Bricks for the box of my cob bench

Bricks for the frame,

O outline my frame for the cob bench, I used concrete bricks. Now I kinda wonder, would this brick good for the box of my bench or they will not allow heat to go through. I need something to allow the heat to go through, right?

http://www.osh.com/Osh-Categories/Tools-%26-Hardware/Building-Materials/Concrete%2C-Cement-and-Masonry/Brick-%26-Building-Block/Mission-Concrete-Products-Concrete-Brick-Tan/p/2416592#contenttab

They appear to be a brick made out of concrete. They should work fine for most of the bench. However, be careful in the hotter areas. I do not recall off hand the temperature rating for concrete, you'll want to find that rating, either for concrete (which will be an average number, given there are different mixes for making concrete), but best would be to get a data sheet from the manufacturer, if they provide heat ratings.

So, in short, do *not* use these in or near the fire box. Use fire brick there, at least 2200 F rated. The manifold is the transition from the bottom of the barrel into the bench. That too can get rather hot, and depending upon the heat rating of concrete brick, you may not want to use them there. If I were going to make a guess (that means I am guessing, and trying to recall what others have reported; better would be to look up the temperature ratings for your materials, and make sure you are safely below their maximum heat / temperature rating), I'd say after you are five feet distant from the barrel, you can make your bench out of whatever thermal material you wish (rock, concrete, clay, cob, adobe, etc).

With regard to thermal mass, any stone or clay material is going to act as a thermal mass. The denser the material the better. To avoid in the thermal mass is air gaps (therefore, do not use sand as thermal mass - it actually has a lot of air gaps and behaves as an insulator for that reason; wet sand will act as a thermal mass, until the water is evaporated, so it doesn't work in the long term). Many people fill in whatever they can get their hands on, be that stone, broken concrete from sidewalks, salvaged bricks, etc, and then they fill in the spaces with a mixture of clay and sand.

Allowing heat to go "through" is a matter of using materials that offer thermal mass, and eliminating air spaces. It is the nature of heat to move to whatever is cooler. So by running hot air through the duct work, the heat is absorbed into the cooler thermal mass surrounding the ducts. If there are air gaps, that stops that from happening (or slows it greatly). Then the heat moves through the thermal mass, always seeking cooler, and eventually radiates from the surface.

How long that all takes depends. How hot is the air in the ducts? How much cooler is the thermal mass in contact with the ducts? How dense is the thermal mass? How thick is the thermal mass? Is the thermal continuous, or are there air gaps or other forms of insulation in it?

For generic cob, the figure I seem to recall the Wisner's stating is that one inch of cob is penetrated per hour. So if you have six inches of cob around a duct, it will take about six hours for the heat that was in the duct to move to the surface of the cob. Other materials have different rates of heat transmission.

The long answer, is to continue your research. Read up on successful builds, paying attention to materials used and how they are assembled, and certainly to the details of the fire box and exhaust routing, as well as typical temperatures.

Not many of us get a good probe into the heart of the rocket. It just is not convenient. In theory, and in agreement with some measured results (read everything by Peterburg over at Donkey's forum: http://donkey32.proboards.com/#category-1) the hottest part of the fire box is going to be in the burn chamber and in the lowest portion of the fire riser: basically right near the 90-degree turn, where the burn chamber transitions into the fire riser.

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 expect it is hotter another foot or so into the burn chamber, where it transitions into the fire riser. But I've not gotten a probe in there to verify that.

Many builds have successfully used 2200 F rated fire brick. My local brick store sells 2500 F rated fire brick, so that is what I am using. I have seen fire brick advertised that is rated at 3400 and 3500 F. Under ideal conditions wood can burn very close to 3400 F. I'm of the opinion that few rocket stoves/RMH get as hot as 3,000 F. On the other hand, I'm very well convinced that temps around 2,000 F happen quite often. But since so many people build with 2200 F rated fire brick, I just doubt temperatures are typically sustained much about that; if they are, why isn't the 2200 F rated fire brick falling apart from the stress?

But the best answer is to build some in your yard and get a good temperature probe in there. Then you'll know what your temperatures are, for the way you built your rocket stove. And that's really more important than theory or other people's builds (although both are very useful in designing yours). Also measure temperatures around the perimeter of your test stove, the sides and below it (easy if you build it on a bed of sand, I found). Then you can use your readings to design safely for you indoor build (and continue to monitor temperatures indoors; thermocouples are pretty inexpensive and digital readers may be bought on Amazon, and I'm sure many other places, for as little as $20 or $30).

Thanks a lot Erik for your explanation and advice. My have another concern please help : I made walls of concrete brick all 4 sides of the bench, the back of the bench, the two head ends, and the front of the bench. I wonder would the heat be able to go into my room from the front of the bench. Somehow I got that the bricks are to prevent heat loss, which means bricks will prevent heat to go out of that confined compartment, right? I am right or wrong here please? If so, then the heat can not get out of that box and get spread out in the room. Do I have to remove the all of bricks in the front of my bench?

Thanks a lot Erik

No, you do not have to remove your bricks, the heat will radiate out of them just fine. I've included some pictures from my greenhouse build. The 3rd layer of brick up from the floor commonly is 140 F when the stove is running and over 100 F the next morning. When you first start to heat your mass it will take time to heat everything up but once you have it fired up for the season those bricks won't cool all the way down till spring. It's all mass, the more you have the longer your heat will hold.

Bacon Lee : As you have already noted, it takes fewer regular bricks to equal the weight of your Concrete bricks. This generally means that they are more
insulating than heavier, denser stones, all this means is that it will take longer for the heat energy to travel through the Shell of your mass, perhaps even
twice as long! You may find that ou will need to start your evening fire sooner, and that you will be a little more dependent on the prompt heat off of Your
Barrel !

Elsewhere I believe you mentioned planning on a bench about 1' high- as at least half of that height is horizontal ducting, you will get heat through that little
bit of mass quite rapidly !

I Think that you may want to increase your Thermal Mass Benches height at some time next spring, both for more storage and to more conveniently
take advantage of the tremendous boon that being able to actively sit on and be immediately warmed by your Thermal mass! Next spring will be soon enough
to decide what adaptations YOU might want to make, after all, we can only propose ideas, you will be the one who translates those ideas and your observations
to future actions !

For the Good of the Craft !

Thanks a lot Thomas and Allen

Thanks a lot Thomas. The photos of your set up made me feel much better. I was so worry I have to break it down.

And I think Allen is right, I will make my bench higher, 14 inches then.

Now I have another questions please advice:

1) The sand for the cob bench, what kind of sand should I use? They have #1 sand and #2 sand which is expensive. They have fill sand which already has some clay in it, and it is cheaper. Should I have fill sand?

