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Small(ish), but not tiny, Rocket Mass Heater for Bedroom in Japan: Is a 4" system feasible?

 
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Hi Everyone,

I am new here, so a quick introduction. My wife and I have a house in rural, central Japan (Temperatures range from -5C to about 35C) and I have been renovating it for the last couple of years. It really was built like a shed, typical for this area of Japan, with corrugated iron outside and very thin plywood (3mm) on the inside of a wooden frame. I have started renovating it, by taken off the plywood and rusty corrugated iron and using the wood frame to make cob/mud walls with tatami mat/straw cores held in place by bamboo. So far I have made one big living space area, the back part of the house, that has a wood burning stove in the middle.

Now I found out about RMH and, since I am not particularly happy with my wood burning stove (which needs constant cleaning and a lot of attention, while still being quite sooty and smokey), I want to make one for the bedroom that I am currently renovating in the same way as described before. I have Ianto Evans and Ernie and Erica Wisner's books but am struggling to make something specific to my conditions  here in Japan (raised floor, small room size).

First problem I am having is finding chimney pipe at 6 inch, let alone 8 inch. Everything here is smaller (and in mm, not inch). I can't seem to find affordable chimney pipes that are bigger than 106mm (4.1inches). Anything bigger seems to be prohibitively expensive import stuff costing thousands of dollars for what I am trying to make.

So my first question, is there any sensible way I could scale down the RMH design to fit 106mm (4.1inch) diameter of chimney pipe for the mass section of the heater? Is there a good practical way to avoid the chimney pipe altogether and only use  (fire)bricks which I can buy for reasonable prices around here if making the pipe system smaller is not a good option?

Thank you all for any input and suggestions. I will be making photos of my project and adding them to this thread soon and am very happy to answer any questions in the meantime.
 
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Hi Paul;
Welcome to Permies! I don't have time this evening to respond but I've linked a post you may enjoy!
https://permies.com/t/122458/Advice-RMH-build-Hokkaido-Japan
 
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Yes 4inch and 3inch is very feasible.
Here is a very informative website. https://batchrocket.eu/en/building#dimension

It sounds like you are USDA zone 9, so a 4inch RMH should be fine.
 
Paul Muthers
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thomas rubino wrote:Hi Paul;
Welcome to Permies! I don't have time this evening to respond but I've linked a post you may enjoy!
https://permies.com/t/122458/Advice-RMH-build-Hokkaido-Japan



Thank you, very interesting article in this thread (http://www.healthyheating.com/History_of_Radiant_Heating_and_Cooling/History_of_Radiant_Heating_and_Cooling_Part_1.pdf) that is making me consider completely overthrowing my current plans and building complete floor heating instead..  I am already planning on putting tatami against the underside of my floor, holding it in place with bamboo and then covering it in mud/cob, like I did at the other part of the house, so it may almost be feasible to just go and turn the whole space into a smoke ducting area... but I am a bit scared of CO leaks especially since we will be sleeping there....
 
Paul Muthers
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S Bengi wrote:Yes 4inch and 3inch is very feasible.
Here is a very informative website. https://batchrocket.eu/en/building#dimension

It sounds like you are USDA zone 9, so a 4inch RMH should be fine.



Thank you, I had come across this website before but must admit I am still struggling with the complexity of it. I am a little overwhelmed, as I am just about getting my head around rocket mass heaters.
The batch rocket seems a little more complicated, but possible better too, but I am particularly unsure how to make a "P-channel", but may be able to find some metalwork place that could make me something suitable..  

Just a quick general question I am not quite sure I understand. Does the list of measurements also, roughly, work for normal rocket mass heaters or do they all require the batchrocket design with the secondary air intake (p-channel)?

And does the diameter of the heat riser dictate the minimum diameter of the chimney pipe system through the heater mass body? Do you remember how they relate? I haven't seen the chimney system mentioned specifically..
The heat riser is not so much a problem for me to obtain/build as the chimney pipe system, especially as it requires a fair amount of it  to run out and back inside the 3-4 meter cob mass heater section I am planning.

