Advice on a RMH build in Hokkaido Japan

Peter Sedgwick wrote: I've got access to tons of reclaimed cinder blocks. Thinking of using those, filled with cob, for the walls of the chamber then cobbing over and plastering with the same material I use on the floor.

A quote from many posts back. Not sure why you switched to filling them with charcoal? The purpose of the blocks is to absorb and hold the heat. The charcoal is fairly insulative which is opposite of what you want. Am I wrong in that these blocks are also acting as a wall to seal up the square space (pit) from the ground to the bottom of the floor? If you were looking at having some insulation between the outside and the inside this would best be done just on the outside of the blocks rather than in the block spaces. Make sense or am I seeing it wrong?

Also, you said you were going to put a cob liner on the inside of the bell so I didn't think twice about the cement blocks not being able to take the heat. After reading this timely post  Bell materials I'm not sure how thick this liner would need to be to protect the cement from degrading? Unless you go with the cement blocks on the base course as you've already done then switch to some refractory material (even a cob and rock lasagna wall) for the rest of the way as Glenn suggested.  

Thanks Gerry,

Will use the scaffolding right side up in that case. Noted on the straw rich clay and then mass cob for double seal.
Should I pre-soak the straw? Seemed pretty stiff in my last test batch.

Read the article on bell sizing again, but still escapes me a bit. Planning to build a 6” CFB J-tube. Is that too small of a “engine” for the volume of the internal space in our “L” shaped bench? I decided to do a 6” based on the size of the room. Bench size was determined by the pre existing scaffolding dimensions.

Dimensions of the bench space at this point are as follows:

120cm on short side of the “L”
200cm on the long side of the “L”
55cm bench width
25cm-30cm hight of chamber throughout

Actually size of the scaffolding top is about 85cm wide, but I’m taking into account the fact that the scafolding is sitting on blocks so that will decrease the internal surface area of the actual chamber. Outside block wall is insulated so that heat won’t escape. So if we subtract the floor and the out side wall from the total thermal mass we are left with the bench top and the inside wall. Can put parking blocks inside to add a bit of mass.

Is this an 8” system scenario?

Should I make the bench dimensions smaller?

Am I over thinking this because it's getting colder and I want to finish and be warm?

The codes don’t apply on this one, just wanted to make sure we are not going to burn the place down. Will get the stove working and then figure out if the wall need anything.

Peter

Peter Sedgwick wrote:Should I pre-soak the straw? Seemed pretty stiff in my last test batch.

The straw only needs to be about 6-8 cm long, enough to overlap any seams or holes and be kept straight so no need to moisten unless your going over uneven terrain and need to wrap it around things. What I have done, is over any holes or joints, put down a thin layer of what is called fire cob first: sand and clay with no straw then a thin layer of slip straw for strength then begin adding mass which is dense 'stuff' (in my case rock) with only enough cob or fire cob to bond it all together.

I don't have much experience with proper bell sizing. Satamax or Peter are the pros in this area. To me, if your close or over the 57sq ft. ISA recommended for a 6" batch box then it may be worth putting up a dividing wall within the bell to make the space smaller but still keep the bench size the same. Perhaps a wall with a flap that can be opened to allow heat to enter the second chamber only when a good draft is established.

Thanks so much for all the insight.

Didn’t realize I missed one of your posts regarding the insulation.

Had some feed back from Matt earlier.

Quote from Matt:

"I think generally you are off to a pretty good start. The first thing that jumps out at me is the tops. That metal scaffolding is not going to work to your satisfaction I'm afraid. It will change size too much with the temperature changes and you will not be able to keep cob on top, it will just become a sand box as it continually breaks apart your top coat. I'd strongly suggest a masonry bench top if possible, stone or cast, or my half-barrel system…”

The scaffolding idea is out the window.

Moving on to a half barrel layout I think, so the one row block wall in the back with charcoal insulation might work.
Leave that as is and build up cob on sides and above half barrel.

Might have to shorten it a bit as well.

Trying to work out the placement of the chimney now.