2) How much sand per clay? One person told me 1 clay and 3 sand, right?

3) I went to the Clay Planet to get Lincoln 60 for the cob bench, and they said they are out of Lincoln 60, they told me to get Lincoln 8, would Lincoln 8 be ok,

4) Someone said that when make the bench, we can not fill it all and finish it in one shot, we have to do it by layers, fill up couple of inches then wait for a few hours. Is it true? If so how many inches should I fill and how long do I have to wait for the next layer

Thanks

Bacon Lee wrote:Thanks a lot Erik for your explanation and advice. My have another concern please help : I made walls of concrete brick all 4 sides of the bench, the back of the bench, the two head ends, and the front of the bench. I wonder would the heat be able to go into my room from the front of the bench. Somehow I got that the bricks are to prevent heat loss, which means bricks will prevent heat to go out of that confined compartment, right? I am right or wrong here please? If so, then the heat can not get out of that box and get spread out in the room. Do I have to remove the all of bricks in the front of my bench?

Thanks a lot Erik

The other guys answered this. But I did want to add a comment:

Bricks do not insulate. (I have used similar mistaken language myself in the past, and will mention that below.) Instead of thinking of them as insulation, think of them as heat batteries (thermal batteries). Note the observation about density (commonly observed as weight). The more mass is packed into the same space, the better it will perform as a thermal mass (heat battery). Water is a good thermal mass; however it is difficult to build a bench out of water so we use bricks, rock, and clay, etc. heheh.

Consider three different bricks: the light concrete brick, a common building brick, and fire brick. Now all of these come in different mixes and will have differences inside their own subset. But generally speaking they also have some common traits, and some of these distinguish one broad group from another.

Concrete brick is made from concrete. Concrete performs poorly under high heat. So it must be used with forethought in a RMH application, making sure it is only used in areas which will not over heat the concrete brick. It would never be suitable to form the fire box, because the concrete could not withstand the rapid expansion nor the high heat. (The same is true of poured concrete, portand cement, etc.) It is light weight, so as compared to the other bricks, it will offer a smaller "heat battery capacity" (lower thermal value) for the same volume.

Common building brick is typically made of some mixture of clay. They are heavier than concrete, and offer a decent amount of mass, so can be used pretty effectively as a thermal mass (heat storage battery). However, they are expensive and are usually only used as decorative outer layers. Since they are heavier, we can (properly) assume they will offer better thermal characteristics than concrete brick. Also, because they are made from clay instead of concrete, they may be used in higher heat areas of our build. But they still will not take the extreme heat of the fire box, so do not use them there. (This is why fire places are lined with fire brick, and not common brick; also masonry heaters.)

Fire brick has a lot of metal added to the mix, and is designed to both withstand extreme heat, and rapid expansion and contraction. These are the primary characteristics that make it "fire brick." It is very heavy (dense) and would make for a very nice thermal mass. However, they are usually prohibitively expensive, so most of us use them where they are in contact or very close proximity to extreme heat.

A few other observations.

If you are using a brick with holes through its core, fill those with clay, cob, or mortar when using it as a thermal mass.

There is another material called kiln brick. This actually *is* insulating. It is very, very light weight, yet can take extreme temperatures. So this is a special category of brick, and I would suggest not trying to think of kiln brick as "brick" and instead think of it as "kiln" material.

My own misspeaking of fire brick as insulating in Dec. 2014 or Jan. 2015, was while talking about a test build I had made outside and the various test points at which I was taking temperature readings over the course of the burn and for several hours afterwards. Fortunately my poor choice of language was immediately corrected. To be clear, like all brick of which I am aware (with the single exception of "kiln brick") fire brick is *not* insulating. It is however, very dense, and if one is careless with one's language, one may be tempted to speak of fire brick as if it were; but one is not seeing it acting as an insulator, instead one is seeing it acting as a very dense (good thermal mass) material.

In my tests, I was measuring temperatures on the outside face of the fire brick, and several inches away from the outer face of the fire brick (buried in sand), and comparing a full size 2.25" fire brick to a split (half size) 1.25" fire brick (I had built the burn chamber with a 2.25" wall on one side, and a 1.25" wall on the other; or possibly both sides were built with 1.25" brick and then I added a full 2.25" brick to one side; right now I don't recall which, and it doesn't really matter).

Basically, the outer face of the thicker (full) brick was half the temperature of the thinner split (half-thickness) brick. If the full fire brick measured 250 F, the split measured 500 F, etc. But here's the important point: This was NOT because the fire brick was acting as an insulator. It's temperature was a reflection of its dense material.

The heat was actually moving through both the thin and thick fire brick at the same rate, it is just that the thinner brick provided a shorter path to the surface than the thick brick (obviously). For a given unit of heat, if it heats twice the material, that material will end up at half the temperature (that's a wildly simplified statement, there are other variables to consider). That's what I saw in the fire brick temperature measurement.

When thinking about the heat being created in a RMH and when it reaches maximum temperatures, what I measured in my outdoor test builds was that the hottest point DURING the burn, was right at the 90-degree turn, where the burn chamber transitions into the fire riser.

This makes sense, I think, because the hottest part of the fire is supposed to be somewhere in this area (I may be mistaken, but my understanding is that it depends upon dynamic variables of the living fire, so the hottest point at any given instant changes; however, it is typically found somewhere in the part of the burn chamber nearest the fire riser, or in the lowest portion of the fire riser. I have not personally taken enough temperature comparisons to speak to this point from experience).

I did measure the outer face of the bottom-most fire brick that formed the fire riser, and which was directly across from the burn chamber (the brick on would see if one looked down the feed tube, through the burn chamber) and found it ran up to the 550-600 F range (2.25" fire brick) during the active burn.

The hottest part AFTER the burn, was below where the coals were piled up to finish burning. Measuring in the sand under that portion of the burn chamber floor, about 1/2-inche below in the sand bed upon which the rocket stove was built, also measured in a similar range, 550 F or so, as best I recall (I'm not looking at my notes, just going by memory). But it took a couple hours for this to reach its maximum temperature. (Having moved the test indoors, I have noted the maximum temperature under this area of the burn chamber (but 3-inches below, and built on clay-perlite, instead of sand) takes about an hour or so longer to reach maximum temperature.)

These both make sense to me. What I *think* is happening is the fire is burning so fast, and hitting the back wall of the transition from the burn chamber to fire riser, that fire brick takes the brunt end of the heat, rising the fastest in temperature. I'm sure that entire area is extremely hot, so one would have to carefully monitor all the brick in that region to really know which one is the hottest, but as compared to other regions of the fire box (feed tube, transition from feed tube to burn chamber, and tallest half of the fire riser) I'm confident to say the transition from burn chamber to fire riser is the hottest portion of the fire box during the burn.

