Ive been researching like crazy for about a week on rocket mass heaters and excited to get started building one for my unheated garage. Being a welder I had planned on using a heavy 6" square steel tube thats 1/4" thick - that I had laying around for the j tube and riser but have stumbled across alot of info here showing metal failing in these heaters when used in the firebox/riser.

Im now planning on scraping the steel idea and trying to locally source some light insulating firebrick to use as my fire box and riser. The more I research on this design...the more questions I have. This site has been a great help and source of info so hopefully the kind community here can help me answer a few of those questions.


1. I plan on stacking the firebrick for my riser thin side facing out to get a larger inside diameter and need of less bricks. Will this be enough insulation and should I mortar the bricks together or just leave them stacked for thermal expansion/contraction?

2.For the base/combustion chamber I planned on using a square wooden form...inside the form I would construct the J chamber out of fire bricks and then fill in the remaining form with regular cement. Would this work or would the cement in direct contact with the fire bricks get too hot and crack?

3. ontop of this cement cast form, that contains the firebrick j channel, I will stack the firebrick heat riser and then a 55 gallon steel drum will sit ontop. What can I use to "seal" the drum to the concrete base that will seal leaks but be removable to allow the drum to be removed for cleaning/maintenance? I thought about building a lip of brick ontop of the cast section along the edge that would hold in 3-4" of sand or cat litter that would seal the drum to the cast fire chamber it sits on.
Is this a workable idea?

4. I planned on a small thermal mass incorporated into the exhaust side. was going to exit the drum with 6" exhaust pipe that would exit 2' and do a u-turn and come back 2' then a 90 degree bend and exiting the garage wall. In this area I wanted to pour a small concrete form in the shape of a single seat or a small recliner sized thermal mass seat for warming your bumm in the winter. From my reading...the exhaust does not get too hot and guessing the concrete would be ok here, but will it hold/radiate heat as well as cob would?



Hopefully this all makes sense...I know pics would help but havent sketched anything up yet. I would really of preferred a steel portable/moveable heater design but would rather use the bricks if it means it will last much longer and not create headaches.
If theres any other points or ideas I failed to mention , please let me know if im on the right track or not.

Thanks all!

Sounds like a neat idea to me. Regular cement might not be good near the cumbustion area but clay and or refractory cement would likely work fine in those areas.

Hi Dan; Welcome to permies ! As you guessed, metal is out. Concrete will only work a ways past the riser. While you are sourcing for light insulative bricks , you will want to purchase hard 1/2 bricks (thin) as well. Use hard bricks ,on edge for your feed tube and the start of the burn tunnel roof. After that switch to lite bricks for the rest of the burn tunnel and possibly the riser. Locate and purchase a sack of fireclay (Lincoln 60) use a thin smear of fireclay to mortar all bricks together. On to your questions #1) yes, stack your bricks thin, use fireclay to mortar. Possible option is to pour an insulated riser yourself, using a 16 gal barrel as an outer form and a piece of sonitube as a sacrificial inner form. A perlite heavy mix with fireclay packed between forms will make a durable insulated riser that can be removed for a rebuild or repair of your core. #2) No concrete will work here, to hot, suggest building the core , then build a "retaining wall" around 3 sides with regular clay bricks leaving a 4" gap ... fill this gap with perlite, or refractory , or insulative cob.... point is to insulate the core , as well as protecting it. #3) You seal your riser to the core with cob. the 55 gal barrel is also sealed with cob. It is suggested to use a barrel with a removable lid rather than removing the barrel. Remember to replace the seal on this lid with a non combustible material. Your idea of sand or litter to hold the barrel is not viable. #4) Without much mass your exhaust will be hotter, using a concrete seat that close is questionable. Is there anyway to add more mass ? The place most prone to issues when building a rmh is the "transition area" from your barrel to the horizontal piping. Think big, go from 10" to 8" then 6", or go into a small bell before going horizontal. If you try to jump sizes to quickly your rocket will not draw and will tend to "stalling " and smoking back. Having a vertical chimney is important, most locations will not draw well with a horizontal pipe. Look for and purchase the book "Rocket mass heaters" written by ianto evans this will help guide you. Also preorder the new builders guide ,written by the wisners, moderators of this forum and acknowledged leaders in building and designing RMH. Good Luck with your build ... you will love your RMH !

