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alternatives to cob?

 
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I'm interested in building a RMH in my garage/workshop area. I'm in an urban setting and I don't have the access to materials to make a cob mass. I have looked online but have been unable to find examples of good alternatives. Thanks for any input you may have.

Dave
 
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Dave Friday : As an experiment, check your yellow pages for an excavation contractor, you want to talk to the guy running the equipment not the secretary who
answers the phone, First thing is to tell him you are looking for clean fill materials.

Since what you want should fit in about 4-6 5gal. buckets you Do Not Want A Truck Load, tell him you are looking to come and get some heavy clay soil yourself !
A couple of phone calls should get you near locations where you can find Clay , You will use enough sand so that you can get it trucked in, but find your clay first
Big Al !
 
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Urban area? How about used bricks? Rather than sculpt your RMH out of cob, if you can score some old solid bricks, you could assemble it LEGO-style. If you want it to be a little more permanent, then you can hold it in place with some refractory mortar (regular mortar with clay added).



There are other YouTube videos of people making quick rocket stoves out of stacked bricks.
 
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How about bags of unscented clay kitty litter?
 
pollinator
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John Elliott wrote:Urban area? How about used bricks? Rather than sculpt your RMH out of cob, if you can score some old solid bricks, you could assemble it LEGO-style. If you want it to be a little more permanent, then you can hold it in place with some refractory mortar (regular mortar with clay added).



I have been thinking along slightly similar lines. In the interest of reducing the cob content of the "mass" section of my RMH (cob is just soooo labor intensive!), I've drafted the following concept. Nothing too original, I'm sure, so I'm hoping that lots of people here will be able to say "yeah, sure, I tried the same and here's what resulted..."

The "mass" bench would consist of an open top masonry box of mortared brick (preferably) or concrete block. I would use refractory mortar and attach pieces of rebar every so often running through the box from side to side in both directions. The rebar's purpose would be to anchor one side of the box to the opposite side, thus providing an internal support structure that would hopefully equal out forces acting on the masonry walls from the weight of the infill, which might otherwise explode the walls outward. The RMH exhaust piping would run back and forth inside the box. Obviously the positioning of the rebars would be chosen so as not to obstruct the path of the exhaust pipe. All of the space inside the box surrounding pipe and rebar would be infilled with gravel. I would use gravel instead of sand as I've read here somewhere (was it Paul's various attempts at a portable RMH?) that sand has already been tried and proves too insulating to let the heat permeate throughout the "mass" bench. The outside and top of the box would be covered with a minimal amount (six inches?) of cob to provide a seamless, smooth, sculptable, and decorative surface.

So, my questions are:

> Has anyone out there already tried this configuration?
> Am I correct about sand being less desirable as infill?
> Might I omit the rebar cross-ties and still build a structurally sound box, or is that asking too much of vertical brick-and-mortar walls?
> The gravel infill will contain a lot more air space than would a monolithic cob "mass," so what should I expect from that? I imagine that the outer surface of the bench will come up to temperature more quickly, since heat inside the mass will flow via convection as well as conduction. I also imagine that, because I am packing in slightly less mass per volume, that I will have to adjust my calculations as to how many degrees of temperature drop to expect per foot of exhaust pipe for a given target temperature of exhaust gas leaving the RMH. Can anyone estimate by how much should I adjust?

A little background on myself: I've watched all of Paul's videos on RMH, read his RMH article, and listened to all podcasts with Ernie and Erica. I just bought Ianto Evans book, but haven't had the chance to read it yet. I've also never had the opportunity to see a real RMH before. So please forgive if any of my questions are hopelessly ill-informed.

Thanks for your input!
 
Matthew Nistico
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I just read the following in another thread:

posted Yesterday 1:14:30 PM by Allen Lumley in the thread 'Heat riser, duct/flue questions'
...All of the stove pipe buried within the R.M.H. / Thermal bench should be considered to be a sacrificial form around which your cob, your Perlite And Clay Slip are placed and a few bricks are stacked. As such we do not care whether they last weeks or years, here we can go with the cheapest grade of thin wall piping we can find, the lighter the better, galvanized pipe having no inherent advantage!