As mentioned, I am struggling to find anything larger than 4.1 inches of affordable chimney pipe and have started wondering about alternatives, such as concrete drains.

 
Paul Muthers
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Paul Muthers wrote:

thomas rubino wrote:Hi Paul;
Welcome to Permies! I don't have time this evening to respond but I've linked a post you may enjoy!
https://permies.com/t/122458/Advice-RMH-build-Hokkaido-Japan



Thank you, very interesting article in that thread about heated flooring (http://www.healthyheating.com/History_of_Radiant_Heating_and_Cooling/History_of_Radiant_Heating_and_Cooling_Part_1.pdf) which is making me consider completely overthrowing my current plans and building complete floor heating instead..  I am already planning on putting tatami against the underside of my floor, holding it in place with bamboo and then covering it in mud/cob, like I did at the other part of the house, so it may almost be feasible to just go and turn the whole space into a smoke ducting area... but I am a bit scared of CO leaks especially since we will be sleeping there....

 
S Bengi
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Yes the riser diameter dictates the exhaust diameter of the system, they should be equal (technically the exhaust can be a bit bigger, but never smaller)

Riser diameter = 4inch
Riser Area/CSA = 12square inch
Bell CSA = Tube CSA times 5 = 60sq inch (aka at least a 8.74inch bell diameter)
Bell Gap = 1in to 2in
Bell Internal Surface Area (top + sides only) = Tube CSA times 300 = 12 * 300 = 3600sq in = 25sq ft
Exhaust/Mass Length = 12ft (after the 1st 90 degree bend, minus 5ft for each additional bend)

I recommend building this system outside and testing it, before putting it inside.

Are you sure that your floor can support the weight of the entire RMH?


You could also go with a radiant in-floor heating system powered by a pellet hot water boiler.
 
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{Paul, do you have the gear to roll your own tubes.
You could use a 'pipe anvil' to make say 48 inch long sections and weld the seams.
 
Paul Muthers
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S Bengi wrote:Yes the riser diameter dictates the exhaust diameter of the system, they should be equal (technically the exhaust can be a bit bigger, but never smaller)

Riser diameter = 4inch
Riser Area/CSA = 12square inch
Bell CSA = Tube CSA times 5 = 60sq inch (aka at least a 8.74inch bell diameter)
Bell Gap = 1in to 2in
Bell Internal Surface Area (top + sides only) = Tube CSA times 300 = 12 * 300 = 3600sq in = 25sq ft
Exhaust/Mass Length = 12ft (after the 1st 90 degree bend, minus 5ft for each additional bend)

I recommend building this system outside and testing it, before putting it inside.

Are you sure that your floor can support the weight of the entire RMH?


You could also go with a radiant in-floor heating system powered by a pellet hot water boiler.



Thank you, the calculations seems a bit complicated but I will get my head round them later. I am coming across a lot of information at once and will need to digest a bit what I have read... I just now discovered that I could make a stratification system (also called "bell", it seems), which would avoid me having to buy expensive chimney piping (even the very weakest, 4.1 inch diameter one will cost me around 200 dollars for the inside of the exhaust mass). Is there some downsides to having an open space, aka stratification system, instead of chimney piping in cob?    

Oh, and regarding your question, my current plan is to remove part of the suspended floor area to build my rocket stove riser and bell and then running the exhaust mass underneath the suspended floor to give some mild underfloor heating, including under the bed, and then having the chimney go up high through the side wall back where the heat riser/bell sits directly on the concrete floor..

Still very much in the planning stages, even though I am hoping to finish this (and all my general cob renovation) by end of November, and am now seriously considering the batch rocket stove that was suggested in the link posted earlier. That way I could fire from outside my bedroom, which was my original plan, and just keep an eye on the burn with a window I am planning to install to make the back entrance/genkan area visible.. Will really need to take some photos and upload them soon, this is all a bit hard to explain in writing and probably difficult to imagine for those not living here in Japan.