We are thinking of running the 15cm chimney through the ceiling of the RMH room uninsulated near the barrel to pick up some extra “exit heat”, then with a 20cm pipe and perlite insulation infill around the 15cm as it goes though the cold attic and out the roof. Basically a double chimney set up.
Go through the ceiling and then make a bit of adjustment with an angle joint to redirect out of the highest roof spine in the house. Big concern with snow drift build up along the edge where the two roofs meet so afraid of sending the chimney absolutely straight out vertically. Fell its safer out the higher roof. (Drawing for clarification)

Any thoughts on this?

Tips on the cob and straw are really great. Arigato!

Cheers, Peter

Peter Sedgwick wrote:Had some feed back from Matt earlier.
Quote from Matt:
"I think generally you are off to a pretty good start. The first thing that jumps out at me is the tops. That metal scaffolding is not going to work to your satisfaction I'm afraid. It will change size too much with the temperature changes and you will not be able to keep cob on top, it will just become a sand box as it continually breaks apart your top coat. I'd strongly suggest a masonry bench top if possible, stone or cast, or my half-barrel system…”

Not sure how one metal differs from another in their expansion/contraction but I would certainly go with his professional recommendation. Thanks for letting me know what he said. A learning experience for all of us!

Go through the ceiling and then make a bit of adjustment with an angle joint to redirect out of the highest roof spine in the house. Big concern with snow drift build up along the edge where the two roofs meet so afraid of sending the chimney absolutely straight out vertically. Fell its safer out the higher roof. (Drawing for clarification)

Doesn't look that sharp an angle to slow exhaust flow enough to worry about, plus the added benefit of getting a little residual heat in that upper room (if you leave it uninsulated of course).

The scaffolding being a large flat expanse will have a lot of heat expansion in one direction. The half barrels being curved will have the expansion oriented in different directions,so there will be less of it to affect the top of the bench. I would have some concern about the thinnest line of cob along the top cracking; perhaps a layer of cardboard (which will eventually char and disappear) between barrel and cob will mitigate this.

For that matter, with the smooth top of the scaffolding facing the cob, and the sides all constrained by room walls, a layer of thin cardboard for a friction releaser may allow the cob top to be unaffected by metal movement. The sides of the scaffolding would need expansion gaps built in so they didn't stress the cob, and the cob top would need to be thick enough to resist traffic stresses.

I’m not sure either, but with the time schedule we have and the night getting colder with each passing day I’m not going to raise my hand and ask questions. If half barrels work I’m going to be cutting some barrels. Plus once the barrels are in place and sealed we can start running heat. Handle the mass part of the build after that.

Will keep plugging along and update as we go. As always thanks for the advice and support.

Cheers Peter and the gang

Thanks for that bit of info Glenn. That sounds very plausible.
The only thing I don't like about the half barrels is the fact that there is a small strip along the top that gets most of the heat while the rest is barely warm. Perhaps this could be mitigated by putting a bit of insulation like rock wool along the top strip so that it would conduct towards the thicker mass areas more evening the heat distribution out?

Gerry Parent wrote:Thanks for that bit of info Glenn. That sounds very plausible.
The only thing I don't like about the half barrels is the fact that there is a small strip along the top that gets most of the heat while the rest is barely warm. Perhaps this could be mitigated by putting a bit of insulation like rock wool along the top strip so that it would conduct towards the thicker mass areas more evening the heat distribution out?

That’s exactly what I was thinking. Only using a mix of perlite and cob, where the ratio of perlite to cob decrease as you go away from the center. Maybe rolled sausages of material laid next to one another. The sausage in the middle has the highest concentration of perlite and the one on each side has a bit less perlite and less after that.

Other option might be to cut a 3/4 barrel and flatten the top for more surface area, but that seems like it would make the barrels difficult to match up.

By the way, if I decide to use this half barrel system, does anyone have any tips on how to cut a 45 accurately on a cylindrical object, so I can make a right turn and match up the barrel seams relatively clean?

This is getting interesting. So nice to discuss this stuff with people that know more than I do and are willing to think outside the box.

Many thanks, Peter

Peter,   A link that might be helpful: half barrel bench  In the post, Thomas gives some links to look over.
Not sure of the best way to make a nice angle cut. If it were me, I might make a template and use it to trace the angle on the barrels and then cut with a angle grinder, cleaning up to make it the best fit possible before sealing the seam.