Hmmm, that could represent a fundamentally different way of thinking about RMH construction that might require considerable modification to my draft design...
 
allen lumley
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Dave F. : Are you still with us !?! There have been many good suggestions here already,You can get Bentonite Clay as a fine dry powder, 2 bags would be more than enough to handle the average Rocket Mass Heater, R.M.H.(#),people have substituted unscented kitty litter, or speedy dry very successfully for
wet, sloppy, and locally sourced and dug clay, old bricks are where you find them, the more urban your location the more likely you are to find exactly
what you want.

Perlite (2 bags ), can be found in Builders Supply places, Please make sure you DO NOT GET HORTICULTURAL GRADE PERLITE !

55 gallon drums are where you find them, I stongely recommend the kind with a removable Top Held on with a Clamping Band !

A close friend only had access to old brick chimneys he could tear down, and most of them were bricks fired at high temps and not as useful as dead soft
orange red bricks, in his case, he was looking for enough soft bricks for the Masonry core in a D.I.Y. Masonry heater, he ended up taking down 5 chimneys!

Builders sand will take somewhere around 5 cubic yards, getting that trucked in and spotted at the end of your driveway should present no problems at all!

I personally recommend everyone going to cobcottage.com to download their PDF copy $15.oo of Evans' and Jackson's Great Book 'Rocket Mass
Heaters, there is STILL no other book in any language that contains as much Rocket Stove/Rocket Mass Heater Family information !
( and i don't make a nickel ).

This will save you hundreds of hours worth of research and mistakes, and allow you a clear understanding of the polyglot of jargon that a successful builder
often sprinkles his conversations with !
For the good of the Craft ! be safe, keep warm ! As always, your questions and comments are Solicited, and Welcome Big AL !
 
allen lumley
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John Elliott : This is not a trolling message, I just HATE all the Jreck videos out there in YouTube Land, and This video is worth picking apart because of what we can learn from It !

Any and all Rocket Stove(s) is/are not Rocket Mass Heaters ! Irreguardless of who you give credit to for the Creation of Early types
of Rocket Stoves, they were Created as Improved Cooking stoves to replace the 3-Rock (often indoors) fires 3 Billion People use
for cooking fires around the world !

Properly made a Rocket stove should be Hot, efficient, and smokeless, using a small fraction of the bio-mass fuels a 3-Rock fire
uses ! It can load its fuel vertically or Horizontally, but as designed has a built in primary Air Channel To pre-warm the Feed Air
used for bio- fuel combustion, This entire part of a successful build was totally left out of the Presented U-tube Video, which is
a little like showing someone an Internal Combustion Engine Build and leaving out the Carburetor !

Please consider googling Stove Tec- or The tin can Built Rocket stove- built by LDS prepper.

1) the rocket stove is for cooking (outdoors, weather permitting) and feeding your family !

2) the pocket rocket is for outdoor heating (and will melt blacktop) good for picket lines, preppers, doomsday/Zombi hunters,
and Ice fishermen !

3) The Rocket Mass Heater is for whole house heating and feeding your Dragon, and is always connected by its exhaust pipe
to a large thermal mass, or it is not a R.M.H. !

For The good Of the Craft ! Be Safe! All comments And questions Solicited and Welcome !












 
allen lumley
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Matthew Nistco :As the mortar used to assemble the bricks is actually used to hold the bricks (universally ) apart from each other, we can
use a Lime mortar, rather than a Portland cement based mortar. Lime based mortar cleans off of old bricks easily, and holds up to the
Rocket Mass Heaters R.M.H.s, high Temperatures with out failure much better than the Portland.

Again, if you are strongly thinking about a R.M.H. in your future, look at my comment above, and get your copy of Rocket Mass Heaters at
cobcottage.com
 
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hey guys and gals. kitty litter is not a good choice and nor is any bentonite clay unless you have very carefully worked with it before. its absorption of water is huge and you have to know how to deal with the huge shrinkage as it dries. use an earthen mortar rather than lime because lime and cement burn out. sand and clay do not.

alternatives are brick, stone, and such, cement is not a good idea as it tends to burn out. there are lots of ways to make an RMH look good, including A nice plaster job. if you are thinking the cob cannot take the abuse then build the mass and ducting under your work benches (often this is not used for any real need.)
or if you are short duct it under your floor so you get the benefit of the heat and the cushion of the cob, build it next to the wall and make it look like a wall, Or since it is a work shop and has wood scrap you would do better with a simple pocket rocket that will burn up you waste wood and heat the place.
if you just use the stove when you use the shop you probably want the quick heat option.
hope this helped
Ernie
 
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allen lumley wrote:Matthew Nistco :As the mortar used to assemble the bricks is actually used to hold the bricks (universally ) apart from each other, we can
use a Lime mortar, rather than a Portland cement based mortar. Lime based mortar cleans off of old bricks easily, and holds up to the
Rocket Mass Heaters R.M.H.s, high Temperatures with out failure much better than the Portland.

