posted 10 years ago
I would suggest starting out by building a test rocket stove outside, if you haven't already. Get a barrel over it, and start taking temperature readings. I set my outdoor test stoves up on a sand bed, and partially buried them in sand, which facilitated sticking my temperature probes into the sand for readings at a number of points. This allows you to build up a temperature profile, as well as to become accustomed to the firing process. Once you have an idea of what temperatures you are dealing with, you are in a position to design a safe installation in your home.
In terms of protecting the floor from heat, I really don't think it is so much insulation, although that is important too. Heat protection is more about removing the heat. Air channels, in other words. One of the best references of which I am aware is the Wisner's plans for the Bonny 8" Convection Bench:
http://www.ernieanderica.info/shop
Obtaining and studying this set of plans, of an actual build that was used successfully for over a year, is a very good investment. (I believe the plans list for $35 USD.)
I'm still prototyping my indoor build, but I can share a few thoughts for your consideration. Currently I favor a double-channel system. My proposed build is a modified version of the one in the Wisner's Bonny plans, in that I added an air channel below the fire box. I did this because I was trying to reduce weight because my prototype is built above a basement, and on a wooden framed floor. The Bonny plans use a lot more brick and/or pavers than I felt comfortable using on my living room floor, and the room immediately below it is a finished basement room, and I really did not want to tear out the ceiling.
But my plans do vary in one really important point from what I understand your plans to be. My full RMH is going to be built on a cement porch this summer, after I enclose the porch. I will then tear out the wall separating the porch from my living room. Therefore, my living room build is only a temporary test, giving me supplemental heat this winter, while allowing me to take additional temperature measurements, and get used to "feeding my dragon."
I'll try to be brief.
Initially I laid out a "deck" on the floor using 2x6s, roughly 3-feet by 6-feet, upon which I laid out 2x4s (flat on their face, creating a 1.5-inch air gap), and upon that I laid out another deck built of 2x6s of the same dimensions. To the upper deck I screwed in 2x4s to provide a wooden "pan" which I lined with heavy aluminum foil and then filled with perlite-clay insulation (that's 3.5-inches of perlite-clay insulation - the height of a 2x4 standing on edge).
Upon this I built my firebox, and insulated with several inches of perlite-clay insulation. However, as I monitored the temperatures reaching the wooden deck, I saw temperatures nearing 150 F. And given my self-imposed temperature limit against wood is 170 F, I deemed this unsafe as a potential fire hazard. Now the odds are, it would be fine given it was only to be up for one winter, and I was building in December, so only on the order of three months of firing. But I was not sleeping well, so I tore it down to the original floor of the fire box, which is set on the 3.5 inches of perlite-clay on top of the wooden deck.
In the second build, I added an air gap immediately below the fire box, framed out with fire brick splits, and with several openings to the room.
I am measuring temperatures as high as 525 F in this air gap immediately below the throat between the feed tube and the burn chamber. However, temperatures near the bottom of the 3.5 inches of perlite-clay (near the wooden deck, or "pan" holding the perlite-clay) has not yet reached 100 F, although they are bumping right against 100 F. This is far below my self-imposed limit of 170 F, and somewhat below what Ernie Wisner recommends as safe against wood (125 F).
Were I building this from scratch, I would make a number of modifications...
1. Extend the air gap under the floor of the fire box (feed tube + burn chamber + fire riser) so that the long axis runs the entire length, from the front of the feed tube, all the way past the barrel and manifold area.
2. Add more openings to the room along the sides, and one at the rear outside the barrel/manifold. (My build has two openings along the sides, one on each side (narrow axis), both below where the feed tube transitions into the burn chamber, and one more opening below the front of the feed tube; the air gap that extends under the fire riser is a dead end, which is not optimal.)
3. For the floor of the fire box, use full fire brick (I used splits, to keep the weight lower).
The above modifications would allow for a better movement of air, and therefore vent more heat into the room before it has a chance to reach the perlite-clay. It would also delay the heat moving through the floor of the fire box until after the burn has been completed, if the floor of the fire box was made of full fire brick instead of splits.
Another consideration, if you can take the extra weight on the floor, is to make the profile of the air gap like a T, so that cooler air is drawn in lower, while the hot air moves more freely from the top of the T profile. To better facilitate this, the air gap would benefit from being closer to 2-inches thick, as opposed to the nominal 1-inch gap I have. (My gap is actually closer to 1.5 inches, because I used splits, which are 1.25-inches thick, and there is another 1/2-inch or so of perlite-clay between them and the floor of the fire box. I think forming the air gap height using two courses of splits would better facilitate air flow, in addition to making it much easier to add more openings in both courses, thus allowing a better flow of air through the air gap; it is the movement of air through the air gap that reduces the temperature gains penetrating throughout the perlite-clay insulation.)
So that's what I did. I find this provides what I consider to be safe maximum heat levels against my wood, yet is lighter in weight than what the Wisner's built in the Bonny system. They were able to carry more weight because they added a built-up 8x8 inch beam under the floor joists. I have posted pictures of this in one of the threads on this site, although I no longer recall which thread.
Another modification to consider would be to eliminate the upper wooden deck and replace it with metal, perhaps corrugated roofing panels. I did not wish to use cement board because I was concerned with its tendency to crack. I felt the shear pressure might result in failure under the weight of the build. One could also replace the 2x6s laying on the floor with 3/4 or 1-inch plywood, which would offer a larger "footing" over which to spread the total weight (32 sq.ft. instead of 18 sq.ft.).
Whatever you build, be conservative in your weight estimates of the bearing weight of the floor joists, and spread the weight of the build out sufficiently to stay well below whatever threshold you deem safe. I think mine is coming in right at 40 pounds per square foot. I would be more comfortable were it lower.
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