 
Paul Muthers
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John C Daley wrote:{Paul, do you have the gear to roll your own tubes.
You could use a 'pipe anvil' to make say 48 inch long sections and weld the seams.



Fascinating, the pipe anvil part I could probably get together eventually with some local scavenging, but I don't know how to weld and don't believe it is something I can pick up quickly. I would certainly love to learn to do it one day, but for this year I probably have enough on my plate already. I do have some very nice local metalwork guys that may well be open to doing some welding like that for me, if I asked nicely and bought them some decent Sake.

I am currently wondering whether to even bother with piping at all (or just the minimum) and making a stratification chamber, i.e. an open space for the hot air to flow into with an exhaust/chimney system connected to the bottom of it. Any experience with this kind of exhaust system? Is it worse than exhaust piping?

Oh, and I just remembered that I have some clay (?) chimney flue sections that I thought I couldn't use because reading about the heat riser I learned that they would not withstand the temperature changes, but maybe they would work for the exhaust system. They are just a bit awkwardly big and heavy, and 2 different sizes, plus no corner pieces, only straight parts... So much to think about, maybe that should form at least part of my actual chimney (i.e. going up to the ceiling... Again, not sure what to do with that yet..
 
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A 4" batch box is easy to build, and would probably even work well without special secondary air supply (at least for the first iteration). I made a dry-stacked mockup of a 4" batch box and it burned beautifully. A properly built one should be even better. A 4" size would need reloading every 15-20 minutes more or less, so I would not put it outside. If you have good draft you will not have to worry about smoke in the room. You would not be running it all night in any case, except maybe on the coldest nights.

I am not sure a 4" system would be big enough to use as an underfloor heater. I think it would work better as a modest bench or "pillar" shaped mass which would radiate across the room. The "cyclone batch rocket" built at Wheaton Labs might serve as a good guide, with modifications based on their experience with it in use. In particular, I would include a metal access panel in the side or front of the mass for some instant heat (also to allow inspection and maintenance without tearing up the masonry.)
 
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Paul Muthers wrote:Just a quick general question I am not quite sure I understand. Does the list of measurements also, roughly, work for normal rocket mass heaters or do they all require the batchrocket design with the secondary air intake (p-channel)?


Despite the similairity in the names the measurements and (simple) formulas on the batchrocket website apply to batchrockets only. Back in the day of, say, 10 years ago I tried to work out a similar set of rules for the J-tube as well. But the then experts on the Ianto Evans' J-tube design refused to cooperate so I gave up.

Paul Muthers wrote:And does the diameter of the heat riser dictate the minimum diameter of the chimney pipe system through the heater mass body? Do you remember how they relate? I haven't seen the chimney system mentioned specifically..


This is one of the few similairities with the J-tube. The j-tube has a consequent cross section area throughout, which is called the system size. In the batchrocket design the system size is the riser's diameter, which in turn is the same as the chimney exhaust diameter.

In general, the batchrocket system is about twice as powerful as the same size J-tube. Since efficiency is roughly the same and raw power can't be produced out of plain air the former is eating away about twice the fuel weight in any given time frame.

There's another difference: the batchrocket system is very picky about restrictions in the smoke path. If ordinary bricks are available I'd recommend to build a bell system instead of a piped bench.
 
Peter van den Berg
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Paul Muthers wrote: Is there some downsides to having an open space, aka stratification system, instead of chimney piping in cob?


For building a bell system other skills are required. But it beats the piped system in simplicity at a physics level. In my opinion, the former is vastly superior in extracting heat provided it's wide enough inside. The velocity of the gases will slow down greatly when entering such an open space, allowing gravity to do its job by separating the hotter gases from the cooler ones. Hot gases rise to the top, cooler gases sink to the floor. As such, it could be viewed as being a displacement system, pushing the gases at floor level out.
As a bonus, due to the low gas velocity in the bell system fine dust will settle quietly on the floor of it instead of clogging up horizontal chimney pipes.
 
