Walker Stoves Superhot J 8 Inch build experience

I'm considering building using Matt's plans for the Superhot J 8 inch.  I have been looking for video and/or discussions of the experience of builders of this design, but have not had much luck.  If you can provide any links or just add to this thread with your experience, it would be appreciated.  I am particularly interested in getting some more information on experiences with the round core 8 inch I.D./10 inch OD core.

Hi William;    
Big Welcome to Permies!   And Another big welcome to the wonderful world of rocket science!
So your looking at building Matts 8" J tube core.
Any 8" J core will roar like a dragon.
I'm not familiar with the "round" core you mention.
Almost all Matts plans  call for ceramic fiber board and of course are square.

Building a RMH can seems so confusing and difficult ,before you have seen one in person and can talk to the builder.
After you build your first one , you will think back and smile about all your worries!
They really are so simple a caveman could build one...
I have built numerous J tubes and batchboxes.  I still remember my first... I worried about everything... I wrote Matt in person to get answers .
I bothered whomever was in charge of the RMH forum here at Permies with question after question....  I attempted to out think the master builders by coming up with "improvements"... Thank goodness they talked me down to earth and I followed a tried and true plan.
I stressed about this mysterious stuff "cob"  and how to make it...  Its sandy clay... that's it.
I had to locate and/or buy all these parts...   firebricks , clay bricks , 8" piping , large flat Rocks for mass  and sand, you need a LOT of sand!

I know its hard, but the best way to build a RMH is to compartmentalize the  project.   Don't overthink ahead of time.
Each issue you come across is easy to surmount as long as you are not worrying about 27 steps ahead.
If you buy Matts plans he is available for consult.   Whether you  buy Matts plans or not,  we are also available here at Permies to help you figure this out.

So tell us a little about your future build.
Are you thinking of a traditional piped mass with a solid rock and cob filing?
Or are you interested in the "bell style" with an empty stratification chamber?
What kind of building is this planned for?  A home , a shop, a greenhouse?
What size and shape is this building?  Is there a basement? Crawl space?  Solid slab floor?

Plenty of experienced rocket scientists hang out here.  
We are glad to help.    We like pictures...

Thank your for the encouragement Thomas, I admit to be a little apprehensive about some of the details of the build.

I am in Southeast Michigan, just north of Detroit, heating season begins late September and runs through mid-May.  Typical winter high/low temps are mid-30's/mid teens.

Below are responses to your questions about the build.

"Are you thinking of a traditional piped mass with a solid rock and cob filing?
Or are you interested in the "bell style" with an empty stratification chamber?"

My thought is to  use a 55 gallon drum bell over Matt's round 8" ceramic wool core,  the heat would exit the lower portion of the bell and enter a fairly short mass (about 8 feet long),  Mass will be granular material washed stone or sand contained by concrete slabs, probably held in place by a steel frame.  8 inch round duct sealed with metal furnace tape running through the mass).  I expect I will do multiple burns during the day during much of the winter.

"What kind of building is this planned for?  A home , a shop, a greenhouse?"

Home, so I'd love to keep things, toasty warm, I will have a furnace to accomplish this goal unless our wonderfully noncompetent government allows or causes a collapse of the grid and/or natural gas delivery.  Then, contingency plans kick in, then I'll probably have to settle for slightly less comfortable environment.

"What size and shape is this building?  Is there a basement? Crawl space?  Solid slab floor?"
Size and shape sucks. Size about 3,400sf, 2 floors.  Main floor about 1,900sf, about 500sf over basement, 1,100sf over crawl, 300sf on slab.

Heater will be on slab in the northwest corner of the main floor, bad that it is in a corner, good that it is in a northern quadrant of the house, good that the main floor is a fairly open plan, good that even thought the plan is a fairly open plan it can be fairly easily partitioned off to reduce heated area if necessary,  Ceiling height varies from 8'3" to 9' on main floor, about 8' second floor.  Old house, insulation not great, lots of doors and windows, infiltration pretty high (not rebreathing a lot of CO2 though).

RMH will be near a return air, which is close to the furnace, I always run the low speed fan on the furnace when I'm heating to keep air mixed and minimize cool spots.  I have enough PV solar to run the fan in the event of a grid failure (unless of course it is due to an EMP, then we will have to see if things still work ... contingency plan #2).  The thought is to pull very warm air into furnace return air and distribute though the house via the supplies.

