Rocket Mass Heater Cast Core Plans

[matt walker]

Well, I've been promising this one for a long time. There's just a ton of info I wanted to pass along, and I've struggled with how to get all of it across. Finally, I just gave up and did the best I could. This could be a really, really long conversation about possible options and whatnot, but I figured I should just get something up as a starting point. So, here it is, how to cast a core like my home RMH uses, with a little bit better tech that I've learned over the last year.

[georgesr]

Matt,
Really appreciate your sharing this information. I read your notes on the video concerning using my own clay. Your giving me much inspiration.
Thanks for sharing

[matt walker]

You're welcome George, I think that article you posted had some great info as well. I have never tried the sugar but I would suspect it goes a long way towards tying the whole mess together. Still, using your own clay with the furnace cement will make a very hard finished product. It's pretty amazing how far that stuff goes actually. I learned it by accident when doing the restaurant build in brooklyn. They bought that instead of fireclay, so I mixed a bunch with perlite for the riser, and when I was running out threw in the dirt/clay we had dug up on site. I couldn't believe how hard it was the next day. Since then I've put little handfuls of mixes in my burn chamber at home, both for patching and experiments. It works great. Kinda spendy, but a pretty good DIY solution since it's readily available.

[GrahamB]

After all the information banding about on the internet about RMHs, I was starting to get a little confused. This is like the video version of RMHs for Dummies, just what I needed. Thanks Matt for doing this, and I'm waiting for lesson 2 with eagerness.

[Nutcase]

Thanks for another very illuminating and useful video. You have a knack for keying in on the crucial issues for getting to a practical understanding these things. For me, one of these crucial issues is the influence of firebox insulation. Another is the importance of "good enough" in an iterative development process.
Anyway, I have a few questions.

Is their any advantage of square over round cross section? If round is okay, couldn't you use a cardboard tube like one of those concrete forms for setting posts?
Is there any practical reason why you would ever have to take the outer plywood off? Even if it were buried in cob?
If you are building this thing in a sturdy box,why does all of the fill have to be solid? You could, for instance, build it with bags of vermiculite slathered with mortar. Well, maybe not that exactly. But you probably get my drift here. Less weight, less drying, less sturdy but could be good enough.

[Lollykoko]

Nutcase, I recently saw a video on YouTube that was showing how to make rocket stoves for cooking. The video was made in a third world country (don't remember which) and the application was cooking only. Anyway, they used sections of PVC pipe to create the burn chambers, then removed the pipe before use.

[Nutcase]

I recall seeing the same or a similar video. Of course just about anything will work if you can remove it after casting...
I'm still mulling about what I will do with my first build, weighing cost, ease of modification, etc. Right now I'm thinking that if I cast something like this, I'd use an 8" cardboard tube for the burn tunnel and riser base, attach a rectangular feed tube, extend the burn tunnel tube back further and cast or cut a removable plug, making a very easy cleanout at a minimum. Unless I learn some reason why that won't work.

My other idea is to just start with rockwool (Roxul) planks in some sort of minimalistic frame. In the long run some parts would have to be mechanically reinforced, channel walls made less permeable and certain areas made with material that can take a higher temp. I also have some concern about the binder in that stuff cooking off, so the minimalistic design probably wouldn't be the end of it. Still, offhand I don't see why you couldn't make a functional rocket very quickly out of that stuff. Unfortunately, I can't just go down to the local building supply place and find the material. Will have to order and wait a bit. Unless anybody out there can tell me why it won't work.

[matt walker]

Glad to hear the video got you thinking, that's really what it's all about. I struggled with making this one, because as you illustrate, there's a million ways to do it, and most of 'em will probably work. I'll address some of your specific questions, but first a comment on your last comment, regarding material sourcing and experimenting. On the last page of the half barrel thread GeorgeSr posted this link to using found clay to make rocket stove cores:

http://www.rechoroket.com/Home_files/EnglishBook_1.pdf

I would highly encourage you to dig up some material from outside and experiment with that in a few configurations before purchasing exotic materials. You'll be surprised what you can do, even if you just dig up some mud from the yard. Don't agonize over the composition, just build a couple mud burn chambers and get a feel for the materials you have, and the way these things burn.

Okay, as for square vs. round, I couldn't tell you definitively, but I'd imagine not too much difference.

The plywood does need to come off, at some point once the whole thing is dry it will start getting hot there over time, not to mention the cob does an excellent job of binding the core together and sealing any problem areas. I believe it moves with the liner and is a part of a composite system. I wouldn't expect this core to hold up too very long without the outer cob shell. I could be wrong about that, I haven't tried it.

As for the solid/light weight issue, I think the inherent problem is trying to get anything that will hold the insulation there if the whole thing isn't solid. Any thin structure for the interior is going to need to go through heavy heat cycling and unless you engineer it just right it's going to eventually fail. This is just a simple way to hold a fairly large amount of insulation around the burn without getting too hung up on the system. It's easy to spin off the deep end and theorize about all sorts of exotic materials, believe me, I do it all the time. This way works fairly well for not too much cash or messing around. For sure there's other ways to do it, but this one really has no major downside that I've found. It is soft, but once you go through a couple patching sessions on the feed as use and wear dictate, it eventually stabilizes. I use a higher ration of fire cement in the patching and it creates a very hard shell where needed.

Lastly, the cardboard might work just fine. However, be aware that the clay holds a lot of moisture for a long time. You need to be able to burn the moisture out, and I would worry that the cardboard would be saturated and soft before the clay could support itself. It would probably work fine, but the hardibacker is really cheap and easy to work with. The whole board, of which I used about half for the interior mold, and another piece for the bottom to make moving the core easier, cost $10.

Good questions and good discussion, I'd love to see you build stuff and add to the knowledge base. I certainly don't think this is the very best way to do it, I just got so many requests for how I built my home system I wanted to share something. If you've read my build thread on the home system you'll know I made plenty of mistakes. I'd love to see a system come out of all this that was affordable, high performance, repeatable, and simple.

[Nutcase]

Thanks for your thoughtful response.

I ordered some 3 1/2 " roxul batts — under $40 for a bundle of 8 2'x4' batts — and some furnace cement. Roxul rockwool isn't something as exotic - i.e. expensive - like the higher temp inswool that I see in YouTube vids being used for small furnaces.
I will do an absolutely minimal build and see what happens. Very cheap and fast, so why not? One feature of roxul of unknown consequence is that it resists wetting. so maybe an issue with stabilizing a surface with cement or whatever.

Thanks for the ongoing prodding and encouragement about backyard clay. Will do, eventually, for the thermal mass if nothing else. Got lots of it, for sure.

The tube is comparable to the backer board , costwise. I think it will hold long enough, but we'll see, eventually.

The part of the plywood box that is under the barrel is of course going to see high temps from both sides. Some of the heat flux will be temporary maybe as the water is being driven out. might be a worthwhile destruct test at some point...

[georgesr]

Here's a couple short video's of the "first firing of the rocket mass heater.


I will be installing the riser insulation after things cool down. At the same time I'll patch some of the cracks.
Thanks Matt for your guidance.
George Sr