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How to make your Rocket Stove more "rockety"  RSS feed

 
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Thanks.
 
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chicken fish solar
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Ernie Wisner wrote:no problem thats why Erica and I hang around here.
have fun experimenting.
and yes you will want the same cross sectional area throughout the stove. (8 inch all the way)



So if I use 8" tube the heat riser should be 24" tall? If that is correct is the barrel going to be cut 2" taller? Also what is the maximum length I can run the bench before its counter productive?

Thanks,

I have your book just not clear on this issue as this is going in my greenhouse.


Lisa
 
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The system diameter does not directly relate to the riser height. It is common to make the feed tube height 1 1/2 to 2 times the system size, so 12" to 16" for an 8" system. Using the 1:2:3 or 1:2:4 ratios, this would give 12:24:36, 12:24:48, 16:32:48, or 16:32:64 feed tube to burn tunnel to riser. (Measured along the outside edges of the components.) The burn tunnel can and should be shorter than 32" in most cases. 1:1.5:3 is another ratio I have seen, which would give 16:24:48, quite practical with standard materials. I would not cut the barrel down in any case; the taller the riser is, the better, within reason. Extend the riser to make it 2" below the barrel top.

An 8" J-tube system can ordinarily support up to 50' of horizontal ducting, minus 5' for each 90 degree elbow.
 
Lisa Riordan
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Glenn Herbert wrote:The system diameter does not directly relate to the riser height. It is common to make the feed tube height 1 1/2 to 2 times the system size, so 12" to 16" for an 8" system. Using the 1:2:3 or 1:2:4 ratios, this would give 12:24:36, 12:24:48, 16:32:48, or 16:32:64 feed tube to burn tunnel to riser. (Measured along the outside edges of the components.) The burn tunnel can and should be shorter than 32" in most cases. 1:1.5:3 is another ratio I have seen, which would give 16:24:48, quite practical with standard materials. I would not cut the barrel down in any case; the taller the riser is, the better, within reason. Extend the riser to make it 2" below the barrel top.

An 8" J-tube system can ordinarily support up to 50' of horizontal ducting, minus 5' for each 90 degree elbow.



Wow 50 feet so two 90's would knock it down to 40' feet my greenhouse is 24x16.

So extend the riser to 1.5 or 2. Inches below the top of the drum and not cut it down, okay. So would it be okay to use 8" elbows to construct the j tube or firebricks are better?
 
Glenn Herbert
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"Metal is Doomed!"

You cannot safely use any kind of metal as the permanent core of an RMH, as the temperatures combined with the atmosphere will corrode it sometimes in less than one heating season.

You need to use refractory materials for the core - feed tube, burn tunnel and heat riser - of a J-tube RMH. This can be firebrick with insulation around it, cast refractory cement, or several variations on the theme.
 
Glenn Herbert
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For the mass, you might want to consider the "half-barrel bell" system (or an all-masonry version of it) instead of ducting. The bell reduces friction significantly and pretty much eliminates the use of elbows.
 
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Soooo, I kept coming up with this thread when searching for why my mass rocket heater became sluggish, with lots of back burn and smoke...inside the house! Yuck! We'd been heating with the system for 3 winters, with each consecutive winter, the system's performance declined. I first thought my 22'run with a 180° turn about mid run through cob bench was too long. I spent many hours tunneling an additional exit flue through the 2' thick cob walls. That helped, but didn't solve the problem completely.
I figured I must have a CLOG! I can run a chimney brush through all the exhaust tubes, so I know there's no rat nests choking the system. The only remaining area that could be clogged was between the heat rise and the barrel. The only way to get to that was to cut the top of the barrel off (I didn't foresee needing to access that area when building...these constructing new system's...be forewarned)
So, using a reciprocating saw, I cut out the top of the barrel, leaving a 1/2" lip on the rim. It was sooooo packed with black residue, lofty soot! I duct taped a  paint mixing stick to the end of a length of pvc pipe, and pushed all that junk into the first clean out chamber. All cleaned out, I ordered a disc of 1/4" thick steel to be cut from the local metal shop. A nice thick braid of stove rope on the the ledge remaining on top of the barrel, with the steel disc on top, closed the system back up nicely. Now the true test....fired up, the system works better than ever!
So, had I to do it again, I'd plan for a removable barrel top from the beginning (the steel is much more beautiful than the top of a barrel, by the way) and a fresh air intake...that's next on the list. The cob house is so tight, we have to open the wood cupboard door a crack for the fire to get it's full of oxygen.
 