Gerry Parent wrote:Peter,   A link that might be helpful: half barrel bench  In the post, Thomas gives some links to look over.
Not sure of the best way to make a nice angle cut. If it were me, I might make a template and use it to trace the angle on the barrels and then cut with a angle grinder, cleaning up to make it the best fit possible before sealing the seam.

Thanks Gerry, I'll have a go through.

Trying to work out the discrepancies in my brick sizes so they fit with Matt's 6" CFB layout without losing any internal surface area in the feed tube and the burn tunnel.
Throwing the dimensions in Sketch-up one time now to double check. Looks like I'll have to shim the difference, with a perlite powder and and clay, as my splits are a bit shorter and narrower than the ones available in the US. Think that seems to make the most sense.

At $75 a sheet I'm a bit nervous...Peter

Numbers are doing my head in...

Peter Sedgwick wrote:Trying to work out the discrepancies in my brick sizes so they fit with Matt's 6" CFB layout without losing any internal surface area in the feed tube and the burn tunnel.
Throwing the dimensions in Sketch-up one time now to double check. Looks like I'll have to shim the difference, with a perlite powder and and clay, as my splits are a bit shorter and narrower than the ones available in the US. Think that seems to make the most sense.
At $75 a sheet I'm a bit nervous...Peter

I don't blame you for hesitating on making the first cuts!
A 6" pipe has a CSA (Cross Sectional Area) of about 28 sq inches. Matt's CFB layout I think is 27 sq inches due to the fact that he's making it work with the firebrick. I think in the Builders guide it just rounds the CSA for a 6" system to 30 sq inches. However, I do recall something Glenn Herbert said a while back in the discussion Tommy's Tea Dome: "When figuring cross section, it is good practice to treat a square as equal to a round flue of the same dimension, because the corner spaces do not help flow but introduce turbulence." To me this means that the dimensions of a 6" system can be 6"x6" which actually turns out to be 36 sq inches CSA. With this in mind, it makes Matt's dimensions kinda small for something that could be much bigger, but again, its being limited by the firebrick dimensions. Of course, the firebrick is needed for the wear and tear experienced in the feed tube so they kinda got us between a rock and a hard place....unless..... you cut another firebrick lengthwise to make up for the extra dimension which is something I have done on my stove.

Filling the gaps with stabilized perlite to me sounds like kinda of a bad idea. It might work for a while but you can bet that the perlite will give out long before the CFB and make for an unnecessary redo.

Alright Gerry,

Now you’ve go me thinking and the 12 hours it took me to reverse engineer Matt’s Emperical PDF plans for the “6” inch J-Tube core to metric has left me with a better understanding of the construction and the internal volume we are dealing with.

It took me a long time to figure out how to fit the fire brick I have into Matt's system without sacrificing too much internal volume. Gerry, you said in your last message that you "cut bricks to fit your system”

I’d like to propose a new plan that involves “make bricks to fit the system” Also I would like to propose a custom size J-Tube design.
We keep taking about a “6” inch design, but in fact it’s not 6" because we are trying to fit dimensions with preexisting materials.

In my mind the one material I can not change the ISA (internal surface area) on is my chimney pipe. It’s made out of metal and I can’t forge a new one:)

So I`m proposing a “15cm” J-Tube build. All dimensions based on the one thing that has a fixed diameter, my 15cm chimney

The other thing I can’t change is the thickness of the ceramic board I have. 1” = 25.4mm
My board is made in metric and it is 25mm, so I was thinking, how about making firebricks that are 150mm wide by 25mm thick.
I have 1/2 bag of blast furnace cement and a 1/2 bag of powdered insulation perlite. It would take me little time to custom make a few molds and cast some bricks that fit perfectly for my build. No more gaps and no more thickness issues.

I could let them set for a few days, while I work on the chimney, half barrels and other bits and bobs.

It would simplify a lot and, I would imagine, make the RMH run better.

Could lay it out in sketch-up with a fixed ISA and then build around to get the precise dimensions needed in both fire bricks and ceramic fiber board.

The only other potential issue is weather my 4 sheets of 60cm x 90cm ceramic fiber board will cover all the parts if we size up, but it should work, seeing that with the present plan I’m only using 3 1/4 sheets and there is a bunch of off cut space when I lay out the flats in illustrator, to scale.

I would fallow Glenn Herbert ’s statement about cross sections. 15cm round chimney means 15cm square fire box and heat riser. All sides equal.

