What are the distinctions of each RMH type?

Hi there,

I am hoping/planing to build an RMH someday… hopefully soon. Maybe this summer or fall. I have matt walkers plans and feel decently confident I can figure it out. But I am also realizing there is just a lot I don’t know.

This is a super basic question, but could one of the rocket scientists here give an easy to understand explanation of each of the styles of RMH? What are the distinctions of a j-tube, batch box, dsr, dsr2, shoebox (anything else I’m missing)? What are the pros and cons of each? I’ll be honest, I’m not super nerdy on this stuff. I just need to know the basic distinctions to evaluate if Matt’s plans are what’s best for our situation. I can find lots of threads regarding specific builds, or intense details and data from experimentation happening within one of those categories, but it would be nice for me (and I expect others) to have a simple description of each. If someone had time to do a summary of each type that would be super helpful. Thank you!!

I'm no expert so others will probably have to correct me.
In essence they are all similar, they produce the 3 Ts: Temperature (>600 °C as a minimum), Turbulence and Time. These are required for complete combustion.

Then as fae as I know there are 3 main differences in concept:
- Cooking: Matt Walker's cookstove is specifically focusing on this feature, so if this is the most important factor, it is probably the most dedicated cooking stove. You could also cook on the top of the firebox of a Batchbox or on the top of the barrel of a J-tube/Batchbox.

- Way of loading: the J-tube has a vertical feed, fuel drops down as it is burned, after which you need to reload. Reloading is more frequent than with a Batchbox design (I believe I read factor x2 - x3 more frequently). For a Batchbox you feed wood in a 'batch', horizontally so you can have a more traditional view on the fire. Matt Walker's stoves are also Batchbox variations.

- Height: the DSR and Shorty batchboxes have lower core height, meaning you could fit them in a location has limited height, I know Peter mentioned one of the test versions was built under the stairs.

Other considerations:
Batchboxes generally require metalwork on the door, making them a bit harder. Thomas has a book to guide you through this though, you could also take that to your local metalworker. For me personally the depth of the bell was important, and I wanted to see the flames from my sofa, so I'm going with a traditional Batchbox with sidewinder.
And lastly, the J-tube rockets are bigger than the batchboxes for the same heat output.

Hi Eloise;
Julian made a good start, I'll try to expand it for you.
We will start with J-Tubes the "original" RMH.
Wood is fed vertically into an open feed tube.
With an 8" build you can expect to add wood every 45-60 minutes.
A J-Tube was traditionally plumbed through pipes embedded in a solid mass of stone and cob usually in a long bench.
With the advent of Bells (stratification chamber) a J-Tube can be plumbed into a hollow bell and made more compact than a long bench.
A disadvantage to a J-Tube is the open feed tube.  Children will drop toys and other things into the fire... some will attempt to reach in and remove the burning barbie doll...
Cats absolutely love any RMH, but with the open feed tube, long bushy tails could become warmer than intended, and your home could be in fire danger if things deteriorate.

Walker stoves)  Matt's stoves feature his "riser-less" cores.
His best-known stoves are cook stoves, although he does offer J-Tubes and bell-designed BBQ ovens.
His stoves are suggested to be built with ceramic fiberboard, although he also allows firebricks to be used.
Ceramic boards are insulating, they allow the heat created to pass quickly through the stove making for an easy almost smoke-free start.
The problem with ceramic boards is how very fragile they become after heating. Adding wood to a fiberboard firebox it is very easy to damage and requires it to be replaced.
Matt recommends lining his ceramic firebox with expensive RA330  sheet metal to protect it from abrasion.
If you choose to build Matt's stoves with firebricks, then you have no worries about abrasion, you will however struggle to build a fire without smoking out the room.
Until your firebricks heat up, the brick riserless core has trouble drafting, a bypass is required to assist in getting things going.
The firebox on a Walker stove is small, so it requires more tending than a Batchbox.

Batchboxes)  although originally developed by Lasse Holmes from Homer, Alaska.
Peter Berg, has further tested, refined, and developed several design variants of the Batchbox.
The first-generation batchbox is the most well-known and the easiest variation to build.
Batchboxes load wood traditionally, they have metal doors.
Dragontech-designed doors for first-generation batchboxes are spark-free and have a small viewing window.
Batchboxes have a large firebox and can be expected to burn and have coals for two-three hours.
Batchboxes are also the hottest burning with riser temperatures over 2000F.
Batchboxes are almost exclusively plumbed into bells rather than a piped mass.

