to be able to stand the Thermal Shock of its uneven heating." Do you mean I need to thin the soapstone down to 1/2 inch thickness?

Many Thanks,

Carlos

Carlos Rodriguez : If you are buying new brick then there is some standardization in sizes, there are a lot of older firebricks out there some with really weird shapes

and dimensions ! However This is at least relevant to what you probably will be working with !

Manufacture In the making of firebrick, fireclay is fired in the kiln until it is partly vitrified, and for special purposes may also be glazed. There are two standard size of
fire-brick; one is 9×4 1⁄2×3 inches (229×114×76 mm) and the other is 9×4 1⁄2×2 1⁄2 inches (229×114×64 mm).[citation needed] Also available are firebrick "splits"
which are half the thickness and are often used to line wood stoves and fireplace inserts. The dimensions of a split are usually 9×4 1⁄2×1 1⁄4 inches

Please understand that I think cutting up your Soapstone is a bad idea, Firebrick is there to assure the rapid increase of Temperatures within the RMHs Combustion core

It will actually take longer to create this effect due to Your soapstones ability to steal and 'wick away ' large amounts of heat.

Outside of the Combustion Core of Your RMH ( Feed Tube, Burn Tunnel, and Heat Riser ) We need insulation surrounding this parts. Every part of the Rocket prior to the
metal skin of the Barrel is insulated to both create the highest and most efficient burn and to protect the rockets operator and the flammable exposures around its Base

The best place for your soapstone is where its ability to absorb and rapidly radiate off the RMHs heat is a tremendous asset - as part of the Thermal Mass . In this

location we will not have to cut up your Soap stone at all !

In another post you mentioned Constructing an RMH out doors, Using Your Soapstone for a Capstone will help protect the cob portions of your Thermal Mass also.

This is where I ask if you have been to Rocketstoves.com to Download Your Brand New 3rd edition PDF copy of Rocket Mass Heaters ?

This is " The Book " And will assure you that when you come back here to Permies you are using the same words to describe the Length Ratios, The simple math behind

finding The Constant Cross-sectional Area C.C.S.A., and the Orientation off the various parts to each other and the whole ! For the Craft ! Big AL

Basicaly, soapstone is a heat trap, so it's no good for the inside of the J tube rocket, or the batch type or else the L type. It is best placed as mass.

Thank you for your comments all.

I went over to rocketstoves.com and downloaded it. I have read it and spent some time examining the dimensions. I have also gone over to donkey32.proboards.com and examined the Peterberg BatchBox dimensions. I am very interested in a batchbox design. My design will be heavily influenced by where I put it. If I go inside, I am interested in doing a batch box with a single bell kachelofen type setup. I think I could incorporate that into my existing fireplace set up without tremendous modification. (if you have any suggestions of where to go to look at how principles of creating the kachelofen bell using stone I would appreciate it.

If I go outside, I have more room to spread out and can do what I want. I live in Virginia Beach so it can be very cold and wet in the winter but the spring/ summer and fall is not cold so most of the year, my rmh would like dormant inside. So, I want to leverage what I build for other functions than simply heating. My question is, if I set up my rmh outside, how do I channel the heat into my house in the winter?

Many Thanks,

Carlos

An RMH is not designed as a remote heating appliance. It works wonderfully as a direct space heater and contact heater. You say you need heat only in the winter; what kind of temperatures do you get, and for how long? Do you often get spells of cold weather, or is it spotty, with cold days interspersed with warm ones? The less consistent your cold spells, the smaller you want your RMH, so as not to carry heat over into a warm morning and bake you out.

We will get spells of cold weather for weeks at a time but it is nowhere like Buffalo or Ohio or something like that. A lot of it is dependent on the ocean. I should not need a massive rmh for my home though it is a two story. It would be nice to figure out how to radiate to thatI am hoping to contain everything to my hearth rather than building a large bench. With the slabs, I have some very dense thermal mass that can be contained in a relatively small space.

Carlos Rodriguez : While there're two schools of thought on supplying air for the Combustion core ether from inside air only, or outside air piped in near the RMH,

There are very few people who have built/use any kind of Outside Rocket for inside heating. This is so universal I don't believe '' The Book '' mentions it at all !


Here in the Hinterlands of Extreme Northern New York, I have no one to partner with for further research and my simple needs are covered with a Basic J-bend

RMH With Thermal Mass. as such I am not a good source for Horizontally Fed Batch Box heaters or Bells.