Does this sound like a good proposition?

If so how do I figure out the proper hight of the feed tube, length of the burn tunnel and length of the heat riser attachment?

BTW started cobbing in the area over the blocks to seal the room. Just too much cold air, blasting through the house at night. Can already feel the difference.

Would love to hear peoples thoughts on this idea. It’s my first build, but seems rational to me based on the general rules of thumb I have gathered so far. Feel free to tell me what you think and where I may be missing something.

Thanks in advance, Peter

Peter Sedgwick wrote:I have 1/2 bag of blast furnace cement and a 1/2 bag of powdered insulation perlite. It would take me little time to custom make a few molds and cast some bricks that fit perfectly for my build. No more gaps and no more thickness issues.
It would simplify a lot and, I would imagine, make the RMH run better.
Does this sound like a good proposition?

With a quick search, I found this on a website: "Portland Blast Furnace Slag Cement is particularly suitable for rivers, ports, roads and tunnels." The name 'blast furnace' sounds good but the description doesn't seem to match its intended use. Can you find any literature on this cement that says its good for over 2000 degrees F? If not, then it is not quite good enough for the high heat that your gonna fire that baby up to. The word "Portland" in their description also indicates to me that it may be similar to regular Portland cement which is only good up to about 400 F (I think) before it starts to spall. Matt has done a cast core video and lists his ingredients here: Rocket Mass Heater Cast Core Build
However, a lot of people have given up a little bit of efficiency in the feed tube by making it from dense materials due to the constant wear and having to patch this area too many times.
I am currently experimenting with making my own neolithic homemade refractory bricks as we speak. Donkey at http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions has done quite a bit of experimenting in the last many years which has been helpful to gleen from also. I'll let you know what I find.....
Sodium silicate is apparently a good thing to add that increases wear resistance and can be homemade quite easily... couldn't find a cheap source of silica gel cat litter so off the list for the moment.

If so how do I figure out the proper hight of the feed tube, length of the burn tunnel and length of the heat riser attachment?

One of the oldest questions but a goodie.... Proportions are 1:2:3 (provided there's an adequate chimney stack). 1:2:4 is generally better because of the improved draft the heat riser gives. Also, the burn tunnel should be kept as short as possible but is given a 2 as a maximum in order to provide enough room for the barrel.

EDIT: Found this post addressing his cast core: A castable burn chamber

An accurate empirical method of making a 45 degree cutline on a barrel, if you have a good-sized tub available, would be to mark the barrel at exactly one diameter up from the end, and submerge it in water so one side of the end is exactly at water level, and the mark on the other side is at water level. Carefully take the barrel out of the water and mark the wet line with chalk, then make a paper template for future use with other barrels.

Glenn Herbert wrote:An accurate empirical method of making a 45 degree cutline on a barrel, if you have a good-sized tub available, would be to mark the barrel at exactly one diameter up from the end, and submerge it in water so one side of the end is exactly at water level, and the mark on the other side is at water level. Carefully take the barrel out of the water and mark the wet line with chalk, then make a paper template for future use with other barrels.

There’s always a simpler answer than you expected. Thank you very much Glenn

I think this is what you’re saying.

As a builder, I would just mark the barrel with a laser but I would think about leaving a flange or tabs so  the barrels can be pop riveted or screwed together.
Probably the easy way to mark the cut is with a piece of wood set out on top of the barrel and just use a  a level to plumb down and mark with chalk.
Anyway I would cut one inch back from he line, make cuts every inch or so and bend up the tabs so they can be fixed together.

Thanks Gerry,

I'm over the idea of making bricks. Think you're right with the cutting brick idea. Gonna take some bricks over to the local grave stone guy in town tomorrow and see if I can get him to cut a few for me. His cuts will be dead on and better than trying to hack through rocks with a grinder.

Working on a new layout now and running it by Matt as well. Will keep you posted. Thanks so much for all you guys have been doing. Really helpful and easy to understand.

Cheers and beers, Peter and crew

Fox James wrote:As a builder, I would just mark the barrel with a laser but I would think about leaving a flange or tabs so  the barrels can be pop riveted or screwed together.
Probably the easy way to mark the cut is with a piece of wood set out on top of the barrel and just use a  a level to plumb down and mark with chalk.
Anyway I would cut one inch back from he line, make cuts every inch or so and bend up the tabs so they can be fixed together.

