Bricks for the box of my cob bench

Bacon Lee wrote:
Erik: When I did the wall of brick around the rocket heater, I have that wall 1/8 away from the wall. Because I was thinking when I have to break down the rocket heater and sell the house, the house is ok on it's own. Now, should I slide a metal sheet into that slot to protect the wall? Would that be good?

Hmmm. I'm thinking I'd *not* do that. Here's my thinking.... stagnant air is a pretty good insulator. And for a heat shield you need at least 7/8-inch of air space behind the shield (and a gap at the top and bottom so air freely moves, carrying the heat away). So 1/8" is not enough to really let the air move, and it would even less because the metal would take up some of that space. So I don't think that the heat shielding will work well between the brick and wall with such a tiny gap.

Now if the barrel is within that 4- or 5-foot radius (or whatever your code dictates) then I'd go ahead and shield the wall above the brick you are talking about, leaving a couple inches of gap at top and bottom so the air moves freely behind the metal shield (or whatever you use). But that is a different question.

For the bricks that are to contain the bench, I think I'd just make sure that it doesn't get too hot near the wall. You control that by the thickness of the cob and the percentage of insulation mixed in the cob. Just like you do for the bench in typical builds.

There is another thought. You might add some air channels. I showed some links to these before.

And of course, it is worth thinking about how hard it is to move those bricks. If you move them so there is a 4-inch air gap between the bench and the wall, then you are within ASTM guidelines (as I recall the standard anyway).

Which you choose to do is dependent upon the work you feel like doing and what you feel is most safe for your home. What is going to let you sleep well at night without worrying about a fire? That is what I'd suggest as the best guideline (assuming it is an informed decision, meaning you have read the codes and have an understanding of what the relevant variables are).

My goodness, what a thorough thread! Everyone is being very helpful, and yet I can see why Bacon is frustrated after 2 tries to find a critical problem still undiscovered.

We do have the thermal mass of our Cabin 8" heater right up against a wall. It is an extra-thick fire-rated drywall, and even so, I think the code guys would be more comfortable to see a 4" air gap there.
The barrel itself is 18" from this wall, with 8" of masonry as a "mantlepiece" behind it. The part of the stonework behind the upper barrel (the hottest part) is hollow.
You may also notice that there is some wood trim on the bench, but the "matching" part just above and behind the barrel is a warm-colored tile.

The closest I would put a barrel to a conventional wall would be about 12", and that's if there is a 4" thick brick wall with a 4" air gap on each side (halfway between barrel and exposed combustible wall). I usually go for 18" with a heat shield, or 36" (3') without, that's about a meter with no heat shield, half a meter with. (I noticed some confusion earlier - we use " for inches, ' for feet, and about 39" = 3'3" = 1 meter.)
Ernie gets tired of hearing me talk about up-to-code clearances - he thinks people should be able to figure out these things for themselves.
But Ernie also thinks that it's a simple job to just replace that part of the wall with something non-combustible. While in some cases you can do that, it's definitely not the easy way, especially if you want to leave your house intact.

The ultimate question is: while you are running the heater at full blast, is the combustible wall cool enough to touch? This includes the paper drywall surface; drywall is not considered non-combustible (because the paper and wood are what holds it in place). If it is too hot to touch (over about 150 F / 65 C) then it is dangerous.

Bacon, you are doing very well. Your first try was already better than a lot of what we have seen here on the forums.
The difference between your first try and the second (third if you count the two tries on the heat riser) is wonderful; you are picking up the design and build and making it happen.

I agree that Ianto's bragging about low cost is misleading. If you stop to pick up bricks and scrap metal on every grocery trip, and don't count the gas, and keep a big pile of these materials in your back lot until the next stove project, then you don't spend much cash. But you also have to wait a long time to build anything, and you have problems like a 2-story cob house whose stove STILL doesn't have a chimney after 15 years, because you are not interested in spending the money to install one properly. Bringing us down to CA will not be cheap either, unless you happen to be near enough to kidnap us from San Diego around the Permaculture Voices conference time. But I don't see much need for rocket mass heaters in that environment, so I'm guessing you are more up in the central or northern part of CA, or in the mountains.

Let's say a few things you do not have to worry about, in my opinion.