Then, wherever you pile up your coals to finish burning out, and especially if that is the same place they were accumulating during the burn, several inches below that will be the hottest part of the build several hours after the burn has been completed. It just makes sense to me.

What I have not measured is below the floor of the fire box, all along the full length, to compare the temperatures across the life of the burn and several hours later. So I cannot offer a profile of how the temperature moderates and travels at different points below the floor of the fire box. I recall Ernie Wisner once observing that 800 F may be expected under the floor of the fire box, and that seems like a reasonable expectation to me.

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??

Bacon Lee wrote:

1) The sand for the cob bench, what kind of sand should I use? They have #1 sand and #2 sand which is expensive. They have fill sand which already has some clay in it, and it is cheaper. Should I have fill sand?

Fill sand. You're going to mix clay with it anyway, so if the fill sand already has clay in it, so much the better! Basically, in the bench you've built it won't matter anyway, because you framed it in brick, which will offer support. And to what degree the grades of sand matter, really? Good question. In theory, sharper sand cuts and holds to the clay better than smooth river sand. But I'm using river sand in my mixes because I have a big pile of it in my yard. If that pile were sharp sand, I'd use that I would use the cheapest sand you can get. I may be mistaken, but I just don't think it is a critical point. Sharper is better than smooth, but cheaper trumps both!

Bacon Lee wrote:

2) How much sand per clay? One person told me 1 clay and 3 sand, right?

It will vary. It depends upon the qualities of the sand and the clay. But generally speaking, the clay is typically found to be most effective when making up about 20% to 33% of the mix. That's roughly from 1/5 to 1/3 of the mixture is clay, and the rest is sand. It depends what you are using it for too, and how critical its potential failure may be. In your case, you are just stuffing it into a brick box, around duct work. Any ratio will work well enough hehheh

To answer your question more directly, just going off the cuff, I think of my ratio as around 1:3 so I would agree with what you are saying is a good rule of thumb.

Bare in mind, sand is for compression strength, and to help reduce cracking as I recall. But it does nothing to mitigate stress cracks (which makes me wonder if the cracking statement about sand is mistaken, and that it is more the ratio of clay (based upon type of clay) to the sand that has be gotten in the proper ratio). To reduce stress cracks you need fibre of some kind. We hear straw being used most often, as that is a traditional cob technique.

But you can use other materials for the fibre. Hair, fiberglass, even cellulose insulation. Throw in your dryer lint! I read of someone who visited a barber and got the hair sweepings. So far I have been using cellulose, because I have it. I haven't gotten any straw yet, and I don't relish whipping it into small lengths. And I'm honestly not likely to get dung and let the horse or cow do the chomping into small bits for me (although that is a time tested traditional technique, and adds to the strength of the cob, from what I've read - something about the digestive process).

But in the bench you do not want fibres. They tend to be insulative, so only use them around the hotter part of the build and in the top coat, where you need the help in reducing stress cracks and fractures in the finish coat.

Bacon Lee wrote:

3) I went to the Clay Planet to get Lincoln 60 for the cob bench, and they said they are out of Lincoln 60, they told me to get Lincoln 8, would Lincoln 8 be ok,

For inside the bench you can use clay you dig out of your back yard! So inside the bench, any clay you get will be just fine.

As mortar for joining fire brick, that is a totally different question, and a use I would personally consider critical. And I do not know if Lincoln 8 works in that use or not.

Bacon Lee wrote:

4) Someone said that when make the bench, we can not fill it all and finish it in one shot, we have to do it by layers, fill up couple of inches then wait for a few hours. Is it true? If so how many inches should I fill and how long do I have to wait for the next layer

I think that is excessive. I have heard that cob walls can only go up about two feet at a time, and then need to dry to the point they will bear the weight of the next layer of cob. I would think that to be a more reasonable estimate. And most benches are less than 24-inches tall, so they can be built all at once.

If your back holds out that long! In *my* case, it may very well be a couple inches at a time, followed by a break! heheh

Plus, you are filling in a brick box you have already built, right? I suspect you will be good to go. If you have enough help or your back can take it, fill it up all at once.

However, it has been advised to only partially cover the duct work, and then get a good fire going. And in this case, periodically burn cold, to produce smoke, because you want to test for air leaks in the duct work. If you find a leak, dig away the clay and fill, clean the duct, and re-tape it with more aluminum tape. That really should seal it. Alternately you can use the 600 F silicone sealant or similar sealants. The clay-cob itself will become a secondary seal, but I am of the opinion that should not be considered the primary seal, just a safety seal "in case" the tested ducting seal fails at some point in the future. That said, it's going to be extremely difficult for gases to move through six inches or more of clay-cob; but I'd still recommend doing the smoke test and repairing any leaks before finishing cobbing in.

But keep the fire going whilst you build the bench and cob over the hot parts of the system. This lets the clay-cob heat up and expand with the metal parts. That will tend to reduce cracking of the clay finish, were you to clay and cob everything and only when all done start firing the RHM.

Also...

Silt is different than clay. If you use clay you dig up somewhere, do the water jar test and figure out how much is clay and how much is silt. Silt is too fine and doesn't help with construction. If anything it hinders it. A little is OK, I'm sure, but if you think you are using clay and it turns out to be 99% silt, the whole bench will tend to want to slid down and form a puddle Not good bench behaviour!

As to filling in the bench only two inches depth at a time.... I wonder, if that was the result of making the clay-cob with too much water? You want to use just enough water, but not too much, as it works against you, both in building up the height of the clay/cob, as well as in lengthening the drying time. And it seems to me a little water goes a long ways (but that's a qualitive statement not a quantative statement, so you can't measure that! heheh).

Bacon Lee : Anything that only allows heat energy through it slowly, ether due to its mass or the way its chemical bonds are made in its crystalline structure
is insulative.

Feathers weigh more than air, a dead air space is insulating, but not as insulating as that same space filled (not packed) with down ! It is the segregation of
the air into 100s of individual packets of air by the Down and not the Down itself that is insulating. Compress or wet the down to remove its air spaces and it
loses most of its insulating ability.

A very dense object with the 'Wrong' crystalline structure can hold massive amounts of heat- its specific heat capacity and it still can be a poor material
to use for Thermal Conductivity !

Chemical bonds and crystalline structure aside, generally a lighter object with low density will have a lower specific heat and probably a low Thermal conductivity

In layman's terms we can say that lighter materials like those Concrete bricks are More Insulating!

Building the bench, the idea is to start at the bottom and work up not start at one end and work to the other, this will allow one area to firm up before you get
back to it to put on the next layer !