I do consider both of these devices to be improvements to the RMH and as limited in their application as is the Basic RMH, we are at the very start of what will be

a great general improvement of all Rocket Types ! I do recommend First building a J-bend style RMH as a Do-it-yourself project, also recommending that someone

tackling the advanced models should attend a workshop or have onsite help for those Models - ( I also am in favor of using inside air as Combustion or "Makeup ''

-Air )

For the good of the Craft! Big AL

How strong and how large is your hearth? And where is it located - internal or against an exterior wall? Is there basement below where a pier could be built for extra support if needed?

If the hearth is capable of holding it, you can definitely make a vertically-oriented bell instead of a bench. Your soapstone will be fantastic as the facing part of the thermal mass. You would want to look at the international building code which has a section specifically for masonry heaters (which this style would be very similar to).
http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_10_sec002.htm

And here is a version of the ASTM E1602 specs for masonry heaters:
http://www.soapstones.com/documents/ASTMGuide.pdf

I would estimate that you could make a bell with 6" batch box RMH in a footprint of 2 1/2' x 4' as a comfortable minimum, assuming the fireplace surround is solid masonry that you can build right up to. You can also use the existing chimney if it is in good shape and appropriate size. You would want to have it checked out by a professional if you are not familiar with its condition. If the existing masonry is insulated or isolated from outside, or maybe on a south wall so it gets some solar gain, you can incorporate that mass into your thermal storage to further reduce the added bulk.

The section of the hearth that is under the chimney overhang is 40.5 front length by 27.5 inches back length by 21.5 inches deep. It is a trapezoid that tapers on both sides from the front to back. It is all made of brick. The hearth extends 28.5 inches more out from the chimney and is 72 inches wide total. It looks very strong but I will check it out underneath.

The firebox area of the hearth gradually tapers upwards to the metal damper which opens the flue so that the top dimensions are 32 in the front and 22 in the back. Height from the floor of the firebox to the metal damber is 41 inches.

The chimney is against an exterior wall and the brick exterior of the chimney is actually outside. It rises all the way of the side of our two story house. There is a cleanout on the floor of the firebox that slopes down to a outer steel door on the outside on the chimney where you can clean ashes. I currently have gas in the fireplace so I don't use that but it could be a lower exhaust for the rocket mass heater.

There is a crawl space underneath so I can access the foundation of the chimney.

A photo of the fireplace would help considerably in understanding what sounds like a non-standard style. The hearth is a total of 72" wide x 50" deep, and the chimney collector overhangs that by 21.5" in the center section? What is the form of this collector? Does the fireplace recess into the back wall, or is it flush?

With an accessible crawlspace, you could easily reinforce the hearth if needed, so we can assume the whole thing is usable. The character of the back wall to the ceiling is the next concern. How much is masonry?

You would want to keep the low cleanout for inspection purposes, but you would definitely not want to use it as an exhaust. An RMH will work much better if it has a good chimney.

Note - I added a link to specs for masonry heaters to my post above so it is all in one spot.

That masonry construction guide is excellent. I am away but will post a couple of pictures on Sunday. The chimney is all brick and is recessed into a back wall with book cases on either side. The attached image shows its layout type.

Thank you for taking an interest in my project.

Carlos Rodriguez : I Am sending you a P.M. Big AL !

I am back in town. Here are some pictures of my fireplace.

Here is the flue and chimney.

So, it is a very traditional house. I would love to incorporate a batchbox 6" into the existing space an perhaps a small bread oven above. Maybe I can take out some front brick if I have to for height. I also have soapstone tile that I could surround it with as an attractive face. I saw another post of a soapstone rmh stove that was attractive because of its size though I am not per say trying to build a stove.

Here is the link: http://donkey32.proboards.com/post/16159/thread

So, thoughts on design? How would I handle exhaust?

Thank you very much!!!

Search through the site with fireplace and satamax as a search, there's exactly what you need drawn in sketchup. I could do even a mixture of two drawings. You can't realy use the outside chimney as mass and bell. But there's other solutions, which would retain the looks of the fireplace, and be more usefull.

Batch box and single bell with oven replacing the front of your fireplace. Perfectly doable.

Hi Satamax,

Are you able to point me more specifically to the relevant sketchup files? I have downloaded sketchup and am getting used to that. Am I to assume that the dimensions are to scale and accurate? I just dont know sketchup.

Thanks for all of you help!