1) The bench fill is going to be easy.
You don't need to worry whether you have the right clay, silt, sand, or whatever for your bench. The bricks provide the structure, so you don't need perfect cob materials.
The firebox area will need good cob if you want to build a hand-sealed manifold, and you are already doing great with the clay-perlite.
The rest can be any non-living mineral soil:
- avoid organic matter: roots, worms, dark fluffy humus - dig down deeper and most places have a paler sub-soil that might be chalky, colorful clay, or an unremarkable greyish blend without many signs of life.
- don't use pure sand, because of the trapped-air insulation thing.
- And you may want a covering on top to keep the neighborhood cats out.
If you are enjoying making cob, and want to practice making dense clay-sand infill cob, you may get slightly more heat storage, and longer life out of the ducting in the bench (because strong cob will hold its shape even longer than the metal, if it is kept dry). If you are tired, just dump some non-living dirt in.
If you will have tanks directly on top of the bench and the mass may be damp a lot from spills, don't bother, because wet cob has no particular structural value. The bricks will hold the mass, and you can put a few stacks of bricks between the two ducts to support the tanks on pillars so they don't crush the pipes.

2) The compacted Roxul is fine. You have already removed this version, but Roxul is a very good insulation and, even compacted, it should perform great. The perlite is not any easier - probably harder - it's just different work. If you want to make the Roxul a little more "super-model" to fit in your barrel, you can try taking it outside and peel the batts in half, so they are a fluffy 1.5 to 2 inches. Wear long sleeves and gloves, and a dust mask or bandanna, because it will itch if you don't.

3) The manifold is fine, both ways you have planned it. As long as you have a reasonably big gap in front of that 6" pipe, it will work out fine. Peter van den Berg and I worked out the same rule of thumb from different directions, for an 8" heater you want about a 4" gap in front of the pipe to avoid restrictions. As Ernie has proved, you can run a stove with a slight restriction there if everything else is perfectly right - your heat riser height and insulation probably made almost all the difference in performance, and the manifold very little in itself.
You can build it with bricks and cob as the guys were showing (works great), or you can use your old cut barrel and cut it a little more for a 1 1/2 barrel setup. If you cut the barrel off about 2" above one of the rings, then make a vertical slice down to just above where the ring starts to flare outward, you can usually slide the

4) The concrete floor is fine. Unless you anticipate the space becoming a dance floor at some point, the amount of deterioration under those 2" or more of perlite-clay should be almost undetectable.
We very often use a 2" pad of perlite-clay on concrete slab floors, with firebrick atop that for the firebox floor pad. We have occasionally done as little as 1/2" for leveling, or even none, and the damage as I recall was very minimal - less than 2 square feet of slightly roughened concrete surface. The temperatures that concrete will tolerate are a lot higher than wood flooring, or any finished floor.

5) The firebox shape is looking good. Both insulation methods look like they will work. You have picked up the trick of lapping bricks for better strength. If you are not careful, you will become a brick-mason by the time you are done with this.
I hope that you are either working with clay-sand now so that cleaning the bricks with the hose will be easy, or if you are still using the strong-bonding refractory, maybe your heat riser came off all in one piece?

What's not working:

Clearances: Eric is correct, this was dangerously close to the wall.
I am glad to see you have already removed the firebox and are getting ready to make it farther from the wall. I imagine everybody is sorry it wasn't mentioned earlier, but not as sorry as they would be if you discovered it by scorching your home! There could also be electrical wires, or other things behind that wall, that could be damaged by heat or fire. You will be very glad if you never have to find out.

Since you are moving the firebox anyway, a possibility: (?)
Where the exhaust goes up through the roof - that vertical chimney will give you better draft if it is warmer than outside air. Sometimes we bring the chimney right next to the barrel, to pre-heat the chimney at the start and end of the fire when good chimney draft is hard to get and matters most.
You might find things easier if the barrel is very close to that pipe. Roughly where you have the cleanout, in fact.
Is there enough room in your greenhouse to attach the firebox at this cleanout, and have the barrel stick out into the room? Then the chimney helps serve as a partial heat-shield.

If the space is too small (and if the ceiling is 7 feet right near the house, I am guessing it is a smallish greenhouse), then it's time to think about other places the firebox can go, or other materials we can use to shield the wall.

If you can bring the barrel out to 12" away from the wall, and stack bricks up beside the barrel to above it (they don't need to go all the way up, just far enough that if you put a little light right on the barrel, the shadow on the wall is 3 feet long - the bricks would block radiant heat where it's closer than 3 feet.)

Clay:
You mentioned you were having problems with the clay not sticking the perlite together. Sometimes when someone asks for fire clay, they find a place that sells already-fired clay, for use in making fire bricks. This is kind of like the ceramic version of sand. Quick way to prove this is not the problem is to stir a spoonful into a glass of water. Does it make the water muddy? Does it stay muddy for longer than 2 minutes? Clay will usually settle out of water over 15 minutes to 24 hours. Silt will settle out within about 2 minutes; sand within seconds; gravel the instant you stop stirring or shaking.

Getting help:
I would like to suggest starting with a copy of our draft plans for greenhouse heaters ($20 by request, I'll email them to you).
The current version includes information not just about how to do a rocket mass heater, but also some details on how to use solar gain and other elements of greenhouse design.

