Proper dimensions for the “J” tube?

Hello All....great forum, been stalking for a few days...

So I am going to embark on my first RMH. Your feedback would be much appreciated.

So, I have a bunch of questions and few odd design criteria’s, but that’s for later............ for now, I wanted some input on the J tube length ratios.

I am doing some drawings before I commit to any mortar. I need some help or guidance with the feed, burn, and heat tunnels sizes collectively known (I think) as the “J” tube. Is the proper ratio 1:2:4?

FYI, I am building my RMH on the smaller side. I’ll explain when my drawings are more prepared. So, onward..

Please see attached, Is that correct? The Heat riser seems tall?

A feed of 8”

Equals a burn tunnel of 16”

And a heat riser of 32”

In my case all measured on the “long” side of the tunnel.

Thanks,
CF

If your sketch is near to scale, it appears that you are thinking of building a 4" system. This is decidedly smaller than the recommended sizes (6" or 8" diameter), and experienced people have found that this smaller size is very tricky to make work well. As you get smaller, you get more surface area to volume, and thus more heat loss with less fuel/flame to make up for it. The surface laminar flow effects which reduce effectiveness begin to outweigh the internal turbulence which makes for good efficient mixing.

Measuring along the outer faces of the elements will give you the most extreme differences in lengths, which will give the most powerful effects for a given overall size (riser height). But if you have a 6" system, the 8:16:32" dimensions will leave only 4" for the top length of the burn tunnel - hardly enough space for the riser insulation and barrel to clear the edge of the feed tube.

Glenn,

Thanks for the input.

So, is the 1:2:4 correct? And as your pointing out, it matters how you apply that ratio.

I understand the ratios are nonlinear. So, how is that ratio supposed to be applied and laid out for construction? Inside edges, center line, etc?

On the cross section, yes I am going to maybe build a smaller unit and see how she runs, but your points on surface area to volume are understood, and perhaps I will just stick with the 6 inch size, not sure.

Thanks

There is no "official" standard on where the dimensions are taken from; I like the "centerline" method of measuring, but it depends on your situation. I have heard it said that the outside method is easier if you are playing with real bricks to come up with your layout, while centerline is easier if you are working on paper and especially if you are planning on casting your core where you can make dimensions precisely what you want. Bricks force you to certain modular dimensions, some of which are easier than others to fit to the theoretical dimensions. The proportions are not ironclad - minor differences can be tolerated, with the proviso that the burn tunnel should be no larger in cross section than any other part, and the feed tube should not be appreciably larger than the burn tunnel. A taller heat riser in proportiion will give a stronger rocket (the original standard for lengths was 1:2:3; 1:2:4 is more reliable.)

Given your example, the 16" = D, and the 32" = E. Were I using these measurements, I would use the 16" = D as you show in your diagram, but I would only measure my vertical fire riser from the roof of the burn chamber, so it would be shorter than 32" the way I'm measuring my system. I would suggest adding height to your vertical fire riser so that the new height, measured from the roof of the burn chamber is a min. of 32-inches (so that, E >= 2D).

With regard to the feed, just make it as small as practical. Mine is two fire bricks tall because that's how the brick stacking works out nicely.

With regard to your first build, I'm just dry stacking the fire brick. It is not perfect but it works and works well enough for you to begin taking some test measurements, and get a feel for what you are doing. Importantly, it is simple to change. I would not use mortar until the final build.

I found a local brick yard and discovered they sold fire brick (2500*F) for about half the price as the big box stores, and a 2500*F brick instead of a 2200*F brick. I have 50 of these which I am using to build my test J rocket stove. I expect I will pick up another 20 or 30 before I start my final build, but 50 is sufficient for now.

That's what I'm doing, so that's what I suggest. But as the more experienced builders have pointed out, there is some wiggle room here, and the smaller you get the trickier the build.