One of our fellow members compares the process to making lasagna ! With your pipe in its final position, you can use clay slip to paint the pipe to assure good
contact and heat penetration from the pipe to the Thermal mass, Then using mostly Cob and dense stone,and being careful to exclude trapped air, we slowly
build upwards, If you do a trial fit then paint or dip the rock in clay slip before final placement of the rock or Urbanite, you reduce both the amount of trapped air
and the amount of cob you must make!

I could be wrong but I believe the Wisners are quoting 2'' an hour for dense rock and 1'' an hour for Cob this affect the height/volume of the Thermal mass

Height of bench : I suggest that you find comfortable chair that you can easily rise from, measure its height from the floor and use that measurement for Your
Thermal Mass Benches height, again the thermal mass is there to serve you, make it something that has as many practical benefits as possible, stacking functions
not as an object that to are liable to trip over and it merely taking up space !

For your Rocket burner and The Burner base you are looking for a stable 'set' to your Cob, here it is important to use a good clay and sharp sand to produce a cob
with low expansion and contraction characteristics !

For the cob in the bench itself use the best materials you can afford, Cob made with a fill sand or bank sand may have a tendency to wear away and hang around
your Thermal mass as Grit (sand) and Dust (clay) but that is what your Structural coat and finish coat are for, any thing inside your bricks will not be a problem !

You will have to determine how much of your clay is Silt, most clay contains silt some can contain way too much ! Due to a little confusion of the language I can not
tell you if you want to Google Soil Sediment test, or soil settlement test! But he test is easy to do and should be done a various depths in your Clay Body

For the good of the crafts Big AL

Thanks a lot to all of you kind and knowledgeable people. it's a nice surprise to see you guys are know so much and you are kindly taking your time explaining things. Thanks a lot

I hope you don't mind I have further question:

1) How do I test for percent of silt in my clay? I use lincoln 60 and some lincoln 8. Does any one already know their percentage of silt?

2) On the top surface of the bench after I finish, what if some time down the line someone pours some water on the bench. Would the clay be running and making a mess? What do you do about the top of the bench so it will not make a mess when contact with water? Can I have a thin layer of cement on top of bench to make sure it will not making a mess

Again thank you very much

I have some more questions please

1) How do I test for leak in my duct work? Just look with my eyes to see if there is any smoke?

2) Why should I wait until fill with some clay to test for leak in my duct work? It is easier? How?

3) Should I test now, burn the stove and look to see if any smoke came out somewhere?

Bacon Lee wrote:
Thanks a lot to all of you kind and knowledgeable people. it's a nice surprise to see you guys are know so much and you are kindly taking your time explaining things. Thanks a lot

I hope you don't mind I have further question:

1) How do I test for percent of silt in my clay? I use lincoln 60 and some lincoln 8. Does any one already know their percentage of silt?

2) On the top surface of the bench after I finish, what if some time down the line someone pours some water on the bench. Would the clay be running and making a mess? What do you do about the top of the bench so it will not make a mess when contact with water? Can I have a thin layer of cement on top of bench to make sure it will not making a mess

Again thank you very much

1) I serious doubt either Lincoln 60 or 8 has any silt. I'm not familiar with 8 at all, so I could be mistaken. Nor have I used 60 (I use a local brand of fire clay instead), but the whole point of using it is to get a pure clay, that is free of impurities which may react poorly when exposed to high heat. Cheat the ingredient list, that may offer a clue. And failing that, call or email the manufacturer. Or find a MSDS sheet or data sheet on their web site.

2) This is not an area I have details to offer, as I've not yet needed to burn that particular bridge yet But look up forums that discuss finishing cob. The idea is to add to the basic clay-cob mix so that it can take bumps and hard hits without cracking, bending instead of cracking, and will include shedding water, instead of becoming a gooey mess.

Bacon Lee wrote:I have some more questions please

1) How do I test for leak in my duct work? Just look with my eyes to see if there is any smoke?

2) Why should I wait until fill with some clay to test for leak in my duct work? It is easier? How?

3) Should I test now, burn the stove and look to see if any smoke came out somewhere?

1) That's what I do. And I use my sniffer (nose).

2) I don't think you have to wait, so long as the duct work is taped, sealed and supported. But you do want to burn the stove when the clay is being put on, so that the parts that are going to expand expand before the clay dries out and gets rigid.

3) I would. I fire it up as soon as I think it won't bellow smoke into the house! It is a new shiny toy! Gotta play with it! On the serious side, I want to know right away if there is a drafting problem, or gaps letting smoke out into the room. And I have to think these are easier to address before claying everything up.

I looked up Lincoln 8, and it is basically the same as Lincoln 60 but with more iron in the mix (it came from a higher-iron section of the mine). It will work excellently for all purposes in RMH building. If you were to make pottery with it, it would be a bit browner with possibly more dark specks on the surface.

The number associated with a particular fireclay may (not must) relate to how coarse it is. Lincoln 60 has only particles that will pass through a 60-mesh screen - 60 wires per inch. There is a variety called Greenstripe 200 which is produced by a different process and has only 200-mesh and smaller particles. This would actually not be advantageous for our purposes, as the larger particles (still clay, not silt) make a stiffer mix that is less likely to shrink on drying.
Another common fireclay in some areas is Hawthorn 40, which is a bit coarser than Lincoln 60, and works especially well for cob ovens according to Sheffield Pottery.
The Hawthorn mines are in Missouri I believe, while the Lincoln mines are in California, so that will affect regional availability.

Thank you all, and I think I will use lincoln 60 and lincoln 8 assuming they are the same. I will try to make my bench around 15 inches. I will leave the bricks on all 4 sides, I already bought them any way. I don't understand much but I am tired of keep learning because it seems like learning forever.

But I need to have one more question please: What do you mean by draft problem? What is that?

A draft problem is when not enough air flows through the system for proper combustion. There can be many causes for this, including too long a path for the gases to travel (too much friction), a constriction in the flue path, poor combustion core design that does not give good heat and thus insufficient push, and many variations. The more you follow established designs, the fewer unforseen problems you are likely to have.

Bacon Lee wrote:I have some more questions please

1) How do I test for leak in my duct work? Just look with my eyes to see if there is any smoke?

2) Why should I wait until fill with some clay to test for leak in my duct work? It is easier? How?

3) Should I test now, burn the stove and look to see if any smoke came out somewhere?

leaks are nothing. just cob it

Glenn Herbert wrote:I looked up Lincoln 8, and it is basically the same as Lincoln 60 but with more iron in the mix (it came from a higher-iron section of the mine). It will work excellently for all purposes in RMH building. If you were to make pottery with it, it would be a bit browner with possibly more dark specks on the surface.