Cool James!

Someone gave me 3 extra barrels yesterday. Might try to work out a 45 link design for the turn and then weld that section together. Then leave the two ends open so I can adjust the length in the room as we build.

Peter

I would count a typical half barrel as about 9 square feet, including one end, and excluding a few inches at the bottom because that may well not get the full exposure to hot air.

Given that I found an 8" J-tube bell system to want around the same ISA as a 6" batch box (I think it can take a little more), I would guess that a 6" J-tube can handle around 30 square feet of ISA more or less, which would translate to at least 3 half-barrels. With absorption by the bottom sides of the barrels not very useful (since there is no lateral exposure of the mass at all), I would consider using four or five half-barrels, and a plunger tube for the chimney so that the bottom can be raised or lowered to use as much of the ISA as turns out to be desirable. If the bottom of the plunger is high above the floor, hot air will escape sooner and not reach low on the bell sides, reducing the heat absorbed. You might want to make another 45 degree joint and extend the last half-barrel across the middle of the room to get more of the floor warmed.

Glenn Herbert wrote: I would consider using four or five half-barrels, and a plunger tube for the chimney so that the bottom can be raised or lowered to use as much of the ISA as turns out to
be desirable. If the bottom of the plunger is high above the floor, hot air will escape sooner and not reach low on the bell sides, reducing the heat absorbed.

When I installed a plunger tube a few years back, I certainly noticed the difference between the tube being lower and higher and its effect on draft and exhaust temps.
However, the results were not long lasting: When put close to the bottom, my exhaust temps were initially reduced but then after about 10-15 minutes came right back up again. I remember Satamax told me that hot air at the top of the bell was re-heating the cooler air inside the plunger tube and therefore not extracting as much as I could be. He recommended to insulate the plunger tube so that it wouldn't do this - It worked and am still using this setup today.
I'd like to hear your comments on this as I really like the idea of being able to move the tube up or down to help with draft (but can't with insulation) during cold starts, windy conditions, shoulder season burning etc. where you need a draft boost, like a bypass.
The only thing I can think of is to do something like this:
 

Peter Sedgwick wrote:I'm over the idea of making bricks. Think you're right with the cutting brick idea. Gonna take some bricks over to the local grave stone guy in town tomorrow and see if I can get him to cut a few for me. His cuts will be dead on and better than trying to hack through rocks with a grinder.

I used a wet tile saw and worked pretty good. One thing I would suggest is to place the seam down the middle and have 2 pieces that are of the same dimensions rather than a full brick and a smaller brick. This much narrower brick I found has a tendency to crack easier and then become loose. Also looks more uniform/balanced. You did get a few extra right?
I like your unintentional pun of the gravestone guy being "dead on" with his cuts. :)

Working out the dimensions in sketch up now. Thanks for the tip on the bricks. I have a few extra and I have to go to Sapporo to get more supplies soon so I can get more then. Also I noticed they sold pizza stones that I might be able to trim down to 25mm. Or a bag of refractory cement for $40 and make the bricks. What ever is cheaper.

Glenn Herbert wrote:I would count a typical half barrel as about 9 square feet, including one end, and excluding a few inches at the bottom because that may well not get the full exposure to hot air.

Given that I found an 8" J-tube bell system to want around the same ISA as a 6" batch box (I think it can take a little more), I would guess that a 6" J-tube can handle around 30 square feet of ISA more or less, which would translate to at least 3 half-barrels. With absorption by the bottom sides of the barrels not very useful (since there is no lateral exposure of the mass at all), I would consider using four or five half-barrels, and a plunger tube for the chimney so that the bottom can be raised or lowered to use as much of the ISA as turns out to be desirable. If the bottom of the plunger is high above the floor, hot air will escape sooner and not reach low on the bell sides, reducing the heat absorbed. You might want to make another 45 degree joint and extend the last half-barrel across the middle of the room to get more of the floor warmed.