Thanks guys for the quick replies!

I could add more mass but hope to move to a more rural area in the next 5-10 yrs.
I originally wanted a small contained metal heater that could go with me.

Im really having to talk myself into this more permanent build but will be fine... I can always build another one later.


I just left rural king and they had the hard/thin fire bricks for $2 ea which I thought was a decent price so picked up 30 of them and a 50 gallon drum, it doesnt have a removeable lid...all the ones they had did not have lids.

Im having a hard time sourcing refractory locally. Probably will have to order it online and pay about double since shipping will be a killer.

I guess I will cast the riser with refractory. So I should mix it with perlite? What ratio refractory to perlite?

would it be ok to make the whole core from this hard fire brick if I insulate it with clay bricks filled in with perlite?

Any idea or rough estimate to how much refractory I will need? Would a 55# bag be enough for the heat riser?

Is COB the easiest/cheapest way to go? I have no idea where to get the clay and hoping there was a cheap alternative like mixing cement or plaster paris with something like perlite

Thanks for all the help guys!

Hey dan; Good price for those firebricks. Not to worry about a permanent build ... cob can be re hydrated ! Don't let making cob worry you, its actually really easy. Your mass is simply clay ,sand & rocks. Clay is everywhere and you should not need more than 5-10, 5 gal buckets full. Sand is the biggie, with my local clay, I mixed 3 buckets of sand to one bucket of clay. You will need a course builders sand not a fine beach sand. Check with your local road crew or a local contractor about the location of local clay, chances are there is some available not to far away. You will need to find a source of fireclay. In spokane its $10 a 50# sack. If you build using fireclay & perlite when the time comes to move ... cob & clay & bricks and the horizontal piping can all be broke apart and reused !!! just add water and cob is as good as new ! Way better than concrete , refractory can be hard to locate and generally run around $50 a bag... trouble with using refractory... if your wanting to move is, it is just like concrete, one time use. Horizontal piping can be hvac pipe available in 5' sections , used clay brick is usually easy to locate , or available new at home depot. I used brick to surround my mass as well as my core. I know you bought a barrel but keep searching for one with a removable lid , it makes install and maintenance sooo much easier! Get the BOOK and read it three times! This is so cool, that its easy to get excited and start building to soon. Make mistakes and you get to do a rebuild. Build A test J tube out in your yard and get a feel for what your building.Read the book. Think out placement in your building, remember to insulate below your mass so as not to try heating the earth. Read the book! Did I mention reading the book... We call this rocket science ... its simple ... but there are certain dimensions that must be adhered to or you will not be happy with what you build. Unless your located down under, winter is over and its spring . You have all summer to do this rite. Like you I discovered RMH in the spring and could hardly wait a day to start !!! I was so excited ! So I did it... found all the stuff , poured a cast core and cast riser , learned the cob dance, got it all built ... and had to wait ... and wait ...almost all summer before it wasn't 100 degrees out there and I could fire it up inside ! There is so many options now that were not invented 5 years ago, read up on batch boxes , learn about bells , matt walkers 1/2 barrel system , casting risers instead of brick risers. Russian masonry heaters. Enjoy your journey on your way to being a true ROCKET SCIENTIST !

Ive done some more reading and decided I dont need the mass battery for storing heat.... I think for my uses it will be better for quick heat when I need it for tinkering in the garage on random winter nights for a few hours.

I really like the looks/ layout of those dragon stoves but cant afford one and would rather build it myself.
I have an old filing cabinet from the 60s the thing is solid and heavy steel. Id like to weld or bolt some heavy duty castors on the bottom to make it semi- portable. Im thinking about building my core out of the hard fire bricks ontop of a layer of fire bricks on the bottom, then filling the rest of the metal box with refractory. For the riser I plan on constructing a metal frame to hold in the hard fire bricks on edge.

The questions I have now are..... inside the metal box.... could I use a homemade refractory? Ive seen Portland cement (1.5 parts), silica sand (2 parts), perlite (1.5 parts) and fireclay (2 parts) used as refractory in diy foundry furnaces for melting aluminum and brass/bronze. Id hate to get big cracks or have it crumble apart but I cant find castable refractory locally so looking at $100 shipped for each 50# bag.