The bad news as Max says is that the chimney is too exposed to the exterior to use as mass, but the good news is that it appears to be solid masonry with no combustibles behind, full width floor to ceiling. Thus you can build your bell right next to it without fire safety issues.

Carlos Rodriguez wrote:Hi Satamax,

Are you able to point me more specifically to the relevant sketchup files? I have downloaded sketchup and am getting used to that. Am I to assume that the dimensions are to scale and accurate? I just dont know sketchup.

Thanks for all of you help!

Well, here's my post with the file.

https://permies.com/forums/posts/list/40/31382#246585

I'd advise you to read the whole thread. In your case, you can't realy use that chimney as a bell.

But see my latest build.

http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

That's how i make cheapo double skin bells. A batch of high power, built fast etc. After, the looks can be much more refined. It all depends on you. The second bell is going to be covered with bricks, the ones you can see on the bottom. It won't be pretty. It's a workshop heater. But with this technique, you can do prety much all you want for the second skin. Bricks, obviously, stones, pebbles and concrete, granite cobble stones. Reclaimed driveway pavers, or terrace ones. Cob, cob or else and tiles. Roof tiles

Now, it's your turn to invent something

Actualy, looking at this pic

https://permies.com/t/51286/a/33629/image3.JPG

I would move the furniture away. You could use uncombustible shelves. But you will have to move heat sensitive aparatus, books and else a bit further away.

But the looks of your chimney makes me think. You could do a nice second skin with your soapstone, mixed with something else May be slate slabs. For a gray and black modern look. May be real clear tan bricks too , not dark red bricks i would say. Nor flamed ones. That wouldn't mix. Or a yellow or red sandstone, mind you, i don't know how thoses cope with heat.

In france, we have pierre du gard, which is nice, and a traditional choice for chimneys.

https://www.google.fr/search?q=pierre+du+gard&rlz=1T4SAVJ_enFR550FR551&prmd=ivnsm&source=lnms&tbm=isch&sa=X&ved=0CAUQ_AVqFQoTCLexiZaOhMkCFUK6FAodS_4BBA

Your stones make me think of this one too!

http://donkey32.proboards.com/post/16159/thread

Hope all of this helps, and you're not overhelmed!

Thank you very much. What brick thickness should I use when building my batch box? Does it matter?

As thin as you possibly can, while retaining strengh. I use 5cm thick slabs. And use Air entrained concrete outside the firebox, to insulate.

I really like this self contained box concept idea for the rocket mass heater. It works well for thinking about plugging into the firebox space even though I may have to open it up. How should I be thinking in regard to constructing the single bell chamber at the top of the riser as I need to think more horizontally than vertically?
From what I have been reading, I need to leave at least a foot of height above from where the heat exits the riser. How do I build in the bell to maximize absorption of the gases and how do I think about how to get the final exhaust out?

For your batch box construction, you want "splits", which are firebrick 4 1/2" x 9" x 1 1/4". Back that up with insulation as Max advised.

As the existing fireplace/chimney is not usable as mass, it would not help to bump out the bell into the fireplace volume. I think the best plan is just to run the bell exhaust from the bottom back of the bell into the fireplace and up, connecting securely to the existing chimney. The bell could be a couple of inches away from the existing masonry to allow heated air to circulate into the room.

All of the existing mantel and shelves would need to be removed. What material is that projecting mantel, and how is it connected? If your ceiling is 8', you could make the bell 7' tall in total and still be 4" beyond the required 8" top clearance. Say this gives 5'-8" interior bell height from the 8-10" raised hearth, and as long as the batch box is set lowish to the floor you will have plenty of vertical room. A 6" batch box wants 43" or so of riser height above the box floor, say 4' above the hearth, leaving 20" clear above the riser.

A 6" batch box is recommended to support about 65 square feet of interior bell surface area (do not count the floor or any part of the walls below the exit.)

Looking at your photos, I can see that your "trapezoid" is the standard recessed fireplace interior, and you have only 28 1/2" of hearth. A 6" batch box interior at 17 1/4" deep plus door plus 10" outer diameter riser needs 28" deep not counting internal clearance to the bell back wall, wall thickness, and some space to the existing chimney facing. It looks like you will have to either bring the bell front out beyond the hearth or break out some of the chimney breast to recess part of the bell.
Also, you will need considerable clearance AND metal heat shielding for your built-in shelves. You might not be able to get sufficient clearance to the built-ins without cutting some of them away, I would have to look at the building code spec again. My reading of section R1002.5, Exception 1, is that if you have a total of 13" of solid masonry (8" wall plus 5" liner), you only need to keep combustibles 4" away from the sides. This would leave about 3' of internal bell width. If the batch box projects from the front of the bell a bit, the bell interior would have to be about 14" min. deep (clearing the riser by 2" front and back), giving 8'-4" wide x 5'-6" high bell interior, or 46.5 square feet plus 4 square feet for the ceiling,total about 50 square feet. This is less than the recommended 65 square feet, but not by a ton. If you can make the bell a bit larger in any dimension, you can get to optimal size.

