Building rocket stove out of soapstone slabs

Carlos, you fill the batch box up to 5cm from the ceiling. Aproximately. 3 to 5 cm from the throat. That's good. You gather a chimney insert door for the feed door. And you're prety much set. Like the one you've seen from the dutch forum. The most complicated part is balancing the heat extraction. I mean, between primary heat, and accumulation. That's where i've failed lately, on the workshop heater. Too much primary heat, not enough accumulation. But if you build all out of soapstone slabs, exept the cooktop. You should be prety much fine. My latest one, with one barrel in primary heat surface, was quite nice. So that's 1.86m² of "radiator" IIRC, your flat's surface is 36m², radiating metal surface should be in the range of .6/.9m². A cast iron cooktop would be more than adequate. Then after that, you need accumulation into mass. About 3 to 3.5 tons of mass, for a small mass heater. Don't know what you already have in soapstone slabs. But you might not need much more.

I have built the firebox and riser out of soapstone. I wanted to now put some insulation on it so I searched for perlite. The stuff I came up with is the fine perlite that is sold at a pool store. It is very powdery. What is the best way to apply it so its holds its form? If I dampen it with water and put it on, will it harden when I heat it? I would hate having to build another form around the box and riser just to keep the perlite in.
Thoughts?

Thanks!

I'm afraid you will need an outer shell to contain the perlite, but it can be as simple as a piece of light sheetmetal or duct a bit larger in diameter than the corners of the riser. The outer shell of the riser sees no extreme atmosphere, just modest heat, and should last a long time. If you dampen the perlite and mix well with a small amount of powdered fireclay before packing the form, it will have enough stability to hold its shape if necessary. The fine granules you describe sound like they may be trickier to handle than the medium-coarse material I have. Make some test bricks with different proportions and heat them in a fire before building anything permanent.

Do you know if soapstone will stand up to 2000 degree (F) temperatures? I don't know its exact properties. I don't think it is an ideal material for the hot face, as it has a high heat capacity and conductance, and will suck heat out of the fire for a while until it is all hot. It is superlative for storage mass, but not for an insulated firebox.

Sorry, my reply has taken so long. Here are the physical specs on soapstone. It really has some incredible properties regarding heat. I believe it is the countertop of choice for chemistry labs for those reasons.



I have a quick Code question. I am building my stove and using the specs from ASTM E1602-1603 per my states code, Virginia. They have also inserted this language about '36 inch clearance to combustibles'. This seems to be quite a difference to 5.2.3 of the ASTM code. I am concerned about the ramifications as the heater sits on my hearth next to my bookshelves. Can anyone give me their interpretation?

Thank you!

Exception 1 is what I referenced in a comment some time back, saying that the way I interpreted it was that you would need 13" of masonry thickness to be allowed a 4" clearance to combustibles.

If the heat channel wall thickness referred to just the walls separating channels, and not adjacent to the exterior heater wall, then you would only need 8" of wall, which is quite reasonable.

allen lumley wrote:
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 ?

Big AL I have the 1st edition RMH book and could you please tell the me the difference between the 1st and 3rd edition?

Carlos Rodriguez wrote: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?

I fill my feed chamber with almost 2 cubic feet of wood and i stuff it full so i dont have to mess with it for hours. but i also have the lower access port so i can feed it without opening my lid and or adjust the wood by poking at it with no smoke.

Carlos you have some very beautiful stone there. i wish i had a source for that. you mentioned you were tight on the resources. i have suggestions since i built my system pretty cheap. i have seen people put ads in the classified section of the newspaper to tear out peoples old chimneys or fireplaces that people do not want any more and you can get some pretty good finds there in type and amount of bricks. if know or see some one with a broken chimney it wouldnt hurt to ask if they want it removed for the brick. i have seen some people even use the old style "chalk like brick" you see in very old homes and fireplaces as burn chambers and RMH firebox lining. they can take higher heat than modern bricks and can make a decent RMH. refractory bricks and insulation bricks may be the best if you have the money but then again we all dont have that luxury and have to deal with what we got. you may check out my system if you are looking for ideas i would be welcome help i am a good idea guy and love to share and bounce ideas around with folks. My first RMH heats, cooks, bakes and heats water