The number associated with a particular fireclay may (not must) relate to how coarse it is. Lincoln 60 has only particles that will pass through a 60-mesh screen - 60 wires per inch. There is a variety called Greenstripe 200 which is produced by a different process and has only 200-mesh and smaller particles. This would actually not be advantageous for our purposes, as the larger particles (still clay, not silt) make a stiffer mix that is less likely to shrink on drying.
Another common fireclay in some areas is Hawthorn 40, which is a bit coarser than Lincoln 60, and works especially well for cob ovens according to Sheffield Pottery.
The Hawthorn mines are in Missouri I believe, while the Lincoln mines are in California, so that will affect regional availability.

Thank you Glenn, I appreciate knowing that. I may forget it, heheh, but I do appreciate that level of investigation. Thanks.

Bacon Lee wrote:
Thank you all, and I think I will use lincoln 60 and lincoln 8 assuming they are the same. I will try to make my bench around 15 inches. I will leave the bricks on all 4 sides, I already bought them any way. I don't understand much but I am tired of keep learning because it seems like learning forever.

But I need to have one more question please: What do you mean by draft problem? What is that?

Bacon, learning is good! It why we eat cows and why cows don't eat us! (Well, that and they are herbivores, heheh)

Perhaps, you missed Glenn's recent post, just above a few posts, but he basically said the Lincoln 8 will work just fine. You're good to go in that regard.

Your comment concerning draft I find a little disconcerting. Is English your first language? Have you studied the Evan's book on rocket mass heaters? I'm hoping this is only a language / vocabulary issue, because if I did not know what draft was, and have some idea what makes it good or bad, there is no way I would build any fire burning device in my home. I get that you are in a rush, I am too. But take your time and be certain you understand *why* you are doing what you decide to do.

I do not mean to be insulting, but I am concerned. Do *not* just do what a pack of strangers online tell you to do, and hope or trust that it will all work out for the best. Your house and potentially the lives of persons inside it are at risk when building a fire inside your home! Far better to take another year of study, or ten for that matter, than waking up dead!

Ok Erik, you are right, English is my second language, I am a woman (I am not a handy man as you all think I would be). My life doesn't have much to do with tools. But I have an aquaponic garden, and my fish pond is 400 gal. I have to find a way to heat the greenhouse or my Tilapia will be in big trouble. The rocket heater is for my greenhouse, but the greenhouse is attached to my home, as you see in the video.

I manage to have it done up to here, now all I have to do is fill the cob bench.

You said the draft problem can cause me dead, which scare me. I thought rocket heater is safe, that's why I do it. I have read online about draft issue. Seems like draft problem is when the smoke has trouble going out the chimney so it went backward into the green house, right. Issue could be due to small flue, the coldness outside of the flue (the portion sticking out the roof)... I will buy the CO detector.
"
I attached here are photos and video of my rocket heater. I added my idea to make my rocket heater simpler. Everyone please take a look to see if I am ok. First I have 2" of perlite/Portland cement/fireclay at the bottom of the barrel. Then I have someone help me cut a rectangular and a circle near bottom of the barrel. Then I build the burning thing outside, by firebrick. I only build the heat riser and the burn tunnel. Then I ask a big guy to put the burning uint into the barrel with the burn tunnel ticking out the rectangle. From the burning tunnel sticking out, I continue to build my feed tube. At the other side of where the circle is, I attached the flue which go for 8 ft then a u-turn it back haft way then and elbow to stand it up to the plastic roof out of the greenhouse.

Am I ok here, please let me know

What happends to my photos and video? here they are, I have to repost photos and video

Photo #2

Here is the clip of my heater. I can't post it here. Maybe too big of a file. So I posted it on youtube. Please click on this link to review and advice please

http://www.youtube.com/watch?v=sfbvls6Kh6U

It looks like you have 6" ductwork, but the feed tube is much larger in cross section. One of the basic principles of the RMH is that the cross section of the system should be close to the same all the way through, except that the space between heat riser and barrel can be larger. Your feed tube, burn tunnel and heat riser should all be 6" x 6" or a hair less if that is the diameter of your duct. We need to know what your system dimensions are to be able to say whether it is likely to work well. Have you tested it yet?

It looks like you have a fairly long horizontal duct run, but probably okay if the combustion core is done right. I can't tell how tall your heat riser is, but it looks like it can't be much more than twice the height of the feed tube. 3 or 4 times the feed tube is the recommended height, so you may have a weak rocket that will not push gases through the duct well enough. Adding another barrel on top of the first one, using the clamp ring to hold them together, would allow you to make a taller heat riser, if your test shows it to be weak. You really need to test thoroughly before proceeding, as it looks like you have some parts that may cause poor performance.

OK, I get that English is your second language. That makes it a little more difficult. But take your time.

My remark about dying was a general statement, what I meant was we are building fires in our homes. It we do it poorly, and don't understand why we do what we do, we increase the risk of causing a fire, burning the house down, and potentially killing someone, or ourselves. All of that is meant to say how important it is to understand what we are doing, and not to just rush to get it built, and not taking the time to know we are building a safe stove.

While it is true that a bad draft might kill, due to poisoning, that really was not what I meant.

I would suggest taking a deep breath and re-reading your books on this kind of stove until you feel you understand why it works.

Moving onto your pictures, as Glenn observed, my guess is you'll need a second barrel on top of the one you already have in place. I also notice it is painted. I find I need to have mine sandblasted, or through some other means, take the paint off, because it bothers me. But I'm sensitive to that sort of thing.

Another thing I noticed is the floor under the barrel doesn't appear to have anything protecting the floor from heat. I personally do not think 2-inches of perlite-clay is enough insulation to protect your floor from damage. It appears to be a concrete floor, so I doubt it will burn, but I suspect it will crack and split apart under the barrel. There are designs where people have make an air passage under the platform on which the fire box sits. That's my favorite approach. In terms of just using perlite-clay insulation, 4-inches might be enough, but I'd be tempted to use at least 6-inches. It is the kind of thing you really only want to do once! And now is the best time, and the least amount of work to re-do it.

But see what other folks think, and look at other builds and ask people how their temperatures are under their barrel.

I just re-built my test rocket stove in my living room. I previously built several outside, and took temperature readings as I figured out where to expect the most heat and at what point in the burning cycle. Then I built a prototype/test rocket stove in my living room, and had 3.5-inches of perlite-clay insulation under the fire box. I pushed a probe into the perlite-clay insulation (while it was still soft enough to do so easily) so I could monitor the temperature under the feed tube, where it enters the throat of the burn chamber.

Temperatures there reached 170 F on the test burns I have run, and the system was getting hotter as the cob was drying out. I really do not wish to exceed 200 F under there, and ideally no more than 125 F. Plus, there is a saturation effect of the heat in the insulation, if the heat cannot move away easily, it is unable to fall all the way back down to room temperature before I am ready to make my next fire. It stands to reason, that would slowly increase the starting temperature of the insulation, and result in a higher high temperature. And I have only been burning once a day, and I want to be able to burn at least twice a day in case it gets really cold, like last year.

