Newbie trying to get started!

Greetings! I’m finally going to make my first rocket mass heater- two in fact! I’m definitely hitting analysis paralysis…
I was planning on 6 inch, j tube style heaters. (I would prefer batch box but my understanding is they are more complex and there isn’t a solid “how to” guide yet?? Any suggestions?)

Both buildings are small, 400-500 square feet. One will be well insulated the other is poorly insulated.
They will be the only heat source in western Washington state weather.

Here is my first list of questions in hopes to at least get started!

(I live on an off-grid island in the Pacific Northwest. Obtaining materials is starting to dictate what and how I do things on this project.)
First off, the firebrick is going to be multiple boat loads. In an effort to reduce the number of bricks I need I’m leaning towards the 5-6 minute riser. Since that goes off script from “the book” I’m struggling to understand how the base of the riser connects with base of the burn tunnel/core inside the manifold? Any links, videos or threads on that part of the construction?

On that note, the manifold…. Barrels are a b @$?ch to get out here (I won’t bore you with all the details but the fewer the better I think?). I’m starting to entertain the idea of building the manifold with regular brick. Again, off script from the book! Any suggestions on if this is difficult to do?! I’m having a hard time finding a clear “how to” on this part. (However, maybe two more barrels are easier to get on island than more bricks?! Phew.) I guess the question here is which material is easier to build from? And any instructions on the brick version would be swell!

On to clay questions- I have an unlimited source of what I think is glacial, marine clay. (Cliff side of land meeting the ocean. It’s light grey when dry and dark grey when wet.) It performs well in my thermal cob samples. I think it’s completely viable for the cob mass but what about the slip and mortar? Is there any way to know if it will hold up to being “glue” between the firebricks in the core? Is it best to play it safe and purchase some known fireclay?

Next up is rmh foundations.
One is going in a room that already has a rock floor. It is beach rock of larger size on top and gets smaller as it goes down. Probably about 3 inches of rock and pebble and then sand and earth below that. Do I need to put an insulated layer under the mass to keep the heat from going to the earth and below? (In addition to the perlite/slip that I will put down under the core.)

The second rmh is going in a cabin currently being built. It will have a wood floor but we can do whatever needs to be done under the heater to protect and support. What suggestions do you all have for how to insulate from the sandy ground and the sides next to the wood floor? Can I build the floor up from the ground with cement or broken concrete blocks to get the heater up to floor level?

(I will try to add photos of both floors.)
My head is spinning but I want these heaters too much to give up yet- any help would be greatly appreciated!
Cheers!
Heather

Welcome to Permies, Heather! Ask as many questions as you want, the more specific the better, and we will be happy to give our best guidance.

If you do a 5 or 6 minute riser, it would simply sit on top of the burn tunnel which would extend the whole length of the core. The riser height for calculation purposes would equal the burn tunnel structure height plus the tube height, so if you want a 48" riser, you would take your 8" or 9" brick structure height and add 39-40" of tube.

The traditional RMH design includes a steel barrel over and around the riser to give instant heat and direct the exhaust gases down and into the mass channels. A more recent practice is to just build a masonry box around the whole core/riser and let the hot gases fill that, give up their heat to the masonry, and exit from a chimney connection at the cooler bottom of the cavity. This box can be any shape desired as long as a few constraints are followed and the interior surface area of the box is sized according to recommendations. See batchrocket.eu for full instructions on different types of bells. You can use this bell information for J-tubes as well as batch boxes; I have found that an 8" J-tube is similar in output to a 6" batch box, for sizing purposes.

For your clay, if it holds up to strength and shrinkage tests, use it. You may need to add sand if the clay is too pure. Pure clay will make excellent mortar for a brick core. Just be aware that it will not act as any kind of glue, just a good sealer and leveler. Your construction must be stable without depending on cement to hold the bricks together.

Dry sand is a tolerable insulator, so you might be okay with just the construction you describe. If you have a horizontally oriented mass, the load may be distributed enough to be stable. If you have a bell that is taller than wide, you likely need a more solid base to avoid uneven settling. Smooth round rocks will not make a stable base; you need flat or rough stones to lock together, or maybe a foot or two deep of round cobbles to give good enough support.

Building up from the ground with broken concrete or blocks is an excellent way to get a good raised foundation. Keep the combustion core away from wood edges and you should have no problems with overheating.

Hi Heather;
Welcome to Permies, and Welcome to the wonderful world of Rocket Science!
For how to books on Batchbox construction see my website https://dragontechrmh.com/
I offer them in hard copy or digital PDF format.

Let me start by saying I believe an 8" J-Tube will make you much happier than a 6" will.
An 8" uses larger wood and once warmed up will only need fuel every 45 -60 minutes.

Barrels) You only need one, preferably with a removable lid.
Or you can build with no barrel at all if you have plentiful clay bricks to create a brick bell instead.

Your horizontal transition area is easy to create with clay bricks. (Much easier than custom cutting a barrel)
The back 1/3rd of your barrel sits over an empty clay brick box.
Hot air leaving your riser sinks around the outside of the barrel it settles down into the brick box.
Your pipes or your hollow bell start from the brick "mini" bell.

You will only want firebricks on the floor, in the feed tube, burn tunnel, and the first 16" of the riser.

A five minute riser is the fastest easiest way to go, it sits on top of the firebrick riser and is sealed with a fireclay mortar (cob).

Your beach clay will work fine as a cob mass, however, a brick bell is a more modern choice than a piped mass.
Fire clay and a screened graded sand work best for mortaring bricks.

A 2x4 frame filled with 3.5" of a perlite clay mix, is enough to insulate under an RMH
A common method to build on top of wood floors involves using clay bricks set flat on the floor.
A cement board is set on top with 3.5 inches of perlite clay as a base to build your core on.

