Rocket Mass Heater Cast Core Plans

[mannytheseacow]

Okay, I think I got them to work!

[matt walker]

Yeah man! That looks awesome! Really nice work, I love the way you incorporated the bricks around the feed. Great idea.

[Nutcase]

Looks to me like that's chipboard around the feed. Of course it's basically symmetrical, so you could move the chimney.

[mannytheseacow]

Well I've got about 18 hours of burning on this thing now and the temps have dropped significantly in Northern Illinois. After the first day of firing it, I woke up the next day to 2" of new snow on the ground. Even covered with a tarp the clay was still kind of mucky on the top. I gave it another 6 hours of burning or so and let the fire go out and moved it inside. It's sitting next to my wood stove now just drying out. I'm really glad I moved it inside, because it was only 6* F out this morning.... brrr. Damn I've gone through a lot of wood this winter.

The OSB fully burned away in the first 45 minutes or so. Here's the feed tube as it looks now:

You can see the brick protection showing a little around the sides, and if you look close you can even see some of the ash pit at the bottom of the feed tube. Not sure how effective this actually is/will be, but it seemed like a good idea to try at the time.

Unfortunately I probably won't do too much more on this project until I shut down my other stove for the season. Looking at the forecast that won't be for a couple of weeks yet, at the minimum. I might try casting the burn tube in the meantime. I found a 12" piece of stainless chimney pipe from an old fireplace to go around the 6" stove pipe, then I'll fill that with the same mixture used in the core. This should be plenty of insulation, plus be a good size to use with my 55 gallon barrel to go on the outside.

[pa_friendly_guy]

Great job Manny, it looks like you have made a great unit there. I can understand waiting to actually install the unit until your heating season is over. It could get pretty cold in there with out heat for a few days while you install the new RMH. Plus mixing the cobb in this cold weather could be a problem. Thanks for posting the pictures, they really help understand what you are doing. Keep up the good work.

[Nutcase]

Ah. Now I see, the brick was buried. Duh.

I still may end up casting something like this, but I still think I'd go with a 8" sonotube for the burn channel and run it to at least one end of the box for an axial cleanout or whatever. Works if one is doing an 8" system, 6" cardboard tubes are harder to come by.

Contemplating your situation, it occurs to me that I could basically take the system I have, swap out the wire mesh barrel stand for a half-barrel and a short length of 8" something or other, and vent it into my existing woodstove. I could do that tomorrow, and undo it, too. Hmmm...I wonder if my wife will go for it...There are a few loose ends to sort out...

[mannytheseacow]

We've had a few rainy days here that have let me finish up some indoor projects and get back to working on my RMH. Istarted doing the mock-up and moved my core into place. It's been sitting indoors after about ~30 hours of firing outside. The end of winter and beginning of spring it sat next to my potbelly to keep it dry.

I went to pull the plywood mold off of the outside and noticed that it is extremely fragile. A little breeze and this thing will totally fall apart. I'm not sure what I did wrong here, I followed the instructions for the mix exactly, aside from incorporating the brick into the entrance of the feed tube. Anyway, I think this core is going to get scrapped and maybe go back to the firebrick method of making the core. Any thoughts out there?




I guess it's better that I waited a bit and let it fall apart than building it right away and having it fall apart when in use.

[matt walker]

Manny, that looks about right, I don't think you did anything wrong. They are really, really fragile. This method makes a core which is intended to be wrapped in cob, and serve as an insulated core inside a cob structure. Used like that, they work great. But yeah, as a stand alone block, they won't last long. It's a bit like an arch in that if it's held from the outside and can't move outwards it will stay together indefinitely, but as a block it will just fall apart.

[matt walker]

It does look like the clay shrunk while the bricks did not, forcing it to have it's largest separation there at the feed. I still think you could use it if you wrapped it up, and put a few patches inside at the worst cracks. It's going to be hard to get into place at this point though, for sure.

[CJ in VT]

Hey Matt, great thread. I'm not sure if you've got a "sticky thread" option if you know what I mean but it'd be great to have this sub-thread right in the beginning of this thread. I had a bit of a hard time finding it.

Have you made a castable core with a window?