Rocket Mass Heater Cast Core Plans

[matt walker]

Hi CJ, I can make threads sticky, perhaps this is a good one for that. It's a good suggestion, thank you.

I have made many cast cores with windows at this point. I played with quite a few methods, and as always the simplest solution has turned out the best for me. I cast the core with an additional piece to make the window void stuck on the side of the mold. Best location in my opinion is a spot that shows the bottom of the fuel, barely, and then has a view into the burn tunnel, on the side of the core.

The window is just captured against the core with the outer cob layer, with some fiberglass rope as an expansion joint around the edge of the glass on both sides. This has worked very well for me. If you need more detail, we could start a new thread and I'd be happy to elaborate on the process a bit for you.

P.S. I'm close to solving the muck issue!

[CJ in VT]

[matt walker]

I gave that specific core to a friend who lives about an hour away, and he hasn't built with it yet after a year and change. I keep meaning to go get it and continue the video series but I haven't done it yet. That said, this is the exact technique I used on my home system, although I added the front window using a similar method to what I describe above, although a little more complex since I was figuring it out at the time. So, yeah, here's a pic of what is possible using this technique. I'm three years in on using it as my sole heat source and it is holding up just great, although it does take regular maintenance to keep the feed area from getting to worn out looking.

[Rhett]

What pigment did you use to color that cob?

[matt walker]

Concrete colorant, which I think is various oxides. The pink was Medium Red 1117, which is my favorite for concrete but kinda turned pink in the cob. The other color was a yellow, I don't know the part number. They are really dependent on your base clay color, so play with them a bit to find what you like.

[hpmer]

Matt,

In the traditional rmh brick construction, the first brick often cracks and must be replaced occasionally due to thermal shock. Have you experienced any similar degradation of that area using your mix? I've often thought it might be due to the bricks and the insulation above them having very different thermal properties and therefore causing problems.

I'm in the process of building a bbq stove and plan to cast it, so thought I'd ask if there are any issues and anything you'd do different in that area before I begin.

[matt walker]

I've never made a core of any type that didn't crack somewhere, but I would say that my mix doesn't fail in the way that bricks do. Once the bricks crack, they typically are loose at that point. My mix tends to just sort of "settle" in the sense that it cracks during drying and then doesn't really break down after that, with the exception of wood abuse in the feed. Be aware, it is a really soft mix, so the feed will wear and need patching. But no, no real dramatic failures over time in my experience.

[freedomlives]

I haven't progressed much on mine lately-- other renovations have taken precedence. However, another person in a city near here is interested in making a RMH so I wanted to send him your video. I am glad someone posted recently to this thread so it was easy to find, but why not make this thread a "sticky" so it stays at the top.

[matt walker]

Good idea Andrew, done.

[mannytheseacow]

What did you use for an outer form on your riser? 16"?