Understood Glenn. Thank you very much

Here is my proposed core layout based on Matt's CFB configuration set with the consent of a 15cm chimney pipe exhaust system.
Bricks are trimmed to 25mm, same thickness as my CFB. The length of the burn tunnel is taken from the suggested length stated for a metric "6 inch" core in The Rocket Mass Heater Builder's Guide.
Because of the size restriction of my CFB I can only get a total length of 90cm for the riser tube section of the unit. That will make the total hight of the riser from the bottom back corner of the burn tunnel to the top edge of the riser bellow the barrel 110cm tall. A bit shorter than suggested in the guide, but longer than the one in Matt's plans.

The thin brick on the top edge of the feed tube my be to thin and be prone to braking, but I thought it was worth presenting as an idea. Would make a clean design and could cheaply/easily be replaced.
The cut brick configuration can change and I may find a way to cut larger fire bricks or pour molded bricks to minimize seams and parts.
Can also adjust the length of the feed tub by stacking losse fire bricks around the mouth of the CFB core top if needed.

Please let me know your thought and if I am on the right track.
I've never even made one of these things and I'm already trying to deviate from the plan. Think it's part of my personality coming through, for better or for worse.

Much thanks for all your input and feed back.

Cheers, Peter

I’m standing here on this pile of earth, mixing with my feet, and thinking to myself “this is going to go in the house, it’s an extension of the earth which is right outside the house. It’s s soon going to turn into a hard brick like substance that fills the room and serves a purpose.”

If you mix sand with cement and you put it in your house that’s normal, if you mix sand with clay and put in your house people look like that you like you’re from outer space. There’s a fine line between inside and outside. It’s the rules and regulations of society that have set us a drift.

I’m definitely in no position to call myself a Permaculturest or professional eco-builder, but at the end of the day this stuff cost me absolutely nothing, will do zero damage to our plant, and it’s probably going to work just fine for many years to come.
Hummmm, which one should I choose...
✌️🤓✌️

I would not encourage you to deviate from the standard tried and tested dimensions however I do believe there is some leeway!

In my own case, I have now built four stoves and only the first one was built to the recommended size.

What I found out for myself based on my own circumstances, was the fire box was to small and the heat riser too tall, so I the made the fire box 8x8” and the riser 30” and it all worked fantastic ... in fact much better than the first model!

However it must be pointed out my stove does not feed any pipe work or barrels etc... it does heat up the hollow base and some mass around the barrel top but otherwise the chimney goes directly out the roof.

There are some similarities to you situation as my stove is in an earth floor outside enclosure with no insulation in the walls.

The lasted one I have built has an even larger feed chamber and a hinged glass lid, it also works extremely well but this might be due the the direct outlet.

My new fire is crazy hot, i can see the top glowing red with in 30 minutes of lighting up and it heats my large room to well over 30c in a couple of hours.
Of course when the fire dies out the heat in the room drops quite quickly.
There seems to me to be a lot of scope for better designs but for a first build the standard dimensions offer well documented and a safe design.
I have this video of mark two....

Thanks James.

My chimney exit pipe is 15cm. That’s .9cm shy of a true 6” ID pipe. That’s the closest I can get here in Japan. My dimensions I have used in the mock up are based on Glenn’s statement about treating a the internal volume of a square the same as the volume of a tube.

I’m more than open to adjust the dimensions. Is there anything you would suggest in particular?

Peter

Well I am reluctant to recommend anything too different as I only have my own stoves to judge by, where as, I assume the normal formula has been tried  and tested by many.
I will say that having  the fire box stepped down a little from the fire tunnel is a huge benefit a it stopes the wood from slipping into the tunnel.
Having some form of pre heated secondary air most definitely improves performance and in the form of a P plate, is very easy to incorporate.

Fox James wrote:Well I am reluctant to recommend anything too different as I only have my own stoves to judge by, where as, I assume the normal formula has been tried  and tested by many.
I will say that having  the fire box stepped down a little from the fire tunnel is a huge benefit a it stopes the wood from slipping into the tunnel.
Having some form of pre heated secondary air most definitely improves performance and in the form of a P plate, is very easy to incorporate.

That's more stoves than I have under my belt for sure.

Noted on your point of the step down. Thanks for that James

Gerry said that the 15cm/5.9" ID chimney pipe I have should not be an issue, but right out the gate I'm starting with a variable that is not in the "normal formula"
Having said that, Matt Walker told me, "You are not changing dimensions enough to effect performance in my opinion. Your slight changes won't have any significant effect either way so don't agonize too much over the small dimensions variances."