GOAL is to have backup heat if there are infrastructure failures, but also to minimize utility consumption ($ outflow).

Plan on building outside first with short chimney and no mass to test and burn off any paint or other nastiness, then disassemble and reassemble inside with mass. One last detail, on final install, chimney will exit mass in single wall stove pipe, take a 90 near the first floor ceiling and transition to insulated chimney, go through wall, take another 90 and head to the sky out side the building.

Looking forward to lots of comments and suggestions.

Thanks in advance to all of you rocket scientists.

Hi William;
Over all sounds like a solid plan.
Your correct that old homes are very safe from co2  buildup.
My home is closing on 100 years now.  No co2 problems here!

So lets talk about your build.
You are calling your 55 gal barrel a bell.
It's not, here's why.
The barrel over your riser is  the start of your "transition area"  (changing gas flow direction from up to down to horizontal.)
A bell system starts after the transition area the same as a piped system does.
The barrel is an instant radiant heat producer.  Some choose to cover a part of the barrel with cob / brick /rock to restrict how much radiant heat escapes and increase mass storage heat.

Yes, you will want several fires a day . When its coldest  you may even find its easier to just keep it burning if/ when  you are home.
You will still use way less wood than your neighbors and  no clouds of smoke billowing from your stack!

Building a test rocket in your back yard is a great plan. We highly recommend it.
Your barrel will only come part clean over the riser.
A propane weed burner (flame thrower) works great to finish one off.
The method I use, is as a burn barrel . Toss in woody debris , and let her rip.
Paint comes off in no time and doesn't even smell that bad.
A long handle wire brush speeds up the process.
You can end up with  quite a good looking barrel.
Speaking of barrels, you really really want one with a removable lid.  
Much Much easier to inspect your riser and ash build up in just a few moments.

Now the most important spot in your whole build is the transition area below the barrel.
It must be large or you will have trouble with flow.
Nothing is as important as easy gas flow.  
Think big large flowing corners heading into your pipes or bell ,  nothing to "stall" the flow.


 Last but not least. Your single wall pipe should angle up to the window not go horizontal at all.

Thomas, thanks for the education of term "bell", also suggestion on smooth - large to small transition at bottom/or below barrel into the piping in the mass.  I had been thinking about taking an 8 inch pipe straight out of the bottom of the barrel.  Also, had been considering using some gas "B-Vent" for the chimney, because I had a bunch from a furnace install I ripped out (I'm cheap), but I'm guessing if I have an 8 inch core I should keep rest of the piping at least 8 inches in diameter.  B-Vent is only 5" I.D.

Hi William,    Generally its best to keep the cross sectional area the same through the entire build, this way there isn't a bottleneck that will slow the flow of gasses down and cause potential drafting problems.

Hi,

Matt's Superhot J 8 inch riser only extends 24" above the core, guessing the riser to be about 34" total, depending on how you measure.  This seems a little short based on some of the stuff I've seen, like 1:2:4 ratio.  Any thoughts on this.  I want to get as much heat generation as I can out of my build.

Thanks.

I don't want to put words into Matt's reasoning for deciding on 24" as the riser height but will guess that the ceramic fiber board from the factory was 24" wide and so practically that's what he used - instead of cutting and splicing another small piece on top for the ultimate ideal height.
I would also guess that since the cf board is already very efficient at maintaining and redirecting the heat back into combustion that this helped out as well with being able to have a shorter riser.

Also, Matt's core internal dimensions are somewhat smaller than a typical 6" system since he is using split firebricks around the feed tube area and so made the rest of the core fall in suit to accommodate them. He addressed this in one of his Stove Chats saying that a smaller front end (the core) is going to draw harder and therefore be more reliable to draft in more circumstances than a larger one.

With all the gives and takes involved, you can see why there are not any hard fast rules but rather guidelines to building a RMH. Each build is unique and there are so many variables that will influence the final outcomes yet still fall within a clean burning finished product.

Hope this helps.

Hi Gerry, Thanks.

Matt just clarified this for me, and I've just purchased his plans for this RMH.  He and I have been going back and forth via email.  I'm sharing our conversation below, with his permission, for the benefit of all.


William/Bill:
Considering  buying plans for the 8 inch J tube.  Does this design put out the most heat?