So, I tore it all down and rebuilt it. I added an air channel under the fire box. I am not done with the rebuild yet, so I have not fired it back up yet, so I cannot yet report on what change this may make.

Your brick work and jointing looks good to me. I may have put the clean out at the U-turn in the ducting, but I don't think that is a big deal. You'll apparently use hose and a shop vac to clean the ducts anyway. And the lid of the barrel looks like it comes off, so that'll make the ash clean out at the bottom of the barrel relatively easy.

I do note there is not a manifold area with a clean out. So you may get more ash down into the duct work than you may have otherwise. Most likely not a big deal, but may require more frequent cleaning. Once this gets up and running I think I'd check the clean out and ducting for ash building up every week the first month, and if is is still ok, twice the month the second month, and if still ok, monthly after that for the first full burning season. The other thing to try to remember, especially since there is not a large manifold to collect the ash, if you notice a reduction in your draft or it suddenly starts drafting poorly, check the for ash build up, both at the bottom of the barrel and from the clean out.

I couldn't see the stove pipe going through the roof very well. It appears to have adequate distance / clearance from the supporting wooden rafters, but that is an important area to pay attention to details, so I would see if I could find some code illustrations, or otherwise get a good idea how that is to be built safely.

That's all I can think of right now.

too tall feedtube.
read the book!

Help please

I will start reading the book today as per your suggestion. But this morning I burn the stove and clamber up the stair to look at the chimney. I saw a lot of smoke, black smoke just like the conventional wood stove. Doesn't behave like the rocket stove with no smoke at all. What's going on with my rocket stove? Is any of you know what cause smoke

Smoke is caused by incomplete combustion, so even if you have draft, it is not running strongly enough to burn hot and clean. How hot is the top of your barrel above the heat riser? How hot is the side of the barrel in various places? Does the fire make any sound as it burns, and rush sideways, or does it move slowly?

I forgot to answer questions.

1) The first time I burned, I remember I heard the rumbling sound of rocket effects. How come this time burning i didn't hear that. The top is not that hot like the first time, why was that?. I don't have the thermocouple. I will go buy one now, I will burn in the afternoon and measure temp in various places

2) Also, my heat riser is 6.5" (square). I made the heat riser out of half firebrick. Two bricks connect the long side make 9" square. I have a half brick standing up at 4 walls. So two end each a half brick 1.25 *2 = 2.5" . 9" less 2.5 is 6.5 Inches. My smoke pipe is only 6"

Bacon Lee :{and Glenn and Erik} As the drum was not burned out before putting it into use, I suspect that that barrel had an Epoxy coating on its inside
and that is what is burning and producing The Black Smoke !

Bacon Lee I am very happy that you ARE so willing to ask questions, relax, take a deep breath -you have come to the right place for help .

You needed to burn or sandblast all the paint off of the outside of the Drum, this is a common mistake but it needs to be taken care off- your drum may
have had a heavy-duty Epoxy liner that is very hard to burn off but we must try !

Below is a link to our Sister site Richsoil.com I will list the steps to do after you have clicked onto that site below :

http://www.richsoil.com/rocket-stove-mass-heater.jsp

Clicking here will take you to a new page where you will have to scroll down the page counting the number of videos you come to .

Stop at the 9th video, Across the top of the video it will say ''rocket flames - rocket mass barrel prep'' This is the video that will tell you how to
prepare your Barrel/drum Good Luck For the good of the Crafts ! Big AL

Everyone :I will be traveling soon and will bye offline, I know you don't need my help, don't expect it ether !

For the Good of the Craft ! Big AL

Burning off epoxy coating inside the barrel is a very reasonable guess for the smoke source. Bacon, can you take the lid off and look inside?

Reading Evan's book a couple times will be a very good start. I expect you will begin seeing places for improvement. And since you have already started the build, what you read should really hit home, and make a good impression, because much of it you will be able to compare to what you have done for yourself. And doing I always find easier to relate to than only theory (both are required, in my opinion).

So I think you are in good shape, and will be getting in better shape with each page you read. And I will reinforce the suggestion that you continue to ask questions here. Just take your time, and relax. Building your own rocket stove, I think anyway, really ought to be fun too. It is a very interesting process, with lots of opportunity for interaction with the system, especially as you work out the problems.

And don't worry about problems -- for sure fix them, but don' worry about the fact they exist. Look at it as a process of running better and better. The system will hit trouble spots along the way. It happens to everyone.

Now, back to your current build...

I can't say that I completely followed exactly what you said about the fire bricks, but what I can say, is it really sounds like the fire riser is too short. You'll read this in the "book" but given your feed tube is three brick widths tall (4.5 x 3 = 13.5 inches), I would expect your burn chamber to be about twice as long (27 inches or so, which is the length of three fire bricks, the same length as the one in my living room), and I would expect the fire riser to close to about twice as tall as the burn chamber (27 x 2 = 54 inches; which is fine, but it can be shorter; mine is 48 inches, for example, and works well); but the fire riser, as compared to the feed tube, ought to be about three times as tall (13.5 x 3 = 40.5 inches).

Obviously, you cannot fit a 40" fire riser inside a 34" tall barrel heheh, so that is why I say, I think your fire riser is too short. *I* would suggest buying a second barrel, getting the paint sand blasted off of it (or burned off, or sanded off; just get it off, how doesn't really matter; and do the same thing for the existing barrel -- yes, I too groaned and moaned when I tore my previous rocket stove apart, so I could take the barrels to get them sand blasted, but I believe it was the healthiest choice, it is not healthy breathing the fumes from the paint burning off, nor perhaps whatever was stored in the barrels, try as I might, I was unable to wash all the residue out of my barrels).

So, other than reading the "book" I would suggest as the next steps:

1) Remove the existing barrel, and have it sand blasted, to remove the paint.

2) Get a second barrel (verifying it is the same diameter as the one you have) and get it's paint removed too.


And while the barrels are off, take some pics for us! I'd like to see the fire brick in side the existing barrel.

3) And as I previously mentioned, I would personally do something about protecting the concrete floor from the heat. Either add more perlite clay (4 to 6 inches, total) or get some brick and/or paving stone, and make some air channels under where the firebox (and barrels) will be; if air can move under there, it will cool down the heat reaching the floor a great deal.


As to why you are not burning as hot now, that is an interesting question. I don't know the answer. It could be a number of different reasons. If one of the bricks fell in the riser that could do it, but I'd expect you would have also noticed a sudden change in your draft, for the worse. But you don't report that, I don't think. If there was a lot of new and wet cob in the system that could be a reason. But you haven't been cobbing yet, so that's not it.