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Barbara, you've found the culprit at last. But why happened this in the first place? One of the reasons could be the top gap between riser end and barrel lid. This could be just right when the thing was built, until due to the repeated heat cycle the top of the barrel dimpled in, restricting the top gap well below minimum value. The result would be improper combustion and a lower gas velocity in that spot, allowing the soot to accumulate and restricting the top gap even more.

The new lid won't dimple in that quick because it's 1/4" thick and besides that, this replacement lid is on top of the remaining rim and a stove rope. What you've done, apart from cleaning, is widening the top gap above its original value so this clogging up won't happen not as easily anymore. Quite reassuring, don't you think?
 
Barbara Dickinson
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Yes, Peter, I think the extra space above the heat riser absolutely helped increase the draw. I think it's working as well, if not better than when it was brand new. Too bad I didn't have the forethought to build in the removable lid from the start. The steel plate is 100% better than the original thin top of the 50 gallon barrel, both in terms of heat distribution across the top, and in beauty.
Happy Hallowmas!
 
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I'm NEW!  I'm curious about variations on the riser.  I'm thinking I'd like to cast one out of 2 PVC forms from refractory cement.  I don't like the idea of a metal in there because of the temperature and chemical reactions degrading it.  55gallon steel drum would be my choice of barrel I think.

I don't see many people casting their own cement riser, is that because it's not suitable or because it's more difficult?


Also, I have another maybe 'weird' question.  Is anyone ducting the intake?  I have a mother that smokes up the whole house with cigarettes, I'd LOVE to pull air from right above her favorite sitting position to feed the RMH!  I was thinking a 120mm fan pulling air into a round flexi-duct and outputting right at the mouth of the stove.    Maybe a quirky question, but it would make the dad's life much nicer!


and...one last thing.   Anyone using the heat mass to support in-floor heat -or- to pipe water to heat exchangers to move the heat away from the RMH to other rooms?  Circulating air is another black-art when the floorplan is set and no forced air ducts are installed or can be reasonably installed.

 
Glenn Herbert
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Your situation is a textbook example for some people who oppose direct-to-appliance outdoor makeup air venting. The Outdoor Air Myth Exposed
Chimneysweeponline disputed a number of this article's conclusions, but the best reason I think remains just yours: better to use the stale/polluted indoor air to burn and replace it with fresh air for the room.

The small duct & fan pulling air from the extra-polluted zone and delivering it for the stove to use as needed sounds good. I would guesstimate that an 8" RMH would draw in the neighborhood of 50-100 cfm, so a fan/duct combo with that capacity range would be optimal.

A refractory cement riser is quite practical and effective, as long as you use insulating refractory. The main downside is that it is expensive material. You can get good results casting a riser with a perlite-fireclay mixture for a fraction of the cost, and as this area is not exposed to abrasion, it should be quite durable.

In-floor heat has been done, though not commonly, and reported to work well. The main issue here would be integrating with existing construction; new construction can be done as desired with proper planning. It may require a greater drop from combustion zone to duct and thus require a better chimney to assure good draft. Circulating water for remote heating has also been experimented with. This requires some degree of expertise to get right, I would think.
 