I think the most important thing you could do is get 30 to 60 fire brick and a wheel barrow of sand, and start some dry stacked builds, and see what you think.

I am assuming you have bought the "book" ("Rocket Mass Heaters" 3rd Ed.) and read it a couple times. If not, I would recommend doing so.

Below is what I'm thinking of building for a J-Tube Rocket core.  I'm looking for advice on optimizing the dimensions.  

1) Assuming the riser height of 42.5" is fixed, would you change the burn tunnel length or feed tube height given the opportunity?  
2) Is it okay to restrict the burn tunnel height to 7" as drawn or would I be smart to leave it at 8" like the riser?

Thanks!

What is your fuel plan? Will you be buying commercial firewood, or cutting your own? You want the fuel to fit completely within the feed tube as much as possible, to make it easy to cover the feed partially for draft control, or completely in case of emergency when you want to shut the fire down. For standard 16" nominal firewood, that means a feed tube 16" tall.

You can easily make the burn tunnel a few inches shorter, say using 1:1.5:3 proportions, which would give outside edge dimensions of 16", 24", and 48". (You have about 14.5", 29.5", and 46" outside dimensions now, or close to 1:2:3.) You would still have 8"+ for burn tunnel roof, which is plenty of clearance for feed tube covering plus airflow space inside the barrel plus riser insulation. The burn tunnel length doesn't contribute to draft, and as long as it is long enough for the fire to develop straight horizontal flow so the sharp angle to the riser generates proper turbulence, I don't think a longer burn tunnel is useful.

I cut my own firewood - probably about 16" length is what I do but I don't use a tape measure.  This core will be for outdoor use, boiling small batches of maple syrup, and other BB tasks.  Are the dimensions in this revised drawing below what you had in mind?  

Should I remove the extra inch of fiber board from the burn tunnel floor to make the tunnel ceiling 8" instead of 7"?  I was worried that the fire bricks floor would be taller than the fiber board floor and would make ash cleanout more difficult without the extra inch of fiber board raising the fiber board floor above the fire brick floor level.  Should that be a concern and how is that dealt with?

Do I need rework the layout to get 1-2" (depending on the above answer) more added to the riser to get it to 48"?  I'm a newbie with J tubes so I have no sense of how sensitive they can be.

Theoretically, the cross sectional dimensions should remain the same throughout, but if the feed tube ends up being a little narrower then that would be fine (as it would help suck air inwards rather than want to back draft.)
The floor is the coolest part of the core so the cf board is not as effective there as it is for the rest. To me, I would replace the board with the firebrick splits all the way as each time you clean it, your going to be dragging a tool of some sort along the floor and abrasion resistance is probably more important here. Of course, keep the cf board under the firebrick floor liner.

In my experience, an inch or two here or there on the riser has little impact on the overall burn. However, if the heat riser height pinches off the top gap (space between top of riser and bottom of evaporator pan/barrel), then an inch or two can make a huge difference.

I only clean the ashes out of my J-tube about once a week, and a pile of ash builds up 2 to 3" deep just inside the burn tunnel, with no apparent effect on the fire. I have strong natural draft so a constriction is not a problem. As you will be letting the exhaust hit the bottom of the sap pan and go sideways, not down, you should have no worries about a bit of constriction from the cf board step; however, I agree with making the whole floor of firebrick splits for abrasion resistance.

Thanks so much for the feedback - keep the feedback coming as it is much easier to redraw this again now than to rebuild it later!  I have updated the dimensions one more time to accommodate the use of fire brick throughout the floor and without having to cut any firebricks and optimizing for 36"x48" ceramic fiber board sheets.  The ratio is currently 1:1.55:3 measuring around the outer perimeter of the inside as drawn below.  Any other suggestions?

Looks like a winner to me.

A P-channel is a good idea to always keep a good supply of air at the front of the burn and help the leading edge of the cf board from getting abraded.