When you say the first time it burned really good, was that by any chance before the barrel was on? Putting the barrel over the fire riser really does alter the system quite a bit. I've had outdoor test burns that ran great, but then started performing poorly after I put the barrel over the riser. In my case, it was a poor draft issue, and adding a vertical piece of 5-foot pipe fixed the outdoor test stove problem.

Or, did this happen after hooking up all the piping? If so, it could be a draft problem, and it could be as simple as there was too much cold air in the exhaust/chimney pipe. That however, would fix itself after everything got hot.

What about the weather? Was it much warmer the second time, when it performed poorly? These can be difficult to burn well when it gets too warm outside (like 60 F, for example).

OK then. That's all I can think of right now.

Keep reading, enjoy it, and come back and let us know what you're doing next.

It surprised me you doctors diagnose pretty good on my rocket stove.

1) You was right, the inside of my barrel also painted. Not the same kind of paint from the outside, but it is kinda clear-red color. Maybe the paint makes the smoke black?
• Instead of sandblast it, is it ok too if I burn it with a torch (That way I don’t have to undo my stove)? If so how long should I burn?
• Oh, and what’s about paint remover? This one has very good review: http://www.homedepot.com/s/paint%2520remover?NCNI-5
• Or what if I burn the stove for the whole day, would that take care of the paint?
It is dark now, but tomorrow I will open the lid and video the inside so you can see. I am afraid that it’s all black and hard to see. Last time I look inside, it’s black I think because of the smoke.

2) You are right about he high of the heat riser too. The high of the heat riser is 27 inches, I have the half bricks by their width look just like the feed tube, but 6 layer of ricks instead of 3. The feed tube, I cut only one one brick (on each layer), so the length and width of feed tube are not the same: the length is 9-1.25= 7.75, while the with is 9-2.5 = 6.5. With the heat riser I did cut 2 bricks (on each layer) so that their length and width are both 6.5. Just look at my feed tube while read this, you will picture it easy.
• So, the heat riser is double the high of the feed tube (6 layers instead of 3 layers). Am I ok or I still have to make my heat riser higher by connecting the second barrel con the second one?

3) About protecting the floor, my goodness, if the floor crack, can I get some cement and fix the crack rather than undo the whole thing? What a big job to undo the whole thing. I am trying to avoid undo it. Would you please think of something else instead of undo it and protect the floor.
• In stead of burning 2 hours straight, can I burn only 1 hour each time and burning it everyday. So that the burning chamber will not be that hot, that way I don’t have to undo everything to protect the floor?

4) Since you said one brick felt into the riser which scare me. I remember when I burn I hear something in the barrel. Tomorrow with the sun light to see better, I will open the lid to see if something happend

5) The first time I burn, I also already put the barrel on and cover and all pipe hooked up.

6) You are right about the weather. Today when I burned, temperature is 62F.

7) I already bought the heat seeker to measure temperature of the stove. Tomorrow I will open the barrel to see what happen inside make a video clip and then if everything is fine, I will burn the stove and taking temperature at different spots

Did I answer all questions. All you doctor of rocket stove, please further reconsider and give me different prescriptions for my rocket stove. Undo the whole thing is so painful

Allen: I already watch the video, seems so easy. Just put some newspaper soaking in clay water, wrap around the barrel and burn. I wish I would have known about this before. Too bad now, do I have other alternative beside undo the whole thing and sandblast it?

Bacon Lee wrote:

1) You was right, the inside of my barrel also painted. Not the same kind of paint from the outside, but it is kinda clear-red color. Maybe the paint makes the smoke black?
• Instead of sandblast it, is it ok too if I burn it with a torch (That way I don’t have to undo my stove)? If so how long should I burn?
• Oh, and what’s about paint remover? This one has very good review: http://www.homedepot.com/s/paint%2520remover?NCNI-5
• Or what if I burn the stove for the whole day, would that take care of the paint?
It is dark now, but tomorrow I will open the lid and video the inside so you can see. I am afraid that it’s all black and hard to see. Last time I look inside, it’s black I think because of the smoke.

You can burn it with a torch, some people do. How long? Until the paint comes off I haven't actually done that, perhaps someone who has can speak to the details. For example, it may be enough to bubble the paint really well and then scrap the rest. Bear in mind that the fumes are not likely to be healthy, so stay out of them as best you can, lot of open doors and windows, and if you start getting a headache or feel bad, get out into fresh air, and try another plan for paint removal.

Paint remover? As long as it works on your paint, and you are able to follow the health and safety instructions. Most likely that is wear gloves and eye protection (which you should any time you work on your stove) and keep the room really well ventilated to carry the fumes out.

Burning the stove? No, sadly that is very unlikely to work. When you burn a barrel to get the paint off, you fill it to overflowing with wood, toss a pipe in there and let it blaze! You basically build a bonfire in the barrel; or more specifically, you are making a really big "toe warmer." I forget what their proper name it right now. Someone might toss that out there for us. I'm drawing a blank on what it is called.

Bacon Lee wrote:

2) You are right about he high of the heat riser too. The high of the heat riser is 27 inches, I have the half bricks by their width look just like the feed tube, but 6 layer of ricks instead of 3. The feed tube, I cut only one one brick (on each layer), so the length and width of feed tube are not the same: the length is 9-1.25= 7.75, while the with is 9-2.5 = 6.5. With the heat riser I did cut 2 bricks (on each layer) so that their length and width are both 6.5. Just look at my feed tube while read this, you will picture it easy.
• So, the heat riser is double the high of the feed tube (6 layers instead of 3 layers). Am I ok or I still have to make my heat riser higher by connecting the second barrel con the second one?

27 inches is only twice the height of your feed tube. That's not going to work very well, I'm afraid. One of the "rules" is to follow basic geometry when making your first build. Break the rules later. It is my opinion for now you ought to be aiming at a well-tested design, and do your best to make it as close to ideal as possible. Among the rules of thumb I would *not* break is having a super short fire riser. I suspect that is going to cause a lot of problems. And you don't need that

In the long run, it is faster and easier to do it right the first time. Therefore, I suggest getting a second barrel (and you can try the clay paper to burn the paint off this time! yay! heheh).

OK, I can picture how you cut the bricks. Technically, the feed tube opening is a little too large. The one in my living room is too. I like the extra room to help with cleaning out the ashes. But it does create some difficulties. Not giant ones, but there are there. Smoke tends to gather in the "dead space", and I do have to be extra attentive to the draw, as it will try to stall out on me, and sometimes it wants to put smoke back into the room. It is not a really big deal in my case, but I also have a pretty strong draft in the right direction. I'm more dealing with eddies, or side currents of air, instead of a primary defect. Or at least that is my analysis of what I see in my living room.