Then if you would like someone to specifically say What Will Work so you can kind of skip to the end of this difficult learning process, we can look at doing a private design consultation from a distance for about $300.
I would want to ask some questions about climate and goals, to make sure that this is the right heater for your needs, and then help lay out safe clearances. There's a slight chance we'd suggest something entirely novel (like turning this bench into a climate-banking system and skipping the firebox altogether, depending on your climate). But if a rocket is the right tool for this job, you won't need much more help. You have already done the hard parts 3 times, it's just a matter of getting the design finalized so that the 4th time is the last one.

- Erica

Erik Weaver wrote:...
QUESTION:

It also leads me to wonder if there is a useful rule of thumb here? My tests have been with a nominal 6" system. And I am seeing temperature extremes ranging up to 600 F. This is basically what I have measured at the bottom-rear of the fire riser, below the coals, and at the top of the barrel (all outer face temperatures; interior temperatures are obviously higher). So I wonder, would one expect to see these areas rising to about 800 F in an 8-inch system, and only 450 F or so in a 4.5" system? Is the rate of change from one system size more linear or geometric??

I would say that the progression for temperatures is definitely non-linear. We have seen nearly-identical fireboxes that achieve different temperatures, probably due to climate or bench configuration. There are non-linear effects all over the place with fire's rate of burn, or even the ability to sustain a flame at all. A 2" "rocket stove" is a fiddly toy that doesn't work well; a 12" rocket mass heater is a terrifying machine that sort of asks the question "Will it Melt" of every substance used in its construction, and the answer is almost always "Magma!" Ok, well, not quite that scary.
There's a theoretical limit to how hot it's possible to get with wood fuels (theory predicts a perfect flame, losing no heat, would achieve something around 3600 F with wood fuels, up to 3900 F with some other carbon-and-hydrogen compounds). To get hotter you'd have to be burning something else, like metals (magnesium, aluminum).

-Erica

In one of my first posts on this thread I said:

"I have measured 1710 F at the throat of the burn chamber, right where the J-style feed tube transitions into the beginning of the burn chamber."

I don't measure this very often, because I now know it is operating in the expected temperature range, but I thought I'd mention that I the highest temperature I have measured at the throat of the burn chamber is now 1925 F.

Erica, thank you for your observations. Given that you and Ernie have built hundreds of these things, I appreciate your input a great deal. In particular, I appreciate your responding to whether or not temperature ranges are linear. Your responds indicates these are somewhat erratic builds, because there are many important variables in the system, that one may note significant differences from one build to the next.

For me this really reinforces the idea that it is very important to carefully monitor one's own build until one knows what to expect from each build, and is able to picture it's temperature profile.

I am monitoring temperatures at about a dozen points, about half of these are on the build itself, and the other half are located at different points in the house. Given my baby dragon is living right in my living room, I consider this very important. It lead me to rebuild the first iteration in my home, and has given me a number of insights which I think will prove quite valuable when I make my permanent build this summer/fall. Since I have found this to be such useful (and good-sleep inducing, heheh) information, I certainly would recommend it to others.

For monitoring temperatures on the build itself, I use good quality thermometers / thermocouples. But for monitoring basic room temperatures, I use very inexpensive thermometers (only $1.50 each on sale at a local hardware store last month, yay! heheh, so I bought ten or so 12" tall cheap plastic wall thermometers).

As one simple example of why I find this useful, I have a temporary heat shield placed between the barrel and my TV. The "heat shield" is just a foil-faced piece of 1/2-inch insulation board (with an inch or two air gap at the top and bottom), that is about 30 inches from the barrel and about 18 inches away from the TV. Typically there is a 10 F difference in temperature from one side of this heat shield to the other (the thermometers are about 8 inches distance from one another) and occasionally as much as 20 F.

Thank you Erica for your input. I am working on cleaning the bricks. The heat riser I did with sand+clay, but the burn chamber i did with refractory cement, so... still cleaning, not done yet.

The guys are great help. They helped me to understand the theory of Rocket heater. But no one would imagine i put the barrel 3 inches from the wall but I did. Ah, i don't trust myself any more, i am afraid of mess thing up to endanger my home, and many things I don't know how to do. I hope you and Ernie would arrange your time to help me here in CA if this is neccessary.

If the bench should be 4" from the wall and the barrel should be 18" from the wall then i think i will undo the bricks box. Since I am not done anything with it yet, I think it is a chance to do it right, should I?

I have a question: For example I burn the rocket heater for until the temperature of the inside 2000 degree and then I don't feed it any more wood. Can fire still be started even if there is no more flame and only the high heat from the burn? The way you said it sounds like there still be fired due to extreme high heat even if there is no more flame, and that make nonsense to me, am I wrong and it is possible that fire start from high heat alone?