I have a few questions:
Have there been many builds of this design? I'm having trouble finding videos.
Is the flue pipe for the core just single wall stove pipe, either black or stainless steel?
Are ceramic wool and ceramic fiber blanket the same material
I'm thinking about a 55 gal drum bell, what are minimum and maximum spacings above the top of the core?
I'm thinking about building this so it can be moved fairly easily, with a small thermal mass (probably gravel), and will probably do multiple burns per day.  Any thoughts on this
Thinking about wrapping the core and mass with precast concrete slabs (like concrete countertops.
If you have links to any builds of the 8 inch Super Hot J,  please provide.



Matt:
Thanks so much for your interest in my plans.

The 8 inch J plans are the plans that put out the most heat. The 8 inch version is a larger version of my 6 inch J. The 8" will be essentially the same, with the exception of the upgraded riser style.

The riser pipe can be single wall black or stainless.
Ceramic fiber blanket, ceramic wool, super wool, kao wool are all names for the same type of material. Any are suitable here.

I have a few customers who have built portable units and are quite pleased with them. Gravel has a lot of airspace and doesn't make the best thermal mass, but it is easy and clean and better than nothing so may be a good choice for you. For portable units water is ideal since you can easily empty to move and fill when located, but of course that complicates things.

If you wrap with concrete make sure to use lots of insulation so that the concrete doesn't see too much heat. Concrete has a hard time holding up to the heat cycles seen in a wood stove like this, but if it's protected it can work.

Here's a video of the 6":



Hope that helps.

William/Bill:
Thanks for the feedback, particularly on the concrete slabs.  I'll have to do more research on that.  I think I'd be OK on the thermal mass, but may have to wrap the burn chamber portion of the J tube with brick or something else, and figure out some insulation adjacent to the bell (55 gal drum).

When you put the ceramic wool in the 10 inch flue does the compression where the edges meet hold it tight against the wall of the flue, or do you use some fasteners or adhesive?

I'm also fascinated by your cook stoves.  Not sure if I'm ready to tackle a masonry project yet though, my past attempts have fallen way short of professional quality. Hopefully brick would be easier that block work though.


Matt:
Sounds like you are on the right track. You have the right idea about perhaps trying something other than concrete in the hottest areas.

Yes, typically the ceramic wool at 1" thickness has enough "body" that when rolled into the pipe it will stay in place just fine.
The brick work in all of my stoves is really forgiving due to the use of non-setting clay/sand mortar. It means you can take your time, and redo sections or change things as you go very easily. I am sure you could build a great stove, even without much masonry experience.

Hope that helps.


William/Bill:
A couple more questions:
The height of riser seems a little short based on some stuff I have been reading, can you give me your input on this?
How can I incorporate outside air feed (if it is advisable)?  I hate the idea of  using heated interior air for combustion, but does pulling in very cold outside air lower the temp of the combusted gases canceling out what I think I'm gaining?  I have an old house with lots of doors and windows so infiltration is pretty high.

I read about ratios like 1:2:4, which would seem to suggest a higher riser, see this Permies thread -  https://permies.com/t/41234/Proper-dimensions-tube      Also, saw dimensions for batch rockets that suggested a riser height of almost 58 inches for an 8 inch riser -  https://batchrocket.eu/en/building

Batch rockets are interesting to me, but the build looks a lot more complicated. I think more than I want to undertake, but I like the idea of being able to load more wood, and various other aspects. Do you have any designs that are simpler to build than Peter van den Berg's?  Is your batch rocket heater core simpler to build?

I like the 24"  high flue in your J Rocket because I could just cut a rectangle out of the bottom of  a 55 gallon drum and drop it over the core, and perhap rise it a couple inches to get required clearance over the top of the riser, as I think  55 gallon drums are only ~34" tall.  Would like the option to easily cook or at least boil water on top of the drum.

Matt:
Great questions, I'll do my best to answer below.

1. The original rocket stove proportions were figured out by Ianto Evans who built using dense high mass firebrick which rob a lot of heat from the combustion zone and hurt efficiency. By using ceramic fiber for the core we increase temperature in the combustion zone exponentially, allowing for less time to clean combustion which allows for shorter riser heights. These designs are tested and perform far better than the traditional firebrick core.
2. Outside air brings a whole host of issues, in my opinion it's not worth the trouble. You will need to heat the combustion air prior to burning, so there isn't much to be gained by pulling in from outside, and there is much to lose due to complexity and pressure issues.