John C Daley
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Good to hear a pipe anvil is on the way.
Maybe you could rivet the tube seems instead.
 
Paul Muthers
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Just testing out uploading some photos of my work site..
IMG_20220907_141052_287.jpg
[Thumbnail for IMG_20220907_141052_287.jpg]
IMG_20220907_141112_099.jpg
[Thumbnail for IMG_20220907_141112_099.jpg]
IMG_20220907_150451_955.jpg
[Thumbnail for IMG_20220907_150451_955.jpg]
 
Paul Muthers
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Paul Muthers wrote:Just testing out uploading some photos of my work site..



Ok that worked pretty well with my phone. So the first picture, IMG_20220907_141052_287.jpg, is showing the probable site for the stove. As I wrote earlier, I am still very much in the planning stages and everything is still on the table, including an entirely different spot. But the current idea is to roughly place the stove where the barrel is right in that first picture. The wooden floor joists can be removed there and it would sit entirely on the concrete below it.

The chimney would probably run out of the wall beside it, near the top. You can see the back entrance, a wide sliding glass door, that I am intending to build a low ceiling over, roughly 50 centimers above the height of the door, basically making a tunnel with cob side walls and wooden joists/flooring on top.

The other current idea is to have a sliding window (just about visible, leaning near the barrel against the wood structure), which I could easily open and get access to the wood storage and possible even the hearth, actually outside the room, behind a mud wall that is yet to be built. It's just an idea, but I really like the thought of having none of the dust and dirt of the fire place in the bedroom, but very close, easily accessible and visible thanks to the big window.

As a bonus possibility for more natural light from the backdoor and extra visibility of the burn area, I have a large curved piece of glass (once a boat windscreen) that I am considering installing into the mud wall right behind the barrel, but not sure if that can handle the possible heat. In the worst case, I guess, it would crack and I would take it out and just make all of it a mud wall...


The second picture, IMG_20220907_141112_099.jpg, is just to give a general idea of what I am doing to my house. It's the other side of the room, the mud wall has had a first layer and is taking a long time to dry (and growing some mushrooms) in the very humid conditions we are having this year. The glass balls are set into the mud, they are old fishing net floats. On the ceiling you can vaguely see the tatamis, set into the ceiling joist and held in place by bamboo, that will get a layer of mud over it later for ceiling insulation.


The third picture, IMG_20220907_150451_955.jpg, is from under the floor, at the front section of the room. Again I have used old tatamis, cut into sections that fit the gap between the joists, held in place by bamboo (more to come), that will later be covered in mud/cob. The bedroom itself ha the same type of floor, but actually sits on a concrete foundation (I was very pleased to discover), unlike the rest of the back of the house that just sits concrete pillars straight on dirt floor.

 
Paul Muthers
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Glenn Herbert wrote:A 4" batch box is easy to build, and would probably even work well without special secondary air supply (at least for the first iteration). I made a dry-stacked mockup of a 4" batch box and it burned beautifully. A properly built one should be even better. A 4" size would need reloading every 15-20 minutes more or less, so I would not put it outside. If you have good draft you will not have to worry about smoke in the room. You would not be running it all night in any case, except maybe on the coldest nights.

I am not sure a 4" system would be big enough to use as an underfloor heater. I think it would work better as a modest bench or "pillar" shaped mass which would radiate across the room. The "cyclone batch rocket" built at Wheaton Labs might serve as a good guide, with modifications based on their experience with it in use. In particular, I would include a metal access panel in the side or front of the mass for some instant heat (also to allow inspection and maintenance without tearing up the masonry.)



Thank you for these very useful links. I think your idea might be the best and simplest, but will have to consider it a bit longer. Very tempted to forget about running anything under my floors, it really isn't that cold here in the winter, a radiant heater on the side of the room would be perfectly fine, for warmth. I just really liked the idea of floor heating, but the practicalities of building it, possibly adjusting it if it doesn't work properly, and cleaning/maintaining it, all under the actual floor joists, seem a bit overcomplicated.
 