My design, at present, with drawing, is merely a place for us to start discussion.
Would love to hear anyone's thoughts on the design, who has experience in this type of build and any suggestions or points to make.

Do need to start cutting a core and building relatively soon, but waiting for chimney parts right now and working out how to cut the half barrels for the bench, so have a bit of time to discuss pros and cons of build designs and dimensions based on the materials I am using.

- CFB firebox and riser good to 1400°C/2552°F
- 15cm/5.9" chimney pipe with 20cm core filled with perlite for insulation
- Chimney comes out the highest point of the roof
- Bell Half Barrel "L-Shaped Bench 58.5cm/23" diameter (one side 170cm/70" long, the other 100cm/40" long)

Plan to put the whole thing together raw and start firing it as soon as I have a working firebox, half barrels and an exhaust. Just seal the barrels at the bottom with cob and see how it runs. Then we can adjust and make barrels longer or shorter add brick hight to the feed tube etc.

Not a lot of info out there on j-tube bell barrel builds with CFB cores, that I can find, so thought this would be a good approach.

Peter

Peter Sedgwick wrote:Thanks James.

My chimney exit pipe is 15cm. That’s .9cm shy of a true 6” ID pipe. That’s the closest I can get here in Japan. My dimensions I have used in the mock up are based on Glenn’s statement about treating a the internal volume of a square the same as the volume of a tube.

I’m more than open to adjust the dimensions. Is there anything you would suggest in particular?

Peter

Made a mistake in my statement about my chimney.
15cm=5.9”
So there is a .1” difference
Not .9cm difference

Peter

Peter Sedgwick wrote:Plan to put the whole thing together raw and start firing it as soon as I have a working firebox, half barrels and an exhaust. Just seal the barrels at the bottom with cob and see how it runs. Then we can adjust and make barrels longer or shorter add brick hight to the feed tube etc.

This is probably the best advice you can give yourself! This experience will teach you SO much and make all the literature you've read, comments made here and visions you've had floating in your head start to make sense. However, for every answer that comes, 2 more will often take their place.

That thin brick can have a much better chance of not cracking if you use a "Peter channel" as Fox pointed out. See here: Peter-channel-dragon

As I read your comment: "I’m standing here on this pile of earth, mixing with my feet, and thinking to myself “this is going to go in the house, it’s an extension of the earth which is right outside the house. It’s s soon going to turn into a hard brick like substance that fills the room and serves a purpose.” I began to grin and thought this is what its all about!  Permaculture at its finest. Seeing the interconnectedness of all life and how we too are apart of the Earth which is something that a lot of our society has lost this connection.  One rocket stove at a time!

RMH
Batteries not included, some assembly required...🚀👨🏻‍✈️🚀

I don't think a step down in the feed tube floor is necessary or beneficial, if feeding is done by the standard rule: lean the sticks forward against the riser side of the feed, not back against the room side. Having the air flow between the sticks lets all of them burn equally, and prevents entering air from bypassing the sticks and entering the burn tunnel without contributing to combustion. A step down will soon be filled with ash and become a moot point unless constantly cleaned out. I never have sticks slipping into the burn tunnel unless I push partially burnt pieces there. (That is a good thing to do in the coaling phase so that the remaining fuel has maximum exposure to draft, including the directed flow from the p-channel with the feed mouth mostly closed.)

Glenn Herbert wrote: if feeding is done by the standard rule: lean the sticks forward against the riser side of the feed, not back against the room side.

I believe that method is more likely to cause excessive Carbon Monoxide as it blocks fresh air from entering the combustion chamber.
However, if you install the Peter channel, then that reduces that problem.



And unblocking the fresh air with the P channel ( or plate )



See https://www.rocketstoves.com/case-studies/peter-van-den-berg/

Thanks for those tips. I will cut it out of this rusty piece of metal. It’s basically just a T-shaped panel that hangs from the side of the feed tube, is that correct?

It's a sort of T shape where the gap between the front of the feed is also the same distance that the bottom of the T goes below the front wall of the feed tube.  The arms of the T stop it falling into the feed tube and I guess there's something that stops the wood from pushing the plate right against the feed tube occluding the air.  It's made of metal so that incoming air becomes preheated.

Roger that. I’ll get one of those in the works as well. Thanks for the details.

Peter