Again, if you are strongly thinking about a R.M.H. in your future, look at my comment above, and get your copy of Rocket Mass Heaters at
cobcottage.com



The mortar performs two functions, it acts as a gap filler for the irregular nature of the brick surfaces,and for an oven, fire box, or flue should be installed with as small an exposure to the flame as possible, the. Other function of the mortar is to provide stress relief, mortar should always be weaker than the bricks it is used with.

A completely lime based mortar is not waterproof until it has aged by fire or time (chemical change). A mixture by volume 3-1-1-1 of fine sand, fireclay, hydrated lime, and Portland cement will be waterproof and stable at high temps. Yes the Portland will burn out at some point but it will keep the mortar waterproof and stable until the lime takes over as the binder.

Chip
 
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Chip Friedline wrote:

allen lumley wrote:Matthew Nistco :

A completely lime based mortar is not waterproof until it has aged by fire or time (chemical change). A mixture by volume 3-1-1-1 of fine sand, fireclay, hydrated lime, and Portland cement will be waterproof and stable at high temps. Yes the Portland will burn out at some point but it will keep the mortar waterproof and stable until the lime takes over as the binder.

Chip



Would you say that your 3-1-1-1 mixture is a good cob alternative for use in a greenhouse? I am building a RMH in a 10x26 greenhouse, the mass will be the soil, I need a suitable substitute to cover the exposed feeding tube and would rather not use cob as the environment will have a high RH value.

 
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I have been buying solid cement blocks that are small enough for me to carry down a flight of steps to the basement where my build will be going on.  I'll use these only for the pedestal the burn chamber will go on and for the bench.  Not sure about today's prices, but last year I could get these at about $70 per ton.
 
Matthew Nistico
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Katherine Burelle wrote:

Chip Friedline wrote:
A completely lime based mortar is not waterproof until it has aged by fire or time (chemical change). A mixture by volume 3-1-1-1 of fine sand, fireclay, hydrated lime, and Portland cement will be waterproof and stable at high temps. Yes the Portland will burn out at some point but it will keep the mortar waterproof and stable until the lime takes over as the binder.


Would you say that your 3-1-1-1 mixture is a good cob alternative for use in a greenhouse? I am building a RMH in a 10x26 greenhouse, the mass will be the soil, I need a suitable substitute to cover the exposed feeding tube and would rather not use cob as the environment will have a high RH value.


I can't really answer your question, and I don't know if Chip will even see it, 8 years later.  I would point out two things, however:

1) What was the point of Chip's comment in the first place?  Why should I need to be concerned with how waterproof is the mortar on my RMH?  It is an interior appliance.

2) See Erica's comment above on strictly avoiding lime or Portland cement in an RMH, in favor of clay/sand mortar or just monolithic clay/sand (i.e. cob).  Anything from Ernie or Erica I would take as The expert opinion when it comes to RMH tech!  Doesn't mean there isn't room for different ways of doing things, but she sounded pretty definite about the dangers of these materials in an RMH build.
 
Matthew Nistico
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allen lumley wrote:Since what you want should fit in about 4-6 5gal. buckets you Do Not Want A Truck Load...

Perlite (2 bags ), can be found in Builders Supply places...

Builders sand will take somewhere around 5 cubic yards...


I am slightly concerned with the impression given here regarding materials.  With the perlite, I'm not sure where he intended that to be used, so I won't comment.  The 5 cubic yards of sand, that sounds about right.  But talking about gathering in-fill rubble (or at least I think he was) in just a few 5 gallon buckets...?