Paul Muthers
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Peter van den Berg wrote:

Paul Muthers wrote:Just a quick general question I am not quite sure I understand. Does the list of measurements also, roughly, work for normal rocket mass heaters or do they all require the batchrocket design with the secondary air intake (p-channel)?


Despite the similairity in the names the measurements and (simple) formulas on the batchrocket website apply to batchrockets only. Back in the day of, say, 10 years ago I tried to work out a similar set of rules for the J-tube as well. But the then experts on the Ianto Evans' J-tube design refused to cooperate so I gave up.

Paul Muthers wrote:And does the diameter of the heat riser dictate the minimum diameter of the chimney pipe system through the heater mass body? Do you remember how they relate? I haven't seen the chimney system mentioned specifically..


This is one of the few similairities with the J-tube. The j-tube has a consequent cross section area throughout, which is called the system size. In the batchrocket design the system size is the riser's diameter, which in turn is the same as the chimney exhaust diameter.

In general, the batchrocket system is about twice as powerful as the same size J-tube. Since efficiency is roughly the same and raw power can't be produced out of plain air the former is eating away about twice the fuel weight in any given time frame.

There's another difference: the batchrocket system is very picky about restrictions in the smoke path. If ordinary bricks are available I'd recommend to build a bell system instead of a piped bench.



Thank you very much for your input ,Peter. I really like the idea of the batchrocket, particularly the ability to load up considerably more wood at a time and to have it heat up even quicker than the normal J-tube design. It's a shame that the experts on the J-tube design did not co-operate with your attempts to make a similar measurement guide, but it is surely a time-consuming and entirely unpaid endeavour, so I can understand it. Therefore I am even more grateful for all the useful information, particularly the detailed size calculator, you have made available on your website. Thank you so much!

In general I feel very honoured by your and all the other knowledgable responses I have been getting on my thread so quickly. Even though there seem to be no sizing guides for smaller RMHs, which was making me worried that I could not build one in my house after all, there is so much information on this forum that people are freely sharing, it really is encouraging me a lot to carry on with this project! So thank you, everyone else, too!
 
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Peter van den Berg wrote:

Paul Muthers wrote: Is there some downsides to having an open space, aka stratification system, instead of chimney piping in cob?


For building a bell system other skills are required. But it beats the piped system in simplicity at a physics level. In my opinion, the former is vastly superior in extracting heat provided it's wide enough inside. The velocity of the gases will slow down greatly when entering such an open space, allowing gravity to do its job by separating the hotter gases from the cooler ones. Hot gases rise to the top, cooler gases sink to the floor. As such, it could be viewed as being a displacement system, pushing the gases at floor level out.
As a bonus, due to the low gas velocity in the bell system fine dust will settle quietly on the floor of it instead of clogging up horizontal chimney pipes.



I am very glad that there is a viable alternative to the pipe system. I was really worried about using small, yet expensive, 106mm chimney pipes under my flooring and how to deal with them clogging up. I was not looking forward to regularly crawling under there to clean out soot.

A bell system sounds interesting, though I may end up not doing either and just going with the earlier suggestions of a nice radiant heating batch rocket, do you know what kind of size restrictions there are for the bell system, attached to, e.g. a 4inch batch rocket system? What would happen if the area was too big?
 
Peter van den Berg
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Paul Muthers wrote:A bell system sounds interesting, though I may end up not doing either and just going with the earlier suggestions of a nice radiant heating batch rocket, do you know what kind of size restrictions there are for the bell system, attached to, e.g. a 4inch batch rocket system? What would happen if the area was too big?


The heater will refuse to start up properly, that's all. Oversizing could also mean that the chimney temperature is too low. Which in turn is causing water condensation in the chimney. This looks like this: at first, the fire is ramping up but within appromately 20 minutes a chimney stall occurs and a tsunami of smoke is streaming into the living space. Keep in mind: burning of wood will produce about half of its weight of water. It's one of the results of the chemical process, the others (assuming complete combustion) are CO² and heat.

