Thursday, November 5, 2020

Discussion of Mini Rocket Stove Mass Heaters: Kang Bed-Stove as Bell Bench heater (Matt Walker)

 Chewfacity

I believe metal as the core is no longer recommended, as it breaks down quickly because of how hot the stove gets.
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that's what I was thinking. So instead they use MORE firebrick I think - to make a tower. Kind of a pain. 
 
Metal, especially heavy metal, has been discouraged as the inside layer of the heat riser for many years now. A 4" system will not get as intensely hot as a larger system, so the metal will last a lot longer here. Heavy metal is still wasted in this system. Thin firebrick (called "splits") with insulation around them are the most durable material for a heat riser though not the most efficient. The most efficient and easiest riser to build is called a "five minute riser" (google it for details), and is made from ceramic fiber insulation rated for over 2300 degrees F. Not cheap but very good. 
 
 
@gjh42 yeah I was catching that ceramic fiber board heat riser and so now I read the concern about fibers burnt off. I have a Silverfire rocket stove - the Survivor - that is awesome for quick concentrated cooking heat from twigs. But as far as the claim for the Mass Rocket heaters being so great - it definitely seems like the designs are still being improved upon a lot. I have a Two Dog with a baffle and just surround it with bricks and then a four gallon pot of water on top - and then the stove pipe goes off at 30 degrees. So by the time it leaves the side wall - it's cooled down a lot. But outside the chimney is rock wool insulated. So I got red glow and blue flames last time I used it - the stove glowed some red. I burn it fully open. So I'm gonna turn it down once it heats up - since the chimney should pull good. But I'm just not sure it's that much worse than a rocket mass heater. I don't get as complete of a burn since it's not insulated but I get probably more immediate heat off the stove. I don't know. Seems like a lot of work to build these things. I've never done masonry - I don't even like the whole "level" and "squaring" and "plumbing." I do like throwing cob against walls. I made a double wattle wall hut with clay slip hay insulation.
Rocket mass heaters are a different animal from rocket stoves. One is made for a superhot fire that then heats a mass, the other is for an efficient cooking fire and has no need for mass. RMH designs are being incrementally improved, though many of the improvements are more expensive, or trickier to build, or sometimes more delicate. The basic design that has been stable for 5-10 years is easy to build, efficient, and durable. You can actually build a J-tube RMH entirely out of cob - I have done it a few times and they work quite well and would be reasonably durable, especially considering the expense.
@gjh42 good to know - and the "manifold" as well out of cob? Like a Chinese Kang bed stove - only cob and bricks. haha. I have a SilverFire Hunter stove. So it has a rocket stove inside it but it can hold a big pot of water for mass heating. So I'm just gonna use that in addition to the wood stove with the baffle and water on top and bricks around it and 30 degree stove pipe. So I can't remember if Silverfire uses ceramic blanket for their riser? He switched materials I remember. Yeah I only have a 9 x 7 internal space to heat - so don't really need to do a lot of masonry work plus I need immediate heat a lot for the very cold winters in northern Minnesota.
Is this for a living space or a shop space? If you only occupy it imntermittently and don't need it to stay warm when you are away, a lightweight rocket stove with some bricks and water tanks could work fine. If you want it to stay warm overnight, you want a rocket mass heater. They can be made very small - one I know of was only about 2' square and 5' high, with a 4" chimney pipe, was safe to touch all around except the firebox door, and would keep a drafty uninsulated cabin (maybe 8' x 12'?) warm overnight with no fire. They only replaced it because it gave no instant heat. A steel plate built into a sidewall would have fixed that.
@gjh42 I built a Bronze Age double wattle wall clay slip hay insulated hut. It is drafty but it's also insulated. haha. So it radiates heat quite a bit. I did an upload on my channel on the hut. So yeah then I have a Wiggy's Antarctic sleeping bag for over night. So I do get quite a bit of heat from the baffle with bricks and 4 gallon water pot - and 30 degree stove pipe that is 6 inches. Actually my ceiling got thermal shock - I had a couple clips holding the poly canvas tarp - the metal in the clip expanded to fast and broke the plastic. So two grip clips suddenly busted off. haha. So the ceiling is breathable though with rock wool insulation. Two layers of rock wool.
So something that holds heat overnight would be useful for you. Here is the RMH I was talking about - thread is full of detailed information. It is built from brick, but could be done in similar shape with cob. https://permies.com/t/71576/tiny-house-rocket-mass-heater Here is a rocket stove I built from cob; it could be surrounded with a cob bell instead of a cookplate and work in your space. https://permies.com/t/52509/Clay-Rocket-core-Bell-RMH#428887
@gjh42 thanks for the link. The video on youtube of that "cyclone" heater does not even have the LINK to that thread (which has much better images than the video). haha. Even still - I don't have a floor. I am on the uneven ground. I have some bricks but they are free and not clean. And I probably don't have enough bricks. Also I do know that the heat riser shown in that thread did NOT hold up - it fell apart from too much temperature. And again such a mass heater would not give enough IMMEDIATE heat for my needs in very cold northern Minnesota. So that means I have several reasons to not build that heater despite otherwise thinking it is awesome. When I first looked that that Cyclone heater my main resignation was the "P channel" as Kirk states: "That metal secondary air channel is the only part of the stove that can not be easily replaced with some kind of neo-lithic mud mixture.. I'm working on an idea, but am not there yet." So when I asked him about that - he claimed you would only lose maybe 10% of the stove efficiency. I don't know. He said it would be fine without the P Channel. Ok so the last post in that thread says the heat riser was not tall enough so they got back smoke. OK - I find all this FASCINATING on a theoretical level. But that is my approach to civilization in general. I study stuff on youtube because I would never want to do it in real life. haha. As I said - I do like throwing cob against walls - which is what i did when I made my Bronze Age double wattle wall hut. I'll look at your second link now. Ok again I really appreciate you sharing this with me. You said you get 400 F. out of the top? So my wood stove gets over 700 since the 3/32 steel glows red. I do surround it with bricks and the water on top of the baffle. So here you have the clay absorbing the heat. I wonder how you would build a "cob bell" over that? It sounds interesting but I really have no idea. I was trying to visualize building some kind of cob covering for my Silver Fire Rocket stove - in other words just incorporate the ceramic fiber riser and stove into a cob "bell" instead of using the top cook "plate" as you state. But to be honest I have no idea how to build this. I tried it first but I did a horizontal pipe like a mass heater "bed" to the side. But obviously that did not work - since it was not air tight - I didn't have a "manifold" - and I didn't have enough pressure drive in temperature difference. So a "cob bell" over it would be ideal. I tried this with my "Hunter" batch stove also - I embedded the vertical chimney into clay. But then I messed it up by having a horizontal chimney extension across my tipi. haha. So the draft smoked me out in back smoke. I suppose it could be like the one I saw - on youtube. Have you seen - he has a baffle attached under the top of the heat riser and then he just surrounds the base with cob for the heat retention. And then the chimney stove pipe is out the top. So he is solely relying on the baffle and cob around the base. This is pretty much what I do already - only he has the riser for hotter temp - or does he? You say you only get 400 F? So I think he records about the same temps that I am seeing in the wood stove. And then again we are back to the problem of lack of IMMEDIATE heat... https://www.youtube.com/watch?v=n2yyaTi_7Vs&ab_channel=VanPowell So this person seems to live down south? Or definitely in a warmer weather area than northern Minnesota. haha. So I'm not ruling out this idea - it's just that as I have continued to study it then I notice that there continues to be "corrections" to the designs, etc.
 