Batchbox variants)
First generation, easiest to build
The DSR variations 1,2 and 3 are tricky to build but have an upper viewing window with beautiful double ram horn flames to enjoy
The Shoebox design is similar but has no window (I think)
Peter's newest design core is the Shorty core, which is not difficult to build, but requires three large slabs to be poured at home.
I built and installed the first Shorty core in the USA.
It utilizes a unique airframe that supplies fresh air to the top and upper sides of the batchbox rather than the air coming in from the bottom like all other wood burners.
This airframe allows the use of a door with a large window, (mine is 7x7)
Note) In 8 weeks of use the window glass has never gotten covered with soot, that is how clean-burning Shorty is!
In my opinion, Shorty is Peter's crowning glory! (I also have two first-generation Batchboxes and a Walker riserless BBQ)
Almost impossible to overfuel, Shorty appears to just be casually burning.
Looks can be deceiving!  Shorty is every bit as badass a dragon as any of the earlier variants, she just does not feel the need to showboat and fly around your valley roaring!  

Thanks so much Julian and Thomas. This is just what I was looking for.

Would either of you be able to comment on ease of use for each of them? Let’s say for instance you were going to be out of town for a week and asked a neighbor to come start a fire in your house to keep things from freezing. How much explanation would each kind require to ensure he could do the job problem free? Assuming he knows how to build a fire and maybe has a typical box stove he’s used to running?

Hey Eloise;
A first-generation batchbox would be the most familiar to a box stove operator.
Nothing special about lighting one, all mass heaters start super easy once the mass is warmed for the season.
One catch would be that to hold your heat overnight, the air intakes need to be closed.
Your friendly neighbor would want to come by to shut it down in the evening.
If your stove has a bypass gate, it must be easily identified and have a clear open and closed operating position that your neighbor is aware of.

They must understand there is no damping a mass heater down, these stoves run wide open all the time until it coals and then the air intakes are closed to keep your heat from drafting out the chimney overnight.

What a good question Eloise, thanks for asking.
And what great answers, thank you Thomas and Julian.

This is a really neat and concise collection of the different RMHs.
I have nothing to add, but like a lot of good questions it makes me have another question.

thomas rubino wrote:
It utilizes a unique airframe that supplies fresh air to the top and upper sides of the batchbox rather than the air coming in from the bottom like all other wood burners.
This airframe allows the use of a door with a large window, (mine is 7x7)

Thomas, would you go into detail about that door? And why does the design allow a bigger window? Or is that part of the door plans you're offering at https://dragontechrmh.com/53-2/?

Hiya Ben;
My spark-free door plans are for a traditional batchbox.
The DSR variants and Shorty use an airframe to supply their air rather than a primary and secondary air supply that the first-generation batchboxes use.
Also, spark-free the air enters below the door and circulates up, releasing the air on the top half of the sides and across the top of the fire.
The door on them is simply a door with no air supply so it can have a large window for fire viewing, and it does provide an outstanding view of a dragon's gullet!
Here is my thread all about it
https://permies.com/t/254292/Airframe-Construction-Shorty-Core
 

thomas rubino wrote:Hey Eloise;
A first-generation batchbox would be the most familiar to a box stove operator.
Nothing special about lighting one, all mass heaters start super easy once the mass is warmed for the season.
One catch would be that to hold your heat overnight, the air intakes need to be closed.
Your friendly neighbor would want to come by to shut it down in the evening.
If your stove has a bypass gate, it must be easily identified and have a clear open and closed operating position that your neighbor is aware of.

They must understand there is no damping a mass heater down, these stoves run wide open all the time until it coals and then the air intakes are closed to keep your heat from drafting out the chimney overnight.

If using a J-tube, they would need to understand that the whole wood feed is the air intake that needs capping after the fire has run it's course, and that while burning the wood should be contained in the vertical opening and not placed in the horizontal burn tunnel.

Also, my experience with an 8" J-tube is that it will burn for about 15-25 minutes with a load of softwood (pine), and about 30-45 minutes with hardwood (mixed).