Horizontal vs vertical burn tube?

hey,

I've been learning about rocket mass heaters and one thing that isn't totally clear to me why the burn tube/riser is always vertical. it seems like running it horizontal should also work (like it's laying on its side) as long as it turns upward at the end to be able to pull a draft.

is there some other effect happing that only works well when vertical?

Thanks

Hi Brandon - good question!

The j-tube layout for a mass heater happens to autofeed to a certain extent. As the sticks of wood burn down, the ash settles and provide a fresh bit of wood. You are limited by the height of the burn tube (which is proportional to the riser so can't be too tall) as to how long the sticks of wood can be. Trying to use sticks too long, could potentially result in them falling out of the firebox, which would be bad (fire hazard)

You might  be interested in batch boxes too. They look more like a conventional stove. All the fuel is laid in a firebox, which again has carefully developed proportions and more complex air feed arrangements. You light it, then can shut the door and walk away.

Both systems have advantages and disadvantages.

edit: This thread may help: https://permies.com/t/41635/Top-Questions-Rocket-Mass-Heaters

I guess maybe the thing that I don't understand is why the lower part of the j must be lower than the burn tube. If the burn tube were sideways, and then you had a regular chimney going up, wouldn't that also draw sufficiently? Or am I mistaken

Hi Brandon;
We may have some terminology mixups, or perhaps I'm not understanding your question.

On a J-Tube
The feed tube is commonly 16" deep, measured from the floor.
The burn tunnel is the horizontal tunnel connecting the feed tube to the riser.
This "tunnel" can be measured wall to wall, or I prefer to measure the burn tunnel "roof", which is 10" - 12" long.
The riser is generally 36"-48" tall, measured from the floor.
All three must have the same area. ( 6x6 , 7x7, 8x8)

Are you referring to horizontal pipes through a mass?
They definitely can be lower than the J-Tube core.

Sorry for the confusion.

I mean the riser. If the lower part of the j and the burn tube stay the same, but the riser (where wood gases burn) is also horizontal but still making a 90 degree bend to the left or right instead of vertical, would it still function as long as long as the chimney went back to vertical after the part where the gases burn? (what is typically the riser, but would now be laying on its side).

so basically, there is a part where sticks go, a part where the initial burn happens, then a bend past which only smoke burns, then a vertical part for a draft. In the standard build, the vertical part is also where the smoke burns. I'm trying to understand if those last two are separable. Could you have the smoke-burnung section still be horizontal and then a vertical chimney where it does not need to maintain high heat but rather just is part of a chimney to let the draft go up and out.

The heat riser (what you have called the burn tube) generates a strong draft. If it were horizontal, that draft would have to come from a very good chimney and building layout, which cannot be guaranteed. Designers have come up with some horizontal cores (Matt Walker is a prime example) which work well as long as they are built exactly as designed, and connected to a good chimney.

Part of the efficient combustion comes from restrictions or sharp bends in the flame path, which again must be built exactly as designed to work.

The riser is not simply for draft. It's where the final stage of super-efficient combustion happens, accelerated by the turbulence created by either the change in direction produced by the upward transition from the burn tunnel in a J-tube, or the venturi effect of the port exiting a batch box. To understand it better, look up the threads that have video clips of the various vortex formations. Fox James has posted several of these.

Just guessing from my understanding of J-tubes, a sideways riser would work and obtain an efficient burn, but would take much more priming to get it going. The vertical riser works as a minor pump for the draft at the start, the vertical exhaust as the major one.

You'd also need to have a priming spot near the exhaust. Normally if the system needs priming, tossing some burning newspaper to the end of the burn tunnel is good enough, but that wouldn't work (without a good chance of smokeback) if the riser were horizontal.

Glenn Herbert wrote:The heat riser (what you have called the burn tube) generates a strong draft. If it were horizontal, that draft would have to come from a very good chimney and building layout, which cannot be guaranteed. Designers have come up with some horizontal cores (Matt Walker is a prime example) which work well as long as they are built exactly as designed, and connected to a good chimney.

I guess I'm just not sure why combustion in the vertical part is necessary for draft. Wouldn't hot exhaust turning and going up a riser of equal length still cause the same draft regardless of whether it's already fully burned gasses?

Glenn Herbert wrote: part of the efficient combustion comes from restrictions or sharp bends in the flame path, which again must be built exactly as designed to work.

Right, I'm theorizing what would happen if you had the same bend but sideways instead of up, followed by an upward section for drafting

Coydon Wallham wrote:Just guessing from my understanding of J-tubes, a sideways riser would work and obtain an efficient burn, but would take much more priming to get it going. The vertical riser works as a minor pump for the draft at the start, the vertical exhaust as the major one.

You'd also need to have a priming spot near the exhaust. Normally if the system needs priming, tossing some burning newspaper to the end of the burn tunnel is good enough, but that wouldn't work if the riser were horizontal.