The numbers given on https://batchrocket.eu/en/building#dimension are pretty accurate. Of course you could oversize but in that case you'll need a chimney shortcut or bypass of some sort. When planning to build a batch box rocket yourself it's sensible to read the whole of the site very carefully.
 
Paul Muthers
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Peter van den Berg wrote: When planning to build a batch box rocket yourself it's sensible to read the whole of the site very carefully.



You are right! I will dedicate some time to fully read and understand your website in the next couple of days and then come back with a clearer idea and (possibly) more sensible questions. Thank you for the explanation on what happens when the bell size (or rather ISA, as I have learned today ; ) is too large.
 
Paul Muthers
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Hey everyone,

due to typhoon reasons I have had a bit of a break from working and been able to read and plan a bit.

I will quite likely build something similar to the cyclone batch of this thread, https://permies.com/t/71576/tiny-house-rocket-mass-heater, maybe with a more simplified batch box like this one, https://permies.com/t/59459/EZ-Tools-Brick-Micro-Batch#505805, thank you Glenn Herbert for those links. I particularly like the casserole door, just pressed into cob. I have a glass frying pan I hardly use that could probably work quite adequately for this (or will find a casserole lid somewhere).

My space, pictured here , is 1200mm by 550mm (47.24 x 21.65 inch). If necessary, but with some effort, I could extend it in either direction. I am currently working under the floor in this area, attaching tatamis with bamboo and now covering everything in cob/mud from underneath. My plan is to sit the whole stove on cinder blocks filled with perlite inside the blocks holes and cover all the wood around it with cob and probably some chalk (shikkui) to further improve heat resistance.

IMG_20220921_082232_327.jpg
1200mm x 550mm (47.24 x 21.65 inch) space for heater, bell and chimney
1200mm x 550mm (47.24 x 21.65 inch) space for heater, bell and chimney
IMG_20220921_082246_330.jpg
Area around stove
Area around stove
 
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In general, my budget is very small. I can get fire and red clay bricks for a decent prize (https://www.komeri.com/disp/CKmSfGoodsPageMain_001.jsp?GOODS_NO=337145, these are rated for up to 1200C), but I am a big fan of recycling and since there is so much of it around my area of Japan (Noto, Ishikawa), I am keen to reuse "waste" (ie abandoned building materials, etc.) that is being left to rot all around here. I am often given things by locals who know that I like to reuse. Hence I have all these big glass balls from fishermen that I am using in my cob walls to add natural light heat (see earlier pictures for an example).

Recently I was given these old chimney parts, which I assume are clay or some kind of pottery anyway (really not an expert on this). I am hoping to use these as my chimney flue, but was actually planning on going out of the side wall. I am really not sure how to connect them to go around the corner at the top. I am considering to stack them straight on top of each other but make them (covered in cob) leaning at an angle and build them right through the cob wall I am going to make, to the outside. With a clean out at the bottom and some kind of rain cover at the top, that might work. I have three of the larger ones that have a 175mm diameter and two narrower ones more that have a 155m diameter. They are all approximately 645mm tall.  

My main question is: Would it be a problem if I my chimney got narrower at the top (going from 175mm diameter to 150mm)?

Overall the chimney flue diameter would also be bigger than my heat riser diameter, since my heat riser will likely be somewhere between 100mm and 150mm diameter. I am assuming a bigger chimney flue is generally better, or is that going to be a problem?
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Paul Muthers
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And further on the topic of reusing. I discovered that my local area is full of diatomaceous earth (keisoudo), apparently 2/3s of the soil here is made up of it. It is not only supposed to be very healthy and a natural insecticide, it has great heat insulation properties. It is sold under the name "Isolite" in this area and I can find old bricks made of it, which are somewhat lightweight and quite heat resistant. They are expensive to buy new (around 5 dollars per brick), but I have an old shichira barbecue, aka konro, made of Isolite and can probably get several more of these (for more info on them, see here https://www.amazon.co.jp/-/en/Diatomaceous-Earth-Shichira-Isolite-Konro/dp/B00ZVU4XNE, amazon pages can be translated to english automatically).