I have considered standing a full brick inside my feed tube, vertically, to take up some of the dead air space. But it hasn't bothered me enough yet to try that, so I can't say if that helps or not. Obviously, if you decide you do not like it, you can either rebuild that side of the feed tube, or even easier, just cut some fire brick and fill in the dead space until you have the same Cross Sectional Area CSA as the fire riser.


QUESTION: What are the dimensions of the burn chamber?

Bacon Lee wrote:

3) About protecting the floor, my goodness, if the floor crack, can I get some cement and fix the crack rather than undo the whole thing? What a big job to undo the whole thing. I am trying to avoid undo it. Would you please think of something else instead of undo it and protect the floor.
• In stead of burning 2 hours straight, can I burn only 1 hour each time and burning it everyday. So that the burning chamber will not be that hot, that way I don’t have to undo everything to protect the floor?

If the floor does crack, your rocket stove is going to be sitting on top of it, and all that mass you are going to add. You won't be able to seal the cracks. That's why I think it is better to try to avoid it happening now, so it doesn't happen. Of course, it may not anyway. I'm just not personally comfortable with only 2-inches of perlite-clay, and no air gap under the perlite-clay. It is my opinion that is a poor idea. But I do not know it will be a problem. Maybe it'll be ok.

But me, I'd rather fix it now, and not worry about, and certain not want to fix it after building the whole rocket mass heater. Now *that* is work! heheh

As to burning one hour instead of two times a day, I don't know about that. Rather dependent on how cold it is, right? See, part of the problem is it can take a day or longer for that perlite-clay insulation to return to room temperature, and if you build another fire before that happens, the low temp is higher than it was originally. So slowly, that heat keeps building up inside the perlite-clay insulation. It will stay in the perlite-clay insulation until it is able to go to a cooler area; the cooler area will be everything it is touching, the floor, the cob, the brick, etc. The heat will take the easiest path, and if it can only go through material that will take a lot longer than if it has an air gap to flow through.

If you look at this page: https://permies.com/t/40996/rocket-stoves/Bonny-Convection-Bench-Rocket-Mass

...and if you look very closely, and know what you are looking for, you just make out some air channels under the bench. Basically you use brick to make these, by laying down rows of brick, and then covering these rows with "bridge" bricks, which make up the platform on which you build the stove. So long as air can move through both ends of the air channels, it will carry heat away from the floor.

I would make the brick "bridge" / platform out of fire brick. I might use fire brick against the floor too, but I think that may not be really needed. For that matter, you may not even need to use fire brick as the "bridge" bricks. As the air flows under there, it will carry away a lot of the heat, protecting your floor, and allowing another route for the heat to move out of the perlite-clay insulation. You don't have to do this. I'm just saying I have, and I will in the future.

As to it being a lot of work to tear down the build and start over, I don't think it is that big of a job. We're only talking about disconnecting the duct from the back of the barrel (and then you can build a larger manifold there, which I think will be beneficial, as well as maybe adding a clean out there too, which will be convenient in the future). Then removing the barrel (you need to get the paint off if anyway, and whether you torch it, or use paint remover, if that is done outside, it will be safer). Then un-build the brick. If you used clay as mortar, that is very easy to clean and reuse.

If you used a refractory cement, then that may be work. If that is the case, I'd have to think about that. I would most likely try taking a layer off the fire rise and see how easy it is. Then decide if I'd take it all down.

And for the air gaps, I'm only talking about under the fire box, from the feed tube to the barrel. I am not talking about doing that to your bench. I think the temps there will be plenty low enough to be safe. I would however, insulate my bench mass from the floor, unless the floor itself is already insulated. The earth sucks up heat! But I would not go to great trouble to do that. I would just lay down an inch of sand, most likely. That will also help break the "wick" effect, and discourage water/moisture from wicking up from the concrete floor into the rocket mass heater.

Bacon Lee wrote:

4) Since you said one brick felt into the riser which scare me. I remember when I burn I hear something in the barrel. Tomorrow with the sun light to see better, I will open the lid to see if something happend.

I said a brick *may* have fallen. I do not know that is what happened. You will know when you look. I rate it as a low possibility. Unless the stove suddenly stopped working right, then I would rate it as much more likely. But you'll know tomorrow when you look inside.

It is surprising how loud even small pieces of debris sound when they hit the inside of the barrel. I heard a really big clunk from mine, which prompted me to take the top barrel off to inspect what had happened. It was just a large piece of the outer surface of the cardboard concrete pier form I used for my outer container of my fire riser. My fire riser is the kind that has 7" metal duct work in the center (which will burn away at the bottom) and a larger 12" cardboard form around that. Between these I packed in perlite-clay insulation, for may fire riser. As it gets hot the clay sets, and should hold together by the time the metal burns away. It has worked for others. Time will tell if I got my perlite-clay mixture right. If it fails, I'll re-build it.

Bacon Lee wrote:

5) The first time I burn, I also already put the barrel on and cover and all pipe hooked up.

Hmmm. Well, then that is more perplexing. Maybe it is just the combined effect of all the extra duct work, with the too short fire riser, no longer being able to push that much air?

Bacon Lee wrote:

6) You are right about the weather. Today when I burned, temperature is 62F.

If it was colder during your other tests, then this too is working against you. If that is the only problem, it is not a concern. However, I believe we may safely say, the fire riser is too short, and this is one known problem that really should be corrected.

Bacon Lee wrote:

7) I already bought the heat seeker to measure temperature of the stove. Tomorrow I will open the barrel to see what happen inside make a video clip and then if everything is fine, I will burn the stove and taking temperature at different spots

I look forward to reading what you discover.

Bacon Lee wrote:

Did I answer all questions. All you doctor of rocket stove, please further reconsider and give me different prescriptions for my rocket stove. Undo the whole thing is so painful

Well, we can revisit this after you tell us what you discover.

Hello

I had such a hard time upload the video from my smartphone. I finally decide to use my ipad to re-record. My ipad is out of battery so now I have to charging it. Not thing break but I saw pieces of computer paper already burn but still intact which is big pieces of ashes. Many of them at bottom of heat riser. It's didn't burn good isn't it?

I forgot to ask you what do you mean by enlarge the manifold? What does that mean?

'
I gave up, can't upload video from my smart phone. So took picture. My burn chamber is still all in one piece, but the ashes from computer paper didn't burn completely, and I can still read it. Many pieces of computer paper ashes. Why didn't it burn completely?

Here are the photos.

More photos. A lot more ashes compare to the first time I check it even the first time I burned 2 hours.

Would someone please explain to me, what does that mean by enlarge the manifold?