i would check your local Brick and cement business for refractory and insulation brick prices. my local brick and cement business sells refractory bricks for $1 each and fire clay for $10 for 50lbs and 1/2 gal buckets of refractory cement for $15. i am getting ready to remove my refractory cement and perlite heat riser for a insulative brick/fireclay/perlite/refractory cement heat riser to get both insulative with a bit of heat retention for faster starts and higher temps. right now i can get 6 to 8 hour burns from an 8'' log and a few smaller pieces in my feed chamber so i hope i can still keep those results with this new heat riser design i am building. i will post pics over there at that link if you are interested in checking it out.

Have you thought about building a portable system that can "plug" into your chimney and then pull away on wheels when spring hits?

i have seen a complete mobile system and i even designed mine on wheels for ease of cleaning and other reasons.

i liked the "p-channel" design and i was going to use that at first until the last moment at build time i changed my mind to a bottom access port instead and im glad i changed my mind. i have a top feed with a "close-able" bottom for access port the same size as my system and it works amazing and gives me many advantages i would not have had if i had not chose that option. My bottom front access port gives me easy start, lighting, cleaning, loading and breathing than if i had top load with p-channel alone. my system is set up i could even add a p-channel later if i so desire but at this point i dont think i need it since it draws the air directly to the bottom of the fire source as it needs it creating great draw for my system. i can close my bottom access port and run my system with the top lid open or close the lid and run it with the access port open depending on needs of the system.

Glenn,

Do the regulations change if you heat shield the walls of the combustible materials so as to make them noncombustible?

F.Styles : A 1st Edition in Good to Excellent condition is now a collectors item worth between $80.oo and $225.oo , The brand new 3rd edition with many updates and new pictures

you can get for a small fraction of that price, especially if you go the download route that allows you to print all the personal use copies you want !

Lets please stay on topic ! Big AL

The codes I know describe the standard woodstove unshielded clearances, and the metal shielding reductions to those clearances (with a steel sheet spaced 1" from the wall on metal spacers.) There are also codes describing masonry heater clearances, and I think I recall some clearance reductions for metal shielding, but not combined with the thick masonry heater wall and the 4" clearance.

Carlos Rodriguez wrote:
Do the regulations change if you heat shield the walls of the combustible materials so as to make them noncombustible?

carlos, they have thin heat reflective material at the hardware store that may put you into code so it may take up less space than thicker material.

al, as you mentioned earlier here and to stay on topic

allen lumley wrote:
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 ?
For the Craft ! Big AL

i could care less how much my 1st edition is worth and did not ask the question to know the difference in price but to know if there is more in there on this topic, and to be more specific if i was to build what Carlos is building would the 3rd edition give me anymore info that would help me in the type of build Carlos is building? Please clarify if you are going to suggest the 3rd edition book i wanted to know what the difference was that may help me on this topic.

it almost seems as though carlos may take advantage of the book The Cabin Stove, by Max Edleson instead of the Rocket mass heaters to kinda get the best of what he has and may want. but what do i know.

Hey everybody, saw this thread and had to get more information. I also have a "huge" amount of soapstone available.
(1-1/4" thick X 22" wide X10 feet long and 1-1/4" thick X16" wide X 10 feet long)

Satamax Antone wrote:Basicaly, soapstone is a heat trap, so it's no good for the inside of the J tube rocket. It is best placed as mass.

I'm confused.... I thought the idea was to get the riser hot and the insulation was to keep it hot to increase the airflow throughout the system. If soapstone is a heat "trap", doesn't that mean that the stone would retain the heat just like the firebrick and insulation would ?? Maybe it would still need insulation to keep from wicking the heat sideways, away from the riser and into the barrel, but it seems that the soapstone could hold more heat than the firebrick. If the firebrick reflects the heat ("refractory brick"?) and the soapstone absorbs it then I "assume" that the soapstone should "stay" hotter longer.

allen lumley wrote:It will actually take longer to create this effect due to Your soapstone's ability to steal and 'wick away ' large amounts of heat.