 
I haven't seen the video you describe. I would say, as far as core durability goes, I think a cob core would do alright. I have built pottery kilns from cob which get to 1800-2000F for several hours at a time, and they only get harder and stronger as they are fired. They have a much larger and more intense fire than any rocket core. I think a steel plate (even a barrel lid) on the top would take the thermal shock of direct and sudden flame better than cob, presuming that you wouldn't want a spacious bell volume at the top to dilute the impact. The steel would give the instant heat while the sides of the bell enclosure made of cob would give the thermal mass. You would have the chimney pipe leave from low on the side of the bell so the hot gases stratify at the top and only the cooled gases at the bottom leave for the chimney. You might be able to simply build a cob enclosure around your existing rocket stove core, leaving the feed exposed, and get full mass storage advantages with little extra work. I don't know how durable your stove would be when enclosed to hold the hot exhaust around the core. The riser in my cob rocket stove is ordinary solid cob, without significant insulating components. It would burn hotter if I had mixed perlite into it, and especially if I had wrapped it in actual insulation like fiberglass or rockwool.
@gjh42 So "donkey" retrofitting this bench into a Bell. https://www.youtube.com/watch?v=jN3v3N4QcE8&ab_channel=SunDogBuilder Really this is the same as a Kang bed stove of the Chinese - only using a rocket stove. So if you look at a Kang bed stove - the wok is on top - so you get your instant heat for cooking. The fire is feed below and an ash clean out below that. So no worries about ash build up in the heat riser. So it seems like you could build your cob rocket stove with some perlite - and so basically just like a Kang Bed Stove only the stove is set up higher like you have it, as a heat riser. So the combustion is cleaner and hotter.
 
 Voidisyinyang Voidisyinyang
https://www.jameshardie.com/products/hardiebacker-cement-board This HardieBacker Stuff. New to me. I like the Bell Bench since it's the closest to the Chinese Kang Bed stove. The only difference between the Rocket. So since you have to replace the metal riser in the Cyclone - and this one is metal. So instead just use ceramic fiber with aluminum foil around it for the heat riser. The only thing is to find a barrel to cover the heat riser. Otherwise this is the EASIEST - no "masonry" like with the Cyclone.
To be fair, the Cyclone was specifically intended to have a small footprint. Also, the cyclone riser wasn't metal exactly. I used perlite/clay inside of an outer metal sleeve. The clay at Paul's place is NOT good at standing up to very high temperatures.
@SunDogBuilder So someone could buy Matt's ceramic fiber heat riser-J tube and then surround it with a cob "bell" - and even a bed/bench. And embed a steel cook top into the top of the bell. Sort of a Kang bed-stove rocket stove mass heater? thanks for your reply.
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@Voidisyinyang Voidisyinyang Totally! That would be a fine collection of choices!

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