Makes sense that it would need priming to restart from cold if there was more horizontal area to cover before turning upward. Thanks for the info

Brandon Hands wrote:
I guess I'm just not sure why combustion in the vertical part is necessary for draft. Wouldn't hot exhaust turning and going up a riser of equal length still cause the same draft regardless of whether it's already fully burned gasses?

Because its function is not solely for creating or enhancing draft, although it does both. It's the zone of turbulent mixing that facilitates complete efficient combustion that we are after. Without a riser, you've just got a partial solution.

For a real-world example, I recently built a hybrid biochar rig that comprises a retort sitting over the exit point of a truncated J burn tunnel -- without a proper heat riser, but sitting inside an insulated cylinder that I hoped  would act as a sort of heat riser and reburning chamber as syngas escaped from the bottom of the retort during the process. It failed. The burn tunnel was working ok, with decent draft once I put a couple sections of flue pipe on top of the whole thing, but the material inside the retort never got hot enough to carbonise. I ran it for several hours before giving up. The design was taken from Kobus Venter's Black Ripple kiln and adapted to the materials available.

My next iteration will have a 5-minute riser, albeit not full height because I don't want to build a stand for the whole thing just yet. I'll see how that goes and decide next steps from there.

Coydon Wallham wrote:Just guessing from my understanding of J-tubes, a sideways riser would work and obtain an efficient burn, but would take much more priming to get it going. The vertical riser works as a minor pump for the draft at the start, the vertical exhaust as the major one.

You'd also need to have a priming spot near the exhaust. Normally if the system needs priming, tossing some burning newspaper to the end of the burn tunnel is good enough, but that wouldn't work if the riser were horizontal.

To reformulate my take on this, I think the 'yank' on the air intake immediately after the feed must contribute to the mixing of gasses. It also is needed to prevent "competing chimneys" of the intake stack allowing backdraft into the room, so a horizontal "riser" would be extremely finicky if it would work at all.

A first step before wasting too much time speculating would be the Ernie Wisner method of building a prototype in the back yard...

Matt Walker's stoves are riserless.    https://walkerstoves.com/
The flame travels horizontally until passing through the port, where it goes vertical.

The design works great in conjunction with the use of a bypass.
When constructed with ceramic fiber board, it starts quickly.
When constructed with heavy firebricks, it is harder to start, and requires building small fires far back in the burn tunnel to heat the bricks enough to start drafting. Smoke back into the room is common until fully warmed up.
An experienced stove operator can learn to minimize that smoke back

Building instructions and dimensions must be followed to achieve good results with a riserless core.

Phil Stevens wrote:

Because its function is not solely for creating or enhancing draft, although it does both. It's the zone of turbulent mixing that facilitates complete efficient combustion that we are after. Without a riser, you've just got a partial solution.

Right the riser serves two purposes, draft and burning of smoke. those two functions don't seem like they must be coupled, just that the typical j-tube design causes it to do both.

If you were to impede the flow of burned gasses or cool them down before going vertical, then I could see that causing a problem for the draft. But what I'm thinking is instead of the riser going vertical immediately, have it turn 90dehrees to the Right (still creating turbulence), then going some distance before turning upward again, still being insulated that whole time to avoid loss of draft. As others have pointed me to, This is basically what the Walker stove batch boxes do.

Hey Brandon;
Matt's stoves require a port into the riser to make them draft horizontally.
J-Tubes are entirely different.  

I would invite you to make an example of your idea (in your back yard, not inside) and see how it works. To be sure it is fully effective, you would need professional testing equipment with sensors that tell you exactly what the heat and chemical output is... but if you can get it to burn totally smokeless after startup, and leave no charcoal whatsoever after burning is done, you might have a usable concept.

Hi Brandon,
would you provide us with the reason behind the „horizontal riser“? There’s so much experience here, someone might come up with a solution to your design challenge that solves the need for the horizontal part with a different tweak.

Benjamin Dinkel wrote:Hi Brandon,
would you provide us with the reason behind the „horizontal riser“? There’s so much experience here, someone might come up with a solution to your design challenge that solves the need for the horizontal part with a different tweak.

At this point, I'm mostly just learning. However, I do have a potential use-case that would need a very compact design because I would want to fit it as much into an existing brick open-hearth fireplace as possible. The regular barrel design would stick out way too far into the room. I also prefer a batch box type of design. So I'm mostly just trying to learn the principles so that if I deviate from the typical, I can make something decent still.

The basic J tube with the correct height riser is a very good and reliable stove, heat rises and it just makes sense to make use of that fact.
Riser less stoves need a very good chimney with very little restriction and often a bypass as well.
I have been developing a very small 4” riser less core that generally works very well but requires a very direct route  to the chimney and it must be a good vertical chimney.
Many of the commercially  available box stoves are getting better and better with insulated internals and preheated secondary air.
Where I live these box stoves are placed inside close fitting fire brick enclosures making them more like a RMH.
You may find my youtube channel interesting as it follows my endeavours and problems developing the tiny vortex stove and also my own version of the J tube
https://youtu.be/vt9kmbhQC1k?si=QzZr7n7sxoqScbJr