Paul Muthers

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.

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!

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'.

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).

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?

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.

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.

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 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!

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.