My idea was that if I got several of these old "trash" konro bbqs and cut the bottom of them, I could stack them and turn them into a heat riser. There are two issues with this plan. One is that they have ridges and more importantly get narrower at the bottom. The internal diameter at the is approx. 150mm and only around 120mm at the bottom. I could maybe sand and file them to a more even diameter and remove those ridges, if necessary, but I actually have never tried cutting or sanding Isolite or Diatomaceous Earth, so I have no idea how difficult that would be. Would a heat riser with these ridge lines (see photo) and narrower sections every 200mm or so be a problem? Would it possibly add to the vortex and improve efficiency of the burn?

Another question would be how to connect them together? I am not sure, but I thought maybe a fireclay and sand mix along the outside could do the job. What do you all think? Also there is an air inlet at the bottom of about 70mm x 40mm. I could cut above it and loose a good third of height (but I think I could get enough of these to compensate), or I could fill in the gap with some perlite or vermiculte and clay mix.

Any input on this idea would be very welcome. Has anyone worked with diatomaceous earth before, particularly in a heat riser? I don't know exactly what temperature these Isolite stoves are rated for but from the Isolite website (https://www.isolite.co.jp/en/products/taika/a_insulation/) I gathered that Isolite bricks are rated from around 900C to 1500C depending on the quality/type of Isolite (A1 to A7).
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Paul Muthers
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I like the cyclone batch rocket mentioned by Glenn Herbert, but would really like a flat top to be able to cook or at least keep tea warm on. I am thinking of having barrel, covered in cob, as a first bell over the heat riser, then  a small second bell made only of bricks next to it before going into my clay (?) chimney going out straight but at an angle out through the wall? I realise I am going to have to draw a sketch of this for anyone to understand, but just as a general question.

Oh and I have one more question on the maths of the bell sizing and the bell in general. Does my calculation of the total bell size, e.g. 5.3sqm for a 150mm riser, need to include the entire internal surface area of my chimney, since that certainly would be a heat absorption area of some kind too?

I wasn't quite clear on that from the website (https://batchrocket.eu/en/building#belltheory), where it says,

"A new round of problems emerged due to the chimney stack being made of bricks, which extracted heat from the exhaust thereby killing the draw. This needed more tinkering and rebuilding of the bell in order to raise the exhaust temperature and restore the draw. The final result was a figure of 5.3 m² (57 sq ft) of 'heat absorption area'.
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My typical 55gallon oil barrel, maybe for first bell, and a section of the clay(?) 645mm high chimney flue (x3 at 175mm diameter and x2 at 150mm diameter)
My typical 55gallon oil barrel, maybe for first bell, and a section of the clay(?) 645mm high chimney flue (x3 at 175mm diameter and x2 at 150mm diameter)
 
Paul Muthers
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Peter van den Berg wrote:

Paul Muthers wrote:A bell system sounds interesting, though I may end up not doing either and just going with the earlier suggestions of a nice radiant heating batch rocket, do you know what kind of size restrictions there are for the bell system, attached to, e.g. a 4inch batch rocket system? What would happen if the area was too big?


The heater will refuse to start up properly, that's all. Oversizing could also mean that the chimney temperature is too low. Which in turn is causing water condensation in the chimney. This looks like this: at first, the fire is ramping up but within appromately 20 minutes a chimney stall occurs and a tsunami of smoke is streaming into the living space. Keep in mind: burning of wood will produce about half of its weight of water. It's one of the results of the chemical process, the others (assuming complete combustion) are CO² and heat.

The numbers given on https://batchrocket.eu/en/building#dimension are pretty accurate. Of course you could oversize but in that case you'll need a chimney shortcut or bypass of some sort. When planning to build a batch box rocket yourself it's sensible to read the whole of the site very carefully.