Well Carlos, Glen covered some of the dimentional data. Depthwise, you could do as the guy you posted photos from did, sideways batch.

You say you want to have more of an horizontal bell, than a tall vertical one. There's a recent discovery by Shilo, Adiel and Matthewwalker, that in case of a batch, the heat riser needs to be only 3 times to 4 times the base number (CSA) in height. So for a 6 incher, that's about 2ft. The top clearance, with such a tall and nice chimney, you can have only 3 inches if you realy want. Tho that gonna be hard on the bit of bell just on top. For an horizontal bell, you see my tanks, just lay one flat. You could even find something better, may be like truck petrol tanks. You will, most certainly, have to make it two or three bells, to reduce the horizontal space, while maximizing the top's load bearing. As if you want someting flat, beddish like, you want lots of mass above.


Well, i will have to give you another option then!

Half barrel bells.

http://s65.photobucket.com/user/mremine/library/NYC%20Rocket%20Stove%20Build/

This one is made as a tunnel, and not bells. Tho, with half barrels, bells can be made quite easily too.

There's this one which was quite good looking

http://technologieforum.forumatic.com/viewtopic.php?f=19&t=27

Here, the dark kitchen one http://www.inspirationgreen.com/rocket-mass-heaters.html is suposedely a J tube rocket. Could be easily made with a batch.

I realy like this one, even more without it's outer plaster

http://batchrocket.hostoi.com/html/foto.html

While rumaging through my linke, i bumped onto this, a little J tube standalone mass heater https://www.youtube.com/watch?v=-jzKKIHhTU0

And i realy like the idea of a wall bell. http://heatkit.com/research/2009/lopez-rocket.htm

Well, more ideas for you!

These are fantastic ideas. I have reviewed and will review some more. An idea is forming.........
1.) So, to clarify. I can modify the Peterberg Heat Riser stove calculation for a 6" from 43 3/16 to 24 inches.
2.) Do I still need at least a foot of clearance from the top of the heat riser to the top of the 'bell'?
3. Latest reading I have says the Bell size needs to be at minimum 53.8 square feet. True?

Is all this correct?

Thank you!!!

Carlos Rodriguez wrote:These are fantastic ideas. I have reviewed and will review some more. An idea is forming.........
1.) So, to clarify. I can modify the Peterberg Heat Riser stove calculation for a 6" from 43 3/16 to 24 inches.

Shilo, Adiel, and Matthew seem to say yes. Mathew has expressed a concern there : http://donkey32.proboards.com/thread/1561/batch-cookstove-inspiration-firespeaking-lasse

Which, incidentaly i didn't come across or paid attention before.

Carlos Rodriguez wrote:2.) Do I still need at least a foot of clearance from the top of the heat riser to the top of the 'bell'?

As i said earlier in this thread, you don't need much in terms of top clearance, you could get away with 2.5 inches. Search for ring projection at the donkey's forum.

Carlos Rodriguez wrote:3. Latest reading I have says the Bell size needs to be at minimum 53.8 square feet. True?

Is all this correct?

Thank you!!!

Nope, that's Peter's latest maximum ISA. Not minimum. Tho, i disagree a bit with him, and think his 6m² previous guess was spot on. But, i work in a cold place, with good chimneys. You can make a smaller bell. But you won't extract as much heat out of your gases. So you're wasting heat. Mind you, doing double or triple bell, with pillars, you can reduce the whole size. Then again, if you don't have enough mass, you won't be able to keep the heat. So, that means mass between the bells. There's something about mass in the library at donkey's.


Hth.

Max.