EDIT:  Looks like your feed tube extension already protects the cf board. Dragon Heaters uses a similar setup with a built in P-channel

Looks good to me. I see a quarter inch step on the inside from feed extension to firebricks - any irregularity will tend to catch wood pieces as you are loading, so I would take pains to make the inside of the feed exactly flush all the way down.

For a P-channel, which is highly recommended, I might just arrange the cf burn tunnel roof so that there is a 3/8"-ish gap between cf and feed steel. That will give the desired secondary air supply. (Check for the exact recommended size of gap, it makes a difference.) The one possible downside to this is what I discovered with my welded-up 1/8" steel feed sleeve/P-channel: the steel at the bottom edge got so hot that it softened, warped, and bowed in, blocking a bit of the feed area and making a minor obstacle to dropping sticks in at the front edge. It has corroded some over four seasons, but still plenty strong. I took it out once and hammered it flat on an anvil, and need to do that again. When it does burn through at the bottom edge in probably several years more, I will cut off the bottom inch or two and bolt on a thinner sheetmetal P-channel tab which will be easily replaceable.

Thanks for the suggestion to fit the feed tube sleeve inside the bricks and to add a P-channel!  I really like that idea for the reasons you mention and also because the sleeve will then also act as a retainer for the stacked fire bricks (which I have no flipped vertical to reap the benefit).  I read up a bit on what a "P-channel" is (Peter van den Berg Channel) and have added it to the drawing as well as a "trip wire" - a projection across the burn tunnel ceiling to disrupt laminar flow also developed by Peter van den Berg.  From what I read, a 1/4" to 3/8" P-channel sounds like the recommended size for an 8" system, and to let it hang an equal amount below the burn tunnel ceiling to add some turbulence.  And the trip wire is recommended to be about 4" from the feed tube and project down 1/4" or less with the sharp edge on the side of the heat riser.  Ideally it would be an 10-15 degree arrow shape pointing towards the heat riser, but straight across is better than not at all.  So I have updated the drawing below with these additions.  Please critique.

Question: I read in one of Peter van den Berg's posts that he said an 8" system has a 50" cross sectional area.  So is an 8" system typically actually 7"x7"?

Off the top of my head, I think I remember something about how a square is not as efficient as a circle in terms of drag due to the boundary layer effect - the corners in particular.

So even though a square has more cross sectional area than a circle of the same size, it also has more drag on the moving gases and therefore needs to be made larger to accommodate this.

How much larger?... can't remember, but do remember that Matt Walker in one of his stove chats mentioned that you can use a larger diameter heat riser with a smaller front end without any problems.

Yes, You sure can!  
I use an 8" riser with my 7" batch and I use a 7" riser with my 6" batch

Those with expertise in aerodynamics, notably Peter van den Berg, say that due to the added drag of air in the corners of a square cross section, you can treat a square as equivalent to a circle of the same diameter. So an 8" RMH system can have an 8" x 8" J-tube core.

Would a a completely round section J tube work as well or perhaps even better than a square section? Or more to the point is there any benefit for casting a round section J tube ?

Hi John;   No, round is not better.   Apparently it has been tested by the innovators as not beneficial.

I would not recommend casting a core that way.

Hi R. Parian,

Did you ever build that J Rocket, I like the design?  If so, how did it work?

I'm thinking about something similar but to heat an interior space so would put some 55 gal drum(s) over the riser and add some mass.

No, I was not able to find a retailer of 36x48 sheets of high temp ceramic fiber board in Washington.

Here is a short excerpt from the new free heat movie about j-tube sizes that might be of interest.

Hi!  This is my second post, and I am new to "forums" in general.  
Thank you all so much for sharing here.  We are learning so much as we plan our RMH build.

Question:  We have an existing 7 inch chimney. New last year.

Has anyone got the J tube dimensions to accommodate the CSA of a 7 inch stove pipe?

