Rocket Mass Heater for my home

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Rocket Mass Heater for my home

Postby matt walker » Mon Jan 23, 2012 2:40 am

Well, I've been planning the rocket mass heater for my home for a while now. I have spent the last few months reading, planning, getting materials, in anticipation of this project. Today, I started physically working on it for the first time. I'm excited, so I thought I'd start this thread as a place holder, and I'll get pictures of the steps up here as I go.

Today I built a two part mold for the combustion unit, as I'm calling it, which consists of the feed tube, burn tunnel, and base for the heat riser. I will be molding those components out of a clay slip with mixed perlite to make an insulative castable refractory. I might line the impact areas around the feed tube with some fire bricks so when I'm banging logs around in there I don't chip away at the refractory.

The mold is made of 3/16 masonite I had laying around, and consists of a piece the shape of the combustion tunnel. That is, a "U" shape, with short horns. Then the outside mold, which is just a box that holds the U with 4" clearance all the way around. I just cut all the pieces out with my hand saw and glued 'em together with hot glue. We'll see how that works. I'll get some pictures up on pour day. Maybe tomorrow!?

I may do the heat riser the same way, I'll see how this goes. Anyway, I'm kinda jumping the gun, but I'm excited!
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Re: Rocket Mass Heater for my home

Postby Lollykoko » Mon Jan 23, 2012 4:55 pm

Waiting with bated breath! Where did you get the perlite? Garden store?
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Re: Rocket Mass Heater for my home

Postby matt walker » Tue Jan 24, 2012 4:56 am

I actually found it at the local, um, I don't know what you call it. Mason supply? With landscape materials, and, well, all kinds of rock products. They had fire clay as well, and it's pretty affordable for small projects like this.

Anyway, someone had ordered two 21.5 cu. ft. bags, and never picked them up. I bought 'em both. If you have a place like that around, they could surely order some. I have heard that there are different grades, and my bags are marked "refractory" or "mason" or something like that. I read somewhere that the horticultural grade is not as suitable, maybe a finer size, I'm not sure.
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Re: Rocket Mass Heater for my home

Postby matt walker » Wed Jan 25, 2012 2:45 am

Okay, well, I got the combustion unit poured today. I'm not sure how it's going to work out, but it's in the mold and drying.

A couple corrections from above, the perlite was in 4 cu. ft. bags. Not sure what I was thinking there! The masonite is 3/8", not 3/16".

So, for a while it was kind of a disaster today. I learned a whole lot of things not to do next time! First and foremost, my mold completely distorted and blew apart towards the end. I should have known, and reinforced it, but looking at those giant bags of perlite, I thought it was going to be a very light load in the mold. It turned out to be quite heavy. I'm not sure about my mix, perhaps it was too wet or has too much clay, but I suppose I'll find out when I fire it if it cracks or not.

At any rate, I was able to empty the mold and reinforce it and refill it. I used the better part of two 50lb bags of fire clay, and all 8 cu. ft. of perlite, minus a tiny bit I used in the heat riser of my first stove.

It was quite a process, and to be sure much more work than the first one I made where I stacked bricks and packed clay around them. I'm hoping the effect of all the insulation around the burn chamber will be enough of an improvement to make it worth the effort.

So, some pics...

The combustion chamber mold.....

Image



I am going to attempt a window so I can monitor the firebox when I have a lid over the combustion chamber. Here I've glued a 4" section of flue to make the mold for the viewing tunnel.

Image




I used mold release, which is just high grade carnuba wax, on the outside mold. It's perched here on the inside mold, but I didn't apply it to that, as that will just burn out.

Image


A full mold. You can see it distorting here, and see my emergency clamps and lines around it, trying to keep it together. This is after I fixed it, it was a total blowout prior to this. I can live with that little bow in the unit, but next time I'd put a frame of 2x4's around the upper and lower mold, or use heavier plywood for the outer mold.

Image



More pics here...

http://photobucket.com/Rocket_Mass_Heater
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Re: Rocket Mass Heater for my home

Postby eeldip » Wed Jan 25, 2012 3:01 am

masonite! if only 1/2" ply was the thing laying around....
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Re: Rocket Mass Heater for my home

Postby matt walker » Wed Jan 25, 2012 3:28 am

I know, I actually have all sorts of plywood laying around! DOH.

Anyway, here's the location where the stove will be. As you can see, most of the mass is already there.


Image


I'm planning on building the outer wall of the bench with bricks, and in filling with clay and rocks around the flue. Then I intend to cap the top of the bench with the same concrete tiles I'm building in my counter top thread. Here's a quick sketch of the general plan.

Image

And the mold concept..

Image
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Re: Rocket Mass Heater for my home

Postby Lollykoko » Wed Jan 25, 2012 5:41 am

We do have a concrete and masonry supply place in town. I'll have to check that out soon. I'm slowly collecting the necessary items to build my first rocket stove.
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Re: Rocket Mass Heater for my home

Postby Lollykoko » Wed Jan 25, 2012 5:45 am

I meant to say, your sketches are very understandable. I hope your casting turns out well. We will learn from your experience and reenforce any molds created. :lol:
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Re: Rocket Mass Heater for my home

Postby dave brenneman » Wed Jan 25, 2012 9:20 pm

Isn't the cob oven model a wet-sand form? would the masonite have held up better if it was filled with a removable substance like that? I know it sure doesn't hold up after getting wet...
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Re: Rocket Mass Heater for my home

Postby matt walker » Wed Jan 25, 2012 10:09 pm

Yes, the oven typically use a sand form, but in this case the inside mold was fine. It was the outside mold which was straining with the load inside. It would have been really simple to just reinforce it before I filled it with 200lbs of sludge. I used the masonite because I've done a lot of composite molding, and masonite works well due to its smooth surface. It releases easily. I'm thinking that isn't a necessary function with the refractory, but it's what I know. Er, knew. Now I know better!
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