3. My batch core is extremely simple to build, but it's true that batch builds in general are more complex than a J due to the needs for doors and secondary air hardware.
4. I think you have the right idea, my cores offer much lower build over height due. Your idea of the drum sounds good, I think it would be a great stove.

Hope that helps.

William/Bill:
Thank you, this is very helpful. Really eliminates my major concerns.   Though l will have some build details I will want to run by you.

I have a thread running in the Permies forum regarding this build, is it ok with you if I share the our email discussion on that thread?  I think it would be helpful to others considering building your Super hot J 8 inch, and hopefully save you from having to answer these same questions for others.


Matt:
Glad to hear it helps. You are welcome to share our conversation, hopefully it will help others as well.

Good luck on the build, feel free to ask me about the build details as they crop up.

If you're going to play with Ceramic Fiber, use a protective mask built for the task.  Don't ask me why I say this. (cough cough)

Cough, cough.  Got it.

William,
Hello from Cleveland. I have built several rocket heaters with Deanne Bednar around the corner from you at strawbalestudio.org and heat my house and sauna and barn and several other buildings here with them. Matt's plans are a very good basis to start your exploration. Please build your test heaters outside while you are still learning about backdraft and flow and where there's a lot of heat and where there is less. Experimenting safely lets you push things, try to break it and understand it and fix it before you depend on it and before breaking would be catastrophic, like not waking up in the morning catastrophic.
--Uncle Mud

Hi Uncle Mud,  My plan has always been to build a test heater, actually (hopefully) my final installed heater outside for testing and burning in, then moving it inside adding mass and final chimney.  I was thinking  for the test heater to just build Matt's core and riser, with some stacked clay brick around it (no mortar), with 55 gallon drum on top, welded steel transition manifold to a very short heat exchange channel in a mass (probably sand contained by concrete blocks), then elbowing up to a chimney that terminates a couple feet above the top of the 55 gallon drum.  Everything sealed to prevent air leaks.  Do you think this would be sufficient for testing?  Thanks.

yes, but give yourself plenty of time to run it before moving it inside.

Rocky Massengale wrote:
I have drawn up a plan for one for our place in Arkansas Mountains, but have Not begun yet. I have a question about the Mass Heater portion of pipe. Does the pipe have to be perfectly horizontal, as I have it drawn with slight incline for each leg, which brings me to the 2nd question. I have approximately 32 feet before chimney entrance which is approximately 30' but have 3 180 deg turns as I have 4 runs drawn into the bench area, with chimney near combustion chamber barrel to help draw. Is this too much length. Please see illustration. Pipe is 6" w/ 1/8" wall

. I need a little advise for this configuration considered before commencement of construction.
You can see that center section is vertical whereas the feed section from combustion into it is slightly angled and discharge end is slightly angled to provide draw to chimney. Feasible ?

Advise on design would be muchly appreciated
I forgot to mention that cleanouts have been added at 180's to clean out Horizontal runs, and that Barrel is Schedule 20 SS 24" pipe 36" long will be building manifold as suggested with IFB-2600 Brick and wrapping with perlite to about 2 - 2 1/2" from the barrel cap at top.

Hello,
I have a few suggestions from my experience.

1. If you bury your pipe any further than about 4" from the surface of the bench the cob mass begins to act more as an insulator/heat sink trapping the heat in the mass and releasing it more back into the pipe than into the outside world. If there is any incline in the pipe it should move gently up toward the final vertical exhaust.

2. The rule of thumb for length of 6" pipe in mass benches is absolutely no more than 35' of horizontal pipe, minus 5' for each 90 degree elbow. The closer you are to the maximum length, the more heat will remain in the mass BUT at the cost of less draft, less reliable performance. If it was mine I would only install one loop of horizontal pipe. I would also use only insulated pipe on the vertical part of the exhaust. I've not been happy with the performance of 6" systems with horizontal pipe longer than 20'.

I've gotten better performance out of 8" J tubes connected to 6" exhaust.

Lastly, the feed tube should be vertical to give more horizontal space for the mixing and burning of the exhaust gasses between the feed tube and the riser in the burn chamber. I'm excited to see your build.
--Uncle Mud.