Also wondered if my hard fire brick riser will need insulation? I know that the soft fire bricks have great insulation properties but cannot find it either, does the hard fire brick have any insulating property to it?
If it does need insulated- Will 1" ceramic wool suffice?

Last question.... does the exhaust pipe need to be anything particular? would regular 6" ducting work?



Sorry for all the questions
thanks again guys!
And I will be ordering the book, as you said....im excited and get ocd on projects and cant get it out of my head untill its done so have to get one built so I can quit thinking about it so much lol.

hey buddy welcome to permies. i see you mentioned a portable unit. i built my unit on wheels and pull my unit outside to cook on in the summer time. i try to live as much off the grid and sustainable as i can.

i agree that a portable unit would be very nice in a garage.

here is a pic of my unit and you can view the building plans on my links below.


the cement in my Rocket Mag Stove does store a large amount of heat and if you built a unit like it for a garage it would work great for instant heat and some storage for longer heating retention.

If you build a portable unit in your old filing cabinet, making the inner core of thin "splits" firebrick and filling all around with a mix of perlite and clay will work well, and be possible to knock apart and reuse if you want to change it. For rigid insulation, you want just barely enough clay mixed into very thin mud to hold the perlite together.

The 1" ceramic fiber blanket will work fine for riser insulation around the firebrick splits. Hold it together with a wrapping of steel wire or chicken wire or the like; in that location it will not suffer from the heat and corrosive atmosphere that happens inside the core.

And definitely build a test core outside to see how it burns! It is quite impressive.

Styles... thats a sweet looking rh! Im sure to get some inspiration from that build.

Im going tomorrow after work and looking to get a gas cylinder for my welder (flux core sucks) and working the metal cabinet to fit my build. I mocked up a couple core designs today and have a good idea for measurments needed for the core/ metal frame.

Will have to order some ceramic wool.
I have a ton of scrap wood piled up ready to get combusted lol.

I dont know if I will be able to find fire clay... .would chimney mortor or chimney cement work to seal riser to core?


Clay and perlite is suggested for a homemade refractory. Is it just ground clay im looking for? I live in a urban envirenment. My yard is about 150x75 ft so dont have a huge area to dig. Ive ran into clay a foot or two down in my backyard before.... im just not experienced in harvesting clay and wondering how big of a hole id need to dig to aquire the amount of clay needed?

Dan Cunningham wrote:Styles... thats a sweet looking rh! Im sure to get some inspiration from that build.

Im going tomorrow after work and looking to get a gas cylinder for my welder (flux core sucks) and working the metal cabinet to fit my build. I mocked up a couple core designs today and have a good idea for measurments needed for the core/ metal frame.

Will have to order some ceramic wool.
I have a ton of scrap wood piled up ready to get combusted lol.

I dont know if I will be able to find fire clay... .would chimney mortor or chimney cement work to seal riser to core?


Clay and perlite is suggested for a homemade refractory. Is it just ground clay im looking for? I live in a urban envirenment. My yard is about 150x75 ft so dont have a huge area to dig. Ive ran into clay a foot or two down in my backyard before.... im just not experienced in harvesting clay and wondering how big of a hole id need to dig to aquire the amount of clay needed?

thanks for the compliment.
i dont use wool but if you like it many people use it. chimney mortar will work if its refractory mortar that is able to handle 2000 degF
ground clay has been used in many rocket stoves but i did not use that in my system and i would not suggest it if you are following my plans.
My system is portable enough i can wheel it out to my garage and use it there if i like. matter of fact i built it in my garage and wheeled it to my house. i use 4.5 in industrial caster wheels. if you have any questions about my Rocket Mag Stove please feel free to stop by my thread links, view the plans videos and ask me anything and ill do my best to help.

Clay for perlite-clay insulation can be just about anything that acts like clay, as long as you can get stones and gravel out. The amount for insulating a core or riser will be just a few buckets at most; if you want to make mass from it, it needs no real refining, and can be broken down and taken elsewhere and rebuilt if you need to move. In that case you would need numerous wheelbarrows full of it.

If clay is a foot or so down, you just need to dig away the topsoil, dig out the clay, and put the topsoil back. Of course you will have a depression if you take a lot of clay, so plan for it and make a frog pond, backfill the hole with rotty or scrap wood for a rich garden bed, or whatever works in your situation.