I agree that the soapstone might take a "bit" longer to reach functional temperatures, but I disagree that it will steal heat because the soapstone has nowhere to "wick" the heat away to. The heat is still inside the riser. I also "assume" that the soapstone would "reflect" back heat like firebrick as it reaches maximum absorption (if there is such a thing).

allen lumley wrote: The best place for your soapstone is where its ability to absorb and rapidly radiate off the RMHs heat is a tremendous asset.

I again "assume" that the reason woodstoves are made of soapstone is that they retain and then "slowly" radiate off excess heat otherwise metal would be better.
What about using the soapstone as just the riser or just riser and burn chamber, and then use firebrick as the feed tube, that way the feed tube stays cooler and will minimize the chimney effect of the feed tube.

In my particular situation, I don't have money for firebricks and I got the soapstone for free so anywhere I can use soapstone instead of firebrick is a bonus.
Thanks for any help

I think we have a case of relative terms here. Soapstone holds more heat and transmits it faster than cob or other stones, but nowhere near as fast as metal.

The soapstone will wick away heat from its surface faster than other stone or brick materials, making the whole mass get hot before the surface reaches desired temperatures. I understand that even hard firebrick, while not ideal, does get hot on the surface long before the far face gets hot. The thinner the soapstone, the less total wicking effect you would get and the quicker it would come up to temperature.

One factor I have not seen addressed satisfactorily is the resistance of soapstone to the extreme thermal cycling the inner face of a RMH would see. It might be able to stand those stresses, but I would want to get some knowledgeable information, or test it for 50-100 complete cycles of 75F - 2000F on one face, before building it in permanently.

You mention using materials on hand and avoiding expenses... The soapstone in the large sizes you describe would make a fantastic exterior for the mass and eliminate much purchase of other finishing material and labor, and make a wear surface that would last about forever. You can use even old soft red brick for the core, as long as you can replace parts when they wear out after some years or decades. This can often be found in the small quantities needed for the core in random demolition or dump areas, perhaps on craigslist for very cheap.

Glenn Herbert wrote:The soapstone will wick away heat from its surface faster than other stone or brick materials, making the whole mass get hot before the surface reaches desired temperatures. I understand that even hard firebrick, while not ideal, does get hot on the surface long before the far face gets hot. The thinner the soapstone, the less total wicking effect you would get and the quicker it would come up to temperature.

So the difference between 1-1/4" soapstone and standard full thickness firebrick would be minimal??

Glenn Herbert wrote:One factor I have not seen addressed satisfactorily is the resistance of soapstone to the extreme thermal cycling the inner face of a RMH would see. It might be able to stand those stresses, but I would want to get some knowledgeable information, or test it for 50-100 complete cycles of 75F - 2000F on one face, before building it in permanently.

Looking at the graph 7 or 8 posts above it seems that the soapstone might be OK . Although I don't understand a lot of what's in the graph.......

Glenn Herbert wrote:You mention using materials on hand and avoiding expenses... The soapstone in the large sizes you describe would make a fantastic exterior for the mass and eliminate much purchase of other finishing material and labor, and make a wear surface that would last about forever. You can use even old soft red brick for the core, as long as you can replace parts when they wear out after some years or decades. This can often be found in the small quantities needed for the core in random demolition or dump areas, perhaps on craigslist for very cheap.

I plan on using the soapstone on the exterior as well, as you described. But I was hoping to "not" have to replace parts of the burn chamber if I can avoid it. What about making the "barrel" part out of soapstone, would it wick heat away fast enough to do the same job as the barrel???

I really have no idea how much difference there would be in performance between 1 1/4" of soapstone and 2 1/2" of firebrick, though I suspect the soapstone would take significantly more time to come up to temperature on the inner face. Only a materials engineer or an experienced builder who has done tests can answer that accurately.

The graph doesn't seem to address the question of whether soapstone slabs would stand up to frequent extreme uneven thermal cycling.

A soapstone "barrel" would certainly not transfer heat as fast as the standard steel barrel, but it might act fast enough to support the push/pull effect, and would probably be a fine radiator which would give heat within a fraction of an hour instead of the multiple hours for transmission through a bench mass.