Thanks Chris,

I can n will upsize my feed tube as well as make it more vertical, can I relieve some of the restriction by using "Bell" transitions versus the 180 Degree turns, as you suggested, my original design had less slope and was not sloped as much as the picture submitted, more like this one, I've changed the 180's to be oversize chambers in this picture to hopefully reduce restriction. Someone mentioned that people are using a 2nd Stratification chamber instead of the pipe runs in mass, have you heard of that being done ?  

I only use stratification benches, not pipes for the heat exchangers in my builds. They are cheaper and easier and more even than the heat from pipes.

Chris,

Thanks, I have received the message loud in clear that the Stratification method is the most proficient from what you have said in your replies. So, now that I have understood this to be the way that is most proficient, here is my dilemma, my Mass Heater is to be under my cabin which is atop buried shipping containers. They  have wood floors so i will need to build up the floor with material (Mass) to isolate (create) the floor of the Stratification Chamber, as well as provide ample room for the Discharge to the Chimney, I have plenty of material for this buildup, but what is your thoughts on the best material to build this up with ? Stone n Cod or What ? The Mass Heater will be closer to one side of the container than the other to provide a walk space beside it . I can build up the wall with Stone to add more mass, but realizing that the heat will be warming the floor of the cabin which is approximately 11' from the floor of the container. I suspect the entire (Man Cave) area will be heated (Approximately 1600 sq ft) with this Mass Heater as well as allowing this heat into the cabin via floor vents. I am still building the cabin above at this time, but am planning on having this Heater to help us in winter bigtime! Cabin has a configuration of containers underneath with 1 set of doors accessible from exterior, Mass Heater will be in center container almost dead center for discharge to chimney .

Thank you,
After Looking at some of the calculations from Peter, do the stratification chambers require a certain amount of cubic footage to work most proficiently or will the cooled gases find their way out not matter what size the chamber is ?

Apologies Attachment not as desired

Rocky Massengale wrote:Thank you,
After Looking at some of the calculations from Peter, do the stratification chambers require a certain amount of cubic footage to work most proficiently or will the cooled gases find their way out not matter what size the chamber is ?

I would say both of your statements are true however, without a certain amount of heat going up the chimney, draft will be compromised leading to all kinds of problems. So yes, it is important to size your bell correctly unless you like breathing smoke.

is that a square foot dimension for bell size, calculated, but will the cubic foot of space also be important or should it be limited to attain a particular stratification in the bell? By the dimensions shown the square foot dim is only 18'6" but the dimension shown for a 6" system requires 57' which is almost 4 times the size shown seems rather large compared to what i see in videos and pictures ? Please explain

Chart from Bell Sizing                                               Does this dimension include all partition sizes as well such as top, walls and floor of bell as well ?

Riser diameter   /   Internal Surface area

12.5 cm  (5")   ISA 3.7 m²    (39.8 sq ft)
15.0 cm  (6")   ISA 5.3 m²    (57 sq ft)
17.5 cm  (7")   ISA 7.2 m²    (77.5 sq ft)
20.0 cm  (8")   ISA 9.4 m²    (101 sq ft)
22.5 cm  (9")   ISA 11.4 m²  (123 sq ft)
25.0 cm  (10") ISA 14.7 m²  (158 sq ft)

Gerry,

I haven't heard back from you yet regarding last question, however after reviewing the chart for the bell size I have redesigned the size of the bell to accommodate all surfaces not including the floor which I don't know if that matters, but after calculating all of the walls and top areas this new attachment shows dimensions for feed, Heat tunnel and riser as well as gap at top for 6" system along with dimensions of bell. Do these seem to be in line with criteria required for an optimum functioning RMH ?

I asked if Gerry would look at that last design drawing, but if anyone (Rocket Scientist) has any input I would appreciated it, the bell actually figures to be 57.8 square feet, I suspect that the 2/3 of a square foot should not hurt too much, as the material to make up the bell wall is 4x8 x3/8" SS rectangle tubing and filled with sand as well as covered in either cod or stone n mortar, sound feasible ? the stainless steel tubing was left overs from a project completed, thought I might as well make use of the drops !  Please Advise if these dimensions are good to go, this is my 1st RMH, work in progress Rocket Mass Attempt !

Rocky Massengale wrote:is that a square foot dimension for bell size, calculated, but will the cubic foot of space also be important or should it be limited to attain a particular stratification in the bell? By the dimensions shown the square foot dim is only 18'6" but the dimension shown for a 6" system requires 57' which is almost 4 times the size shown seems rather large compared to what i see in videos and pictures ? Please explain

The chart Peter gives on his website were intended to accommodate a batch box build, not a J tube core, so the numbers may not be exactly the same for both (although it is a good starting point).

