My Rocket Mass Heater Project – Casting Core / Propane Tank

[NateH]

I don't normally post to forums, but I really wanted to share and get some opinions.

So here is the project so far.

I decided to try to cast my core, so I used 3/4 plywood to create a box.



I used pocket holes so secure sides to bottom.





Then put a couple in the ends as well.

[NateH]

Before assembling I coated all sides that would be exposed to the cast with tungsten oil so the cast wouldn't soak into the plywood. (Hoping)



Everything turned out nice and square up to this point.



I proceeded to create the feed / burn tunnel mold.







Once completed I noticed my math was off a tit in a place or two. No more than an 1/8th anywhere, so I just went with it.

[NateH]

I am casting a little different then I have seen anyone else do, I hope it works.

I am using insulated brick encased in insul-stick (insulating refractory cement). I hope to create a strong super insulated core.

To start I poured a 1/4" of the insul stick in the bottom of the box/mold.



After it started to set I laid the first layer of bricks in place.



Poured to cover bricks and have 1/4"above them, adding bricks around feed / riser areas.



Poured over that layer and added the final insulated brick to the correct levels, the bricks over the burn tunnel run the complete width (sorry no picture of putting them in.



The final layer I made pretty dry so I could just mold it with my hands.

[NateH]

So it is still drying and getting REALLY Hard, I am pretty happy with how it is turning out.

I do worry about how fragile it is and I have a couple questions.

Is there a product that I could put really thin over the entire core that would help with strength, or should I not worry and try to incorporate the strength with the rest of the build.

I am thinking of covering the walls in the stove corner with cement board, then building a wood box to encase stove and ductwork.

Here is the corner I want to put it in, how would you do it?

[mannytheseacow]

Looking great there, Nate! Really good, man, that's awesome!!!

I like those pocket joints... gonna get one of those tools myself one of these days....

I'm not sure about anything thin you can put around it for strength. You might want to just use the rest of the build for that.

I'd worry about that chipboard wall. Covering it is a good idea, but I'd still leave an air gap between that and your heat. I'd love to see a wood box holding everything together on one of these builds but you might worry about heat on that, too. Especially with an 8" system. Oddmar used wood and it seems to work great for him- so check him out for some tips. I love wood, though... could be really gorgeous. Earth is non-flammable though and more forgiving!

[NateH]

Thanks for the reply Manny. This is a workshop that sits about two feet away from my house.
So safety is top priority. Looks are not as important to me.
My end goal is to pump the hot air into my house and offest out high electric bills as much as possible. If the workshop gets too hot while working out there, I flick on the duct fan that would push heat into my house.

I guess I never gave dimensions on anything really. This is a burn tunnel of around 5" to 5.25", and I will be using 6" ductwork. The workshop is only around 300sf. So I can see needing to bring the temperature down while burning.

Has anyone tried using high heat paint on wood? I wonder if that works? Not that I would put flame to it or anything. Or what if I did the cement board right on top of osb (chipboard) Then paint that with high heat paint. Some paints are rated over 1000 degrees. Just not, sure how it would adhere to it.
I want to be safe, but be compact, as space is limited. Not sure how I'm going to do this.

[mannytheseacow]

You've got a neat idea there, pumping the heat into the house. You could probably run a small fan on a tiny little solar panel and small battery and have that thing off the grid.

I wondered about your dimensions because your form looks very skinny. Are you sure you have enough insulation around your burn chamber? I used 4" of insulating refractory around my 6" system (though I tend to overkill everything).

You will definitely have plenty of heat in a 300ft. area, assuming you've got some good mass. Don't depend on too much radiant heat from the barrel- I'm surprised by how little my barrel gives off compared to the bench. I don't have any experience with heat-stop paint so I can't comment on that but from the little experience that I do have an air gap will give significantly more insulating value than anything else. Even if all you can do is an inch.