As you may have noticed there are quite a few upgrades and changes that are now common in RMH construction that are not in the Wisner's Builders Guide.
You can find information on new innovations in the RMH forum or on my website.

thomas rubino wrote:Hi Heather;
Welcome to Permies, and Welcome to the wonderful world of Rocket Science!
For how to books on Batchbox construction see my website https://dragontechrmh.com/
I offer them in hard copy or digital PDF format.

Let me start by saying I believe an 8" J-Tube will make you much happier than a 6" will.
An 8" uses larger wood and once warmed up will only need fuel every 45 -60 minutes.

Barrels) You only need one, preferably with a removable lid.
Or you can build with no barrel at all if you have plentiful clay bricks to create a brick bell instead.

Your horizontal transition area is easy to create with clay bricks. (Much easier than custom cutting a barrel)
The back 1/3rd of your barrel sits over an empty clay brick box.
Hot air leaving your riser sinks around the outside of the barrel it settles down into the brick box.
Your pipes or your hollow bell start from the brick "mini" bell.

You will only want firebricks on the floor, in the feed tube, burn tunnel, and the first 16" of the riser.

A five minute riser is the fastest easiest way to go, it sits on top of the firebrick riser and is sealed with a fireclay mortar (cob).

Your beach clay will work fine as a cob mass, however, a brick bell is a more modern choice than a piped mass.
Fire clay and a screened graded sand work best for mortaring bricks.

A 2x4 frame filled with 3.5" of a perlite clay mix, is enough to insulate under an RMH
A common method to build on top of wood floors involves using clay bricks set flat on the floor.
A cement board is set on top with 3.5 inches of perlite clay as a base to build your core on.

As you may have noticed there are quite a few upgrades and changes that are now common in RMH construction that are not in the Wisner's Builders Guide.
You can find information on new innovations in the RMH forum or on my website.

Well hell! Just when I was feeling slightly educated enough to pull the trigger on a j-tube, vented mass stove. Now I’m taking a second look and diving deep into the batch and bells. Which definitely make more sense for me…. But damn… do I have time to start over in the planning department?!
One thing I’ve noticed within the forums is how quickly one great idea is no longer….. but the no longer part is not always easy to find.Hence why I was thinking of sticking with tried and true. However, the bells and batch models appear to be sticking around. Between your site and products, walkers and Peter I ought to be able to figure it all out?!
🤷🏻‍♀️
I need to learn more about the “skins” and how to do that with cob or rocks instead of 600 bricks.
One question for now-
Are walker’s CFB cores still a good choice? If remember correctly, you didn’t like them in the feed tube due to the abrasion -but I like the simplicity of them. Do they have the same concerns as the hazardous ceramic blankets?

I will go back down the rabbit holes and return with a slew of new questions I’m sure!

Thank you!

Bells and J-tubes are fully compatible, so you don't need to go to the complexity of batch box building if you don't want to. Bells are actually simpler than channeled masses, and have much less friction to worry about.

It is simple to make a double-skinned bell with brick and firebrick in the inner layer and cob in the outer layer. I would recommend around 4" of cob over brick. Mine is 6-7" of cob, and while it holds a lot of heat, it takes hours for the exterior surface to get hot from a cold start. 4" will be much more responsive. If using a cob outer layer, give it rounded corners, as sharp corners will put that surface far from the hot interior and never get more than lukewarm.

You can make the whole bell wall from cob, though it may not last as long as using some brick.

Hey Heather;
A bell can be a recycled metal tank (Fuel oil tank) or a sheet metal box.
You then add a cob and rock "mass" around or even inside, of the tank.
Another method is the Walker half-barrel bench system.
55 gallon barrels are split in half lengthwise.
All but a few inches of the ends are removed, these half barrels span a gap between rock/brick / concrete blocks or cob stub walls.
Cob is then applied over the metal portion of the barrels.
When finished you have a heated hollow bell/ bench to sit or sleep on.
The result is the same as a piped solid rock and cob mass, a heated bench.
The biggest difference is with a bell you will never have your pipes fill with ash and restrict the exhaust flow.

About CF board. Away from abrasion, it works great as an insulator.
I still prefer heavy firebricks for their long-lasting durability and heat holding.
As far as being concerned about safety.
Only after it has been superheated does Superwool or CFB become hazardous, similar to asbestos insulation.
Not all CFB is created equal, quality can vary.
It is hard to have shipped and rather expensive to buy.

Check for a masonry supply yard in a bigger city near you.
Chances are good they will sell heavy firebricks and also fireclay.

Hi Heather, glad you made it over to this forum.
For a high use rocket stove there is not really many choices of material to build the core from, fire brick is number one and cast refractory number two.
Perhaps the riser is best built from insulating fire brick rather than ceramic fibre or you could use split fire brick wrapped in ceramic fibre from the outside and sealed somehow.
If you have good cut stone available then you could build a bell from stone and mortar, clay brick would be easier to build with.
A big J tube like Thomas suggests would be a good choice and split barrels as suggested would also be an option.
A batch box would be better but will require much more work and knowledge to build and also more fire bricks.
I have built a sort of highbred J tube with a glass front and a firebox lid that could also be a compromise..

Hi Thomas, hi Fox,
as always a pleasure to read your answers, thank you. I'm picking up a lot and I'm exposed to new interesting things.

Fox James wrote:
I have built a sort of highbred J tube with a glass front and a firebox lid that could also be a compromise..

Can we see that J-Tube somewhere here on the forum?

Yes I have a youtube channel but there are some videos here as well … https://permies.com/t/226771/messing-vermiculite#1948654