Not sure where you got 18'6" from as it is not the total ISA of your bell - That's approximately just the roof ISA. Add another 4 walls and there's your explanation.
I see in a later post you have also updated the size of your bell and have said it is roughly in line with the recommended chart which appears correct.

Although bells can be pretty much any size or shape, bends, restrictions and obstacles will impede the natural flow and stratification may suffer. Your rectangular bell is simple and should have no issues. If floor surface area is in limited supply, columns can add to ISA and also help to support the bench top.

Rocky Massengale wrote:Does this dimension include all partition sizes as well such as top, walls and floor of bell as well ?

Yes, however the floor is never included into the ISA calculations.

Rocky Massengale wrote:Do these seem to be in line with criteria required for an optimum functioning RMH ?

From my understanding, the tube within the bell is only necessary for longer runs (or bends) where the heat has a harder time getting to the other end of the bench and heating it unevenly. Otherwise, its just another thing that can clog up with ash and in your case, no real way of cleaning it properly.
The exhaust exit on the bottom of the bell could also be a problem in this same manner. Eliminating the extra 180 degree turn and just have it come straight up from the bell would be better.

Thanks Gerry

Revisions to Bell
I had originally planned on a CO on the far end, but my concerns were mainly with the J Tube Dims and the Dimensions needed for the bell ISA, which you have addressed those questions.
I have also removed the 180 and incorporated a Sump and approximately a 3" elevation change to bottom of Chimney 90 so that the Sump is not to deep. Your thoughts ?
have come a long way from original prototype Design ! shown as well.

Rocky Massengale wrote:I have also removed the 180 and incorporated a Sump and approximately a 3" elevation change to bottom of Chimney 90 so that the Sump is not to deep. Your thoughts ?

From your picture, the opening to the exhaust looks cramped to me. The transition from the bell to the vertical pipe needs to be large enough as not to restrict the gas flow. A funnel shape is ideal.

Bell Revision 4

Gerry for your view of Rev 4

J tube Dims appear to be good ?

Cool, see Rev 4 picture ? For Funnel implementation

Rocky Massengale wrote:J tube Dims appear to be good ?

Just now looking at your core dimensions.

Top gap (space between top of heat riser and barrel) could be made more spacious as this is where ash tends to build up.
Instead of raising the barrel height, I'd just lower the heat riser height by at least 1.5" since its already slightly taller than needed.
3" has been found to be a minimum for top gap in a J tube, a bit more if you can manage it.

In regards to the height of your feed tube being 14" tall.
It is recommended not to have any wood sticking out of the feed tube (mostly for safety reasons) so if your wood is cut 16" long, you may consider a shorter fuel supply. You don't want to raise the height of the feed tube too much either as it can lead to it being a competing chimney and also make it that much harder to be able to reach in to scoop out ash.

Burn tunnel could be made shorter.
Closer to 24" is preferred. The heat riser doesn't  have to be in the center of the barrel so it can be moved a tad closer to feed tube.

Your update to the exhaust exit looks much better. Depending on your space and layout preferences, you could also have it exit straight out the top of the jut-out you've created eliminating the 90 degree pipe bend.    

So Gerry,
From what i have seen is that the burn tunnel is supposed to be 1/2 the size of the riser as is the feed tube to be 1/2 size of burn tunnel 1:2:4 ratio is that not correct ? So lower the chimney pipe closer to floor level ]without restricting the flow, so that only the coolest air is sent up the chimney ?

Located this in a forum chat a while ago :

I read about ratios like 1:2:4, which would seem to suggest a higher riser, see this Permies thread -  https://permies.com/t/41234/Proper-dimensions-tube

Glenn Herbert
Rocket Scientist
Posts: 4736
Location: Upstate NY, zone 5
673  5

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 proportion will give a stronger rocket (the original standard for lengths was 1:2:3; 1:2:4 is more reliable.)

My system is a 6" System:

I read this criteria online


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.

If you want a hot spot on your barrel top for heating a pan or kettle then stick to 2.5”. Otherwise make it more.
Some designs use two barrels on top of each other for space heating, meaning you could have a top gap of 48” if you wanted to!