So I am planning to build a batch box now and have read most of the site pretty thoroughly. There are a couple of questions that I hope you could answer for me:


1) Regarding the p-channel. I understand the measurements, but was wondering if the rectangular shape was absolutely necessary. Would a repurposed round pipe possible work well too? I have some old water pipes that seem of strong enough metal and have the length and curve I would need. Does the round shape make it less effective?

2) Regarding the heat riser. I understand the internal surface of the heat riser should be smooth, but would a few narrowing sections (possibly up to 5) every 20 cms, i.e. sudden bumps reducing the diameter from 150mm to 120mm, maybe help the mixing up of combustible gases and efficiency of the burn? I am considering using the isolite round barbecue grills in the attached pictures, stacking 5 or so on top of each other and cutting the bottom of them, in order to save material cost and re-use local rubbish. Do you think this is worth a try?

I believe I saw some designs with bricks slightly sticking into the heat riser to improve the gas circulation, but I can't find the links right now, which is why I am asking this question..

Thank you for any advice!

shichira-1.jpeg
Isolite mini barbecue identical to the ones I could repurpose
Isolite mini barbecue identical to the ones I could repurpose
shichira-2.jpeg
Isolite mini barbecue identical to the ones I could repurpose
Isolite mini barbecue identical to the ones I could repurpose
 
Peter van den Berg
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@Paul, there's a distinct difference between a p-channel and a floor channel. The p-channel blocks a little bit of the port at the top so no air can get around it. The floor channel's vertical part is far enough from the port to create half a port's width left and right. Regarding a round duct: using it for a p-channel the answer is no, for a floor channel it's a maybe.

The riser works best without restrictions, the whole of the design is very picky about friction in the smoke path, as a matter of fact. So, what you propose means 5(!) orifices in the riser's length. The odd chance that it would work looks very slim to me.
 
Paul Muthers
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Peter van den Berg wrote:@Paul, there's a distinct difference between a p-channel and a floor channel. The p-channel blocks a little bit of the port at the top so no air can get around it. The floor channel's vertical part is far enough from the port to create half a port's width left and right. Regarding a round duct: using it for a p-channel the answer is no, for a floor channel it's a maybe.

The riser works best without restrictions, the whole of the design is very picky about friction in the smoke path, as a matter of fact. So, what you propose means 5(!) orifices in the riser's length. The odd chance that it would work looks very slim to me.



Thank you for explainig the p-channel in more detail. I understand the shape of it now and will do more sourcing work to try and get a proper p-channel piece.

Regarding the riser, if I understand correctly, by orifice you mean the air inlet at the bottom? I was intending to either fill that opening or cut just above it so that the 5 (or more if necessary for the correct height) separate bbqs could sit snugly on top of each other. My question was regarding the narrowing of the internal diameter, as the bbq's diameter becomes narrower at the bottom (120mm) than at the top (150mm). I do gather from your answer that it probably would not be beneficial, so I will look into how easy the bbqs is to sand/file down and even out, before deciding whether to make a real attempt of the idea I had.

Just wondering, have you ever made or come across a riser built with diatomaceous earth (or even "Isolite") before? It seems to have a number of uses and health benefits, as well as being heat resistant and highly insulative, and since it is apparently quite abundant in this area, I will try to find a good way of integrating it in my batch rocket stove.
 
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Paul, as I understand the Batchbox workings, the air and fire mixing is done Specifically at the base of the riser in the form of the double vortex (rams horn effect) there after that the flow should be as unrestricted as possible based on the system size.
So any restriction of gas flow in the riser is unlikely to be of benefit, whether or not this has been experimented with I don't know ?
You have to bear in mind that temperatures in this area will be very high and of all the places in the whole stove, this will be the place to invest in suitable long lasting materials.
You can use fire brick or half fire brick backed with insulating material or you could do away with fire brick and use ceramic fibre products.
Perhaps you can search for kiln materials as most places in the world we have some sort of outlet selling kiln products……  
 
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