In the 8 years since this thread started, I have actually participated in an RMH build, and my take-away was that you cannot underestimate how truly massive it is, and how much material you will need!  We used many many 5 gallon buckets worth of in-fill rubble, and could have used twice as much.
 
Matthew Nistico
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I would also like to post an addendum to my original lengthy post, above, of 8 years ago.  Having since been part of an RMH build, I don't think I would do it in the future for myself quite like I imagined it in the past.  I would likely stick to a monolithic cob design, albeit one with a LOT of large rubble in-fill in order to minimize cob volume, and NOT the gravel filled masonry box I had previously proposed.

For one, the heater we built didn't use metal flu pipe snaking back and forth inside the mass bench.  I had previously thought this was the only way one plumbed an RMH mass.  Instead, it used a single, "half-dome," self-stratifying exhaust chamber inside the bench, on the ground, running the full length of the bench.  It was composed of two or three halves of 55 gallon metal drum - I forget whether two or three, and in any case you could extend as necessary to meet the dimensions of your heater and your room - laid cut side down, end to end, with vertical walls remaining only at the two far ends.  Combustion gasses from the heat riser entered at the near end, filling the chamber.  At the the far end, an open flu pipe collected the now-somewhat-cooled gases, and piped them back the length of the chamber to exit into the base of the chimney.  This flu pipe ran on the floor inside the larger half-dome chamber.

Not sure if that is easy to picture from my description.  But it impressed me as an easy way to plumb an RMH mass.  I should mention that this was an Uncle-Mud build, and he knows his stuff when it comes to RMH tech, so I'm sure he had good reasons for choosing this design.  Some of those reasons may have been site-specific; I couldn't say.

However, I don't see how this type of plumbing would be easily compatible with my gravel-filled masonry box design.  Moreover, while my old design was focused on ease and speed of construction, I think perhaps it would have a hard time achieving the solidity to serve as a functional bed or bench or whatever.  Not sure.  But not sure I'd be willing to chance it, either.
 
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Paul really likes the "pebble style" as he calls it. It has several features which may or may not be advantages depending on the situation. It is often quicker to construct than a solid cob mass; it heats and cools a bit quicker than solid cob; it tends to be a bit lighter for a given size mass; it is easier to deconstruct and modify or move if desired.

All these features may be positive or negative depending on conditions. He likes to make a wooden frame to hold the pebbles, with venting so air can move through the pebble bed, and often a loose stone slab top.

The bell type heat exchanger you participated in (a "half-barrel bell" to be precise), or any other mass that consists of a large hollow box, is generally easier to build than the original duct-in-cob style, and has much less friction so draft may be better.
 
Matthew Nistico
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Glenn Herbert wrote:The bell type heat exchanger you participated in (a "half-barrel bell" to be precise), or any other mass that consists of a large hollow box, is generally easier to build than the original duct-in-cob style, and has much less friction so draft may be better.



Yes, that's what it was called!  Thanks, I'd forgotten that.
 
Katherine Burelle
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Matthew Nistico wrote:

Katherine Burelle wrote:

Chip Friedline wrote:
A completely lime based mortar is not waterproof until it has aged by fire or time (chemical change). A mixture by volume 3-1-1-1 of fine sand, fireclay, hydrated lime, and Portland cement will be waterproof and stable at high temps. Yes the Portland will burn out at some point but it will keep the mortar waterproof and stable until the lime takes over as the binder.


Would you say that your 3-1-1-1 mixture is a good cob alternative for use in a greenhouse? I am building a RMH in a 10x26 greenhouse, the mass will be the soil, I need a suitable substitute to cover the exposed feeding tube and would rather not use cob as the environment will have a high RH value.


I can't really answer your question, and I don't know if Chip will even see it, 8 years later.  I would point out two things, however:

1) What was the point of Chip's comment in the first place?  Why should I need to be concerned with how waterproof is the mortar on my RMH?  It is an interior appliance.

2) See Erica's comment above on strictly avoiding lime or Portland cement in an RMH, in favor of clay/sand mortar or just monolithic clay/sand (i.e. cob).  Anything from Ernie or Erica I would take as The expert opinion when it comes to RMH tech!  Doesn't mean there isn't room for different ways of doing things, but she sounded pretty definite about the dangers of these materials in an RMH build.



Thank you Matthew I will reach out to them
 
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