Greetings, I have looked over designs and I am think I am going with a design that looks like the one below. (HAve already begun on the batch box and heat riser) It will not have the height that this one has. I am looking at building it so that the heat riser is actually in the chimney cavity and I will build the bell over it and out onto the exposed hearth. So the heat riser will hit the back of the bell at about 26 inches and the gases will move forward into the rest of the bell which will come out of the chimney cavity and rise up to 35-45 inches in that trapezoidal shape. My question is. How best to deal with the exhaust. I believe my bell will have about 65 square feet of isa. The goal is for the gases to transfer their heat, to cool and then to exit out the bell. Should I be looking to create pathways that lead to an exhaust that I retrofit into the cleanout area on the bottom of the chimney floor (which I much prefer), or should I be looking to vent the exhaust through the upper flue of the chimney?

Many Thanks,

Carlos

Carlos, could you draw what you mean?

A side view cross section would be good.

Are you ready to insulate your chimney's massonry from the outside?

If you're talking about running your exhaust directly out the inspection port and terminating there, that will definitely not work. You need a proper chimney for a batch box setup, according to Peter van den Berg. It would be silly to run a new chimney up beside the existing one, so yes, you do need to get into the chimney from inside. As you are making the bell to include the space under the chimney, what you need is called a "plunger tube". It is a duct or pipe that goes straight down through the top of the bell to an opening near the floor, so the exiting gases must be at floor level.

Here is the best representation I can do.

The bell will have depth and butt up to the chimney cavity. Even though it is a one
chambered bell, the back part of the bell will drop down to a lower ceiling and
recess into the chimney Cavity. In that section in the bell in the
cavity is where I will put the plunger tube that will go up through the top of the bell,
through an iron plate into the flue.



Questions I have:
1. What diameter plunger tube should I use?
2. What material should it be made of?
3. How do I retrofit the flue for maximum insulation?
4. What design issues should I be thinking of?


I am currently building the 6 inch batch box core. All seems to be going well. Hopefully I will be doing some outside test firings by Thursday

Carlos

If you are making a 6" batch box, you should probably make the plunger tube the same size.

You say the firebox door will be on the side of the hearth. How will you reach it to feed it, and how will you protect the cabinet from the heat? Building code requires at least 4 feet I think from a firebox door to any combustibles. If the door is in the front, this is easy. In the side, it would require removing a large section of cabinet, and probably still need some metal shielding to the sidewall. Also, how will the riser get into the back of the fireplace if the door is in the side of the hearth? It would have to be angled for that to work. The "sideways batch" design is not a regular batch box turned sideways door and all, it is a special design with the door in what would normally be the side of the firebox. I think there are several variations of this that have been done, and you should probably review them before proceeding. You may find most of these at Donkey's forum.

If you do make a sideways batch box, the riser will be in the main bell area on the hearth, and height will not be an issue. The bell can still expand backwards into the existing fireplace with the plunger tube.

The door will be on the angle or side of the RMHeater. So instead of loading in the front. You will load it on the side. I am talking with a knowledgeable person on code issues to see if that is workable. Currently in a holding pattern......

I'm still not certain where the door will be when you are standing in the room looking straight at the hearth. Will you see the door from there, or will you have to go around to the side? If you are considering code issues, I can tell you that that will plainly raise a lot of clearance issues to cabinetry and maybe even the outside (wood framed) wall. If the RMH core (batch box plus riser) is oriented sideways on the hearth so that the riser is out in the main bell space, and the door is in the side of the firebox and the front of the hearth, that is likely to be safe and convenient.

I've got two options. One is to build the rmh core so it sits sideways on the heart parallel to the fireplace opening. This would require modifying my current batchbox design to load wood through thr sidewall. Does anyone have information on how to modify the p-channel tube and the primary air intake for a side batch?
Other things to think about?


The alternative is to make the heat riser tunnel so it sits in the fireplace cavity, but it will be too tall. Can I curve the burn tunnel so it initially rises vertically but them continues at. 45 degree angle so it will end outside of the fireplace cavity out in the Hearth area.

Thanks for helping.

Carlos, sideways is the way i would think. So,

you do your primary air as an "airwash around the bottom and sides of the door. Leaving a tiny gap, like 5mm. The door closing properly only on top and may be 1/3 of the vertical on top.

And for the P channel, there's plenty of ways.

Check how i've done it here.

http://donkey32.proboards.com/post/18913/thread

Excellent. Will incorporate the p-channel design! Any other tips on altering to a side style? I would assume you need to load up the box with wood and then really can't mess with it until the burn is done because you would not want to open the door. Sounds dangerous to do so. How high to fill the chamber?

Do you increase the width to make room for the door so as to have a little clearance or just make the door the sidewall?