I realize I can calculate it with pie r squared etc.  (Explained here by Richard Freudenberger- Living Web Farms: https://youtu.be/MNKgkgukmkc?si=LV_dbc3JOM3JbGzz
but I thought there might be a chart which doesn't jump from 6 inch to 8 inch.  

TIA.

Edit : I've determined 6.25 inch square equals 39 and that's close enough to the CSA of a 7 inch @ 38.465 sq in.  Correct?  

So now, the upright dimension can be anything as long as it stays within the 1:2:4 guidelines.  Or is it 1:1.5:3.5?  

Thanks again.

Hi Connie;
Seven inches is an odd size for a chimney, but no problem building a 7" J-Tube.
6.5" x 6.5" square. 16" deep feed tube with a 6.5 x 6.5 square burn tunnel " apx 10-12" long (roof measurement) with a 6.5 x 6.5 riser base.
For a riser, I recommend a Morgan Superwool 5-minute riser, but firebricks also can be used.

Everything is "odd" here in Newfoundland!
lol.
We just came out here to the wilderness 1.5 yrs ago.

Thanks so much. I'll look into the Morgan Superwool 5-minute riser.

My system is a 6" System:

I read this criteria online, What is the prime target Dimension? are is there testing to be performed to determine that ?


For an 8-inch rocket mass heater system, the recommended gap between the top of the heat riser and the underside of the barrel is generally 2 inches, with 2–3 inches being ideal for optimal performance.

This dimension ensures proper airflow and heat transfer while avoiding restrictions. Some builders suggest that a gap as small as 0.8 inches may be too narrow due to boundary layer effects in airflow, and that a 2-inch gap is good, and more is okay.  The key principle is maintaining consistent cross-sectional area (CSA) throughout the system, and a 2-inch gap helps achieve this efficiently.

Additionally:

The barrel diameter is typically 22.5 inches (standard 55-gallon drum).
The side gap between the barrel and riser should be 1.5–2 inches to allow hot gases to flow evenly around the riser.
For best results, use Kirk Mobert’s (Donkey) method: test the stove without the barrel, then lower the barrel until you hear the "sweet spot" — the loudest rocketing sound — then back off slightly for optimal draft and efficiency.
✅ Bottom line: Use a 2-inch gap from the top of the riser to the barrel top for an 8" system, and adjust slightly based on real-world testing.

For a 6-inch rocket mass heater (RMH) system, the recommended gap between the top of the heat riser and the bottom of the barrel is typically 1.5 to 2 inches.
• This dimension ensures proper constant cross-sectional area (CSA) and allows for efficient hot gas flow into the barrel.
• A gap of 1.75 inches has been successfully used and reported to work well in practice.
• The calculation based on CSA supports this: a 6" diameter riser has a CSA of ~28 in²; dividing by the circumference (π × 6 ≈ 18.85") gives a gap of ~1.5 inches, which aligns with real-world builds.
• While slightly more gap (up to 2") is acceptable and may be used for ease of construction, keeping it within 1.5–2 inches maintains optimal draft and heat transfer.
Note: This gap is critical for maintaining draft and preventing bottlenecks. Ensure the barrel is properly supported and the riser is centered to avoid uneven stress or heat loss.

Hey Rocky,
let me try to understand. You’re asking for the exact size of the gap between riser and barrel for a 6” J-tube?
And then you give an (AI generated?) answer for it in your own post?

First, the original question was asked, the 1st line States :   I read this criteria online, What is the prime target Dimension? Then the info online was posted about 6" and 8" Risers. i was attempting to get a definitive answer, seems as though someone responded and said that some folks has put an entire barrel atop 1st barrel to accommodate the heating of an area, while also stating that if you wanted to be able to cook or have a hotspot for warming put the barrel top to the minimum distance. Surmising that different applications of the system entirely determine this distance. Therefore, I looked online to obtain criteria and then posted that on my question post about the distance. Does that explain how and why the answer was shown at the bottom of the post ?