Converting regular woodstove to a "rocket" or "rocket mass" heater?

so, I've been learning about rocket stoves and rocket mass heaters and it seems like the main things you need are:

1. a "burn tunnel/riser" where the unburned smoke from the main combustion chamber swirl around in a highly insulated riser, which causes a forge-like effect that causes the temp to get very high and efficiently burn the smoke
2. a "bell"/barrel at the top of the insulated riser that allows the burned exhaust to be directed down in order to send it through some masonry/cob to extract the heat before it goes out the chimney.

so, it seems like it would be possible to have a regular wood stove then feed its smokey exhaust into an insulated riser (burn tunnel) after it exits the normal wood stove, which would ignite the unburned gasses, and the you could add a barrel at the top of that.

this seems very similar to what people refer to as a "batch box" in the forums, but it seems like all of the batch boxes are fully custom/DIY, rather than just having that first combustion chamber be a regular old wood stove (which could still have some fire brick to increase temp). then, you could put a bell/barrel at the top of that riser in order to redirect the smoke through some mass.

I searched the forums and saw a couple of mentions of the idea, but didn't see any examples of it actually being done. it seems like the "J-tube" design helps with draft/draw, but isn't totally necessary.

is this something people do? is there a reason it does not work?

thanks, in advance

You would need to cut or form a port in the firebox that would create gas velocity increase that would create turbulence that would combust the exhaust in the riser. Also, the proportions of the firebox would have to meet some minimums for this to happen. It may be more effort than building from scratch, but you are welcome to try and report here.

Hey Brandon;
It can and has been done.
Here is why it is not a popular idea.
To contain the heat and direct it up a riser, you must DIY a batch core inside your box stove.
These stoves run wide open, and even a contained firebrick core will superheat your metal box unless you insulate it.

A batchbox core is not that hard to build.
A brick bell could not be easier to build.
Build a bell, stick a core inside, put a bypass gate at the top of the bell, and exit the exhaust out the bottom.

I sell two comprehensive books on my website https://dragontechrmh.com/
On how to construct a batch core and build a door for it.

thomas rubino wrote:Hey Brandon;
It can and has been done.
Here is why it is not a popular idea.
To contain the heat and direct it up a riser, you must DIY a batch core inside your box stove.
These stoves run wide open, and even a contained firebrick core will superheat your metal box unless you insulate it.

wouldn't the riser be the part that got particularly hot? the regular burn area seems like it wouldn't get as hot, especially with air inlets restricting air flow.

thomas rubino wrote:
A batchbox core is not that hard to build.
A brick bell could not be easier to build.
Build a bell, stick a core inside, put a bypass gate at the top of the bell, and exit the exhaust out the bottom.

the main reason I ask is that it might be nice to start with something like a normal cook stove to get burners and an oven, and then have a burn riser at the output. most cheap wood cook stoves do not burn efficiently, so if I could get a "rocket" section attached to the back to feed in some burn air and burn up the remaining wood gas, that would be great.

Yes, inside the riser temperatures are the hottest, close to or over 2000°F
The Batchbox core runs well over 1000°F
Without insulation, your metal stove will warp.
Batchbox incoming air is not restricted; they are run wide open to burn hot and clean.
Most box stoves do not have a large enough air inlet from the factory to feed a batchbox.
By not building to specifications, not running wide open,  you will have a normal creosote-making, smoky wood burner.

If you are looking for an RMH  wood cook stove that burns super hot and clean, then you should investigate Walker stoves.
https://walkerstoves.com/
Matt developed a riserless core RMH.
He has many options, but his tiny cook stove or his large cookstove might be more to your liking.

Think of an RMH as a top fuel drag race engine.
You could put one in your wife's grocery getter, but it would not be easy, and it would not perform as well as it did at the race track.
Soon, it would carbon up and quit running...

thomas rubino wrote:Yes, inside the riser temperatures are the hottest, close to or over 2000°F
Most box stoves do not have a large enough air inlet from the factory to feed a batchbox.
By not building to specifications, not running wide open,  you will have a normal creosote-making, smoky wood burner.

thanks for explaining that. do people make batch boxes that have a "secondary air" so that you can separately control the speed of the fuel burn and the burn tube? it seems like it would be advantageous to control each separately, especially for builds where you don't have a ton of mass and want a longer burn time but without making creosote and sacrificing efficiency. .

Batch boxes run best at full speed, and should never be throttled down. There is no need to throttle a batch box that has mass connected; you just run a larger or smaller load, or two loads in extreme weather, and the heat is stored to release slowly over hours or days. The colder your climate, the thicker you make the thermal storage walls so that heat takes longer to escape and overnight heating is more even.

There is a primary air intake as well as a secondary air intake.
If the stove is lit, both remain wide open.
Once the stove has died back to a few coals, both air intakes are shut until the stove is lit again.
This keeps the heat inside the bell, rather than exhausting it out the chimney while not burning.