Author Topic: 2 burner cement block rocket stove using L blocks  (Read 6523 times)

Offline Aunt

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2 burner cement block rocket stove using L blocks
« on: May 31, 2014, 04:02:07 PM »
Cement block rocket stove.  Built with 3 cement blocks and 2 L cement blocks.


Start with two cement blocks.  A portion of these blocks will be the base of the burn chamber.


Add the first L block.


Add the second L block.  Place your sticks to burn into the stove in this block.


Add another block on top and a grid for you pan to sit on.


Right now I have two grids from an old stove on top.  Any suggestions for something for the pots to sit on that would fit better and allow good air draw for the stove?  Thanks.

Offline flippydidit

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Re: 2 burner cement block rocket stove using L blocks
« Reply #1 on: June 02, 2014, 05:03:30 PM »
My only concerns are that if the cement blocks have enough moisture content that they might explode when heated that hot.  If cinder blocks are used, I'd imagine they wouldn't last long with such intense heat.  Have you tested this stove to see if there are any issues?

Offline Aunt

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Re: 2 burner cement block rocket stove using L blocks
« Reply #2 on: June 03, 2014, 04:18:44 PM »
I have one that I have used a little.  Never thought about the block possibly exploding.  Guess I will burn some sticks and see what happens.

Originally I found this type of rocket stove on YouTube where they used an H block instead of the two L blocks.  I used the L blocks because they are much easier for me to obtain.

Offline flippydidit

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Re: 2 burner cement block rocket stove using L blocks
« Reply #3 on: June 04, 2014, 06:25:27 PM »
The concern is usually with concrete (as opposed to just cement), and usually in a thicker volume.  The concept is that the moisture is trapped within the concrete and it vaporizes with the high temperature.  That trapped steam violently peaks in pressure and shatters the concrete into shrapnel.  It's why you will almost never find concrete in any kind of "stove" application.  When you do it will almost always involve firebrick, refractory cement, or some other way to keep the heat from exploding the concrete.

I'm very curious (and nervous for your sake) to find out if this thinner cement-only stove will allow the moisture to escape fast enough (or not retain enough) to cause the same problem.

Please be safe!

Offline Aunt

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Re: 2 burner cement block rocket stove using L blocks
« Reply #4 on: June 04, 2014, 06:42:58 PM »
Thanks flippydidit.  Do you think a fire using sticks would get hot enough to make steam?  If I made a stoneware lining for the firebox would that help make it safe?

Offline Coctailer

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Re: 2 burner cement block rocket stove using L blocks
« Reply #5 on: June 04, 2014, 06:47:54 PM »
Subscribed.
Interesting.

Offline Aunt

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Re: 2 burner cement block rocket stove using L blocks
« Reply #6 on: June 04, 2014, 06:53:53 PM »
flippydidit on youtube there are many rocket stove designs using cement block.  So far I have not found a reported problem.

Thank you very muck for pointing out the possible danger.

Offline flippydidit

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Re: 2 burner cement block rocket stove using L blocks
« Reply #7 on: June 04, 2014, 07:08:37 PM »
No problem Aunt.  Keep in mind that I'm in no way trying to speak from the point of expertise in the construction of rocket stoves.  I've only dabbled with it myself.  I'd guess that part of the reason that we don't see any issues is that maybe there aren't any.  Though there are probably a myriad of reasons why that might not be the case either (people in the hospital, never used the stove after building it, etc).  My only concern came from my experience with building firepits with my company (PermEscapes).  For our firepits we use the metal inserts sold by Lowe's or Home Depot.  That has proven to be efficient in deflecting the heat away from the concrete for our installations.

Keep in mind though, that a firepit is not intended to work the same way as a rocket stove.  A rocket stove is designed to use a high temperature chamber to efficiently burn wood near-completely before exiting out the exhaust pipe.  Many of those stoves are generating temperatures between 900-1400 degrees Fahrenheit.  Some more, some less.

If it was my project, I'd consider lining the inside of the stove with refractory cement (at least 1/4" thick).  Of course I'm super paranoid about anything that might go KABLOOEY.  Do you have an IR temperature gun that can read the temperature from a distance?  They are available at Harbor Freight pretty cheap.  That would at least tell you the temperature range that your stove is capable of.

After saying all that, I'd still come back the opinion that it's probably fine as you have it.  The cement-only blocks are pretty porous.  They allow moisture in and out pretty easily when you compare it to an aggregate, sand, cement mix.  Combining that with the fact that the blocks are pretty thin (relatively), it probably isn't a serious concern.  I'm probably just playing Chicken Little here.  I really do like your design and am hoping it works marvelously.  Mostly because I want to copy the design and use it myself.  I've got a cast iron stove with a hairline crack that I "inherited" from a friend.  Fortunately the cooktop is pristine.  I'm hoping that I can repurpose that cooking surface to your rocketstove design.

d3nni5

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Re: 2 burner cement block rocket stove using L blocks
« Reply #8 on: June 04, 2014, 07:31:06 PM »
 :popcorn: :popcorn: :popcorn: :popcorn:

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Offline OutWestTX

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Re: 2 burner cement block rocket stove using L blocks
« Reply #9 on: June 04, 2014, 08:54:45 PM »
Aunt, I built mine using "fire bricks"....you know, the ones that are approved for fireplaces.  You might try those instead.  Mine is also a twin-burner.  We use it all the time in the summer (as long as there isn't a burn ban) because I hate to use heat up the kitchen.  It takes practice to cook over a fire or on a wood burning stove. 

d3nni5

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Re: 2 burner cement block rocket stove using L blocks
« Reply #10 on: June 05, 2014, 06:06:02 AM »
Did you cut or buy your "L" blocks?   I have a few leftover blocks, and think I want to try this too.


Offline Aunt

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Re: 2 burner cement block rocket stove using L blocks
« Reply #11 on: June 05, 2014, 06:23:30 AM »
OutWestTX please post some photos of your rocket stove.  Sounds like an excellent stove.

flippydidit Thank you.  I appreciate your comments!  Right now I do not have an IR gun.  I added it to my list.  But my list, of course, is longer than the number of federal reserve notes that I possess.

Here is a rocket stove that, flippydidit, you could use your stove top with.  http://helpsintl.org/programs/onil-products/onil-product-selection/onil-griddle-stove/  This stove can be used inside and vented to the outside.  When you come up with alternative design ideas please post them.

Mountain State Prepper I purchased the L blocks from a local hardware/building supply store. 

I think a rocket stove using an H block instead of 2 L blocks would be easier to clean the ashes out of.  I will need to disassemble the stove to get the ashes out of the bottom of the burn chamber.  It looks like with an H block the ashes can be swept out without disassembly.  I have not yet tried the H block.  My local hardware/building supply store does not carry H blocks.  They have ordered some for me.  On youtube there are videos of cement block rocket stoves using an H block.