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[size=18]When designing these stoves, builders should try to bear in mind that rocket stoves are efficient for TWO basic and combined reasons. [/size] 

First, there is a highly efficient burn, giving maximum heat from a given fuel quantity.   
This is achieved by way of the primary (fire box) , secondary (lower burn tube with re-oxygnation) and tertiary (middle and upper burn tube) burns being almost perfectly reflectively insulated (ideally, with ceramics).   
Also, this insulated  burn process is appropriately (sic) oxygenated .... and re-oxygenated, allowing the process to 'accelerate' and reach a maximised 'almost plasma' stage.  This should involve (NB) an adjustable restricted supply at the firebox to generate as much gas as possible there, and then, again adjustable, preferably pre-heated, re-oxygenation of the second and third stages to maintain the accelerating burn process as temperatures rise and the particulates and gasses are progressively combusted.
Second, these stoves offer maximum heat extraction This is because, uniquely, rocket stoves should, and usually do (very often by accident of design) produce positive exhaust pressure and, therefore, do NOT require a vertical flue.  (This positive pressure is created due to the proportions of the 'throat' design , effectively creating a restriction in the fire path feed to the secondary combustion chamber, such that that the oscillating pressures in the final burn process works as positive 'downstream' pump, exactly the same as a WW2 German V1 or doodlebug engine. )  Anyway, that positive pressure is very useful as it allows the use of a very long, low level, horizontal and serpentine exhaust cum heat exchanger design, such that almost all the heat of the (already very efficient combustion) may then be extracted.   QED.

'Rocket stoves' that do not incorporate effective throat-to-riser proportions, or really effective insulation of the entire burn, or a serpentine exhaust, are all rather missing the point, many unwittingly no doubt, harking back to Biblical technology.  The basic 'rocket burner' design has been used for years in larger tankers, crude oil fed, to very efficiently produce heat (for warming oil tanks etc.) as well as a relatively cool, totally clean and inert exhaust gas for pumping into void and ullage spaces.
 
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Hello everyone!  Just let me say this, I have searched the world over and I cannot find an answer to this question.  I am all but certain you will tell me it's impossible, but, here goes...

I wish to build an 8" batch box style rocket mass heater in my 2000 square foot basement.  I spend a fair amount of time down there and the RMH area will become a family area.  I have a fireplace adjacent to the burn chamber area with a 24 foot chimney that I will outfit with a flue liner.  I would like to design the thermal mass as a bell system, as I believe that will be the least restrictive for the exhaust gas flow.
The downside to my situation is two fold.  The ceiling is only 7 feet high.  Considering that I am an older person and I do not wish to get on my knees to load this thing, I'll need to make the burn chamber floor 2 feet off the floor.
I feel that this will put the top of the barrel dangerously close to the wooden ceiling joists.

I seek a radical design change that will allow me to shrink the height of the riser without degrading its performance.
What I propose is creating a hexagonal cluster of 7 -  4" risers, 27 inches tall.  This cross section should be roughly equal to the 57 inch tall, 8" riser as found in batchrocket.edu. and still fit in a barrel leaving a larger cross section for the exhaust.
While I realize my box may have to shrink somewhat and other tweaking may ensue, my question is....

Has anyone ever built a rocket stove with multiple risers?  And if so, what were the results?
If it failed, can you describe the failure mode.
Thank you for taking the time to read through my crazy.
 
Peter van den Berg
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Thomas Tipton wrote:Has anyone ever built a rocket stove with multiple risers?  And if so, what were the results?
If it failed, can you describe the failure mode.
Thank you for taking the time to read through my crazy.


Around 2009, I built a rocket stove of some sorts with twin risers. Results were meager, one of the risers tended to heat up first while the other was letting unburned gases through. There wasn't a pattern in which side heated up first, it seemed to be completely random. Escaping unburned gases means smoke for extended periods during the burn, not what we want in a properly running rocket heater.

I'd suggest you don't use a barrel but a masonry bell around the combustion core, with lots of insulation in and on the top deck. Further more, the riser could be 46" high without repercussions but there are people who did go down to 35" and still had a properly working batch box rocket. An aluminum radiation shield on spacers above the heater could further add to the safety against scorching rafters.
 
Thomas Tipton
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Thank you Peter.  I don't think I'll find a more qualified opinion than yours, so while I have your attention perhaps you might indulge me on why you attempted the twin riser design.  Were you attempting to lower the riser height requirements as I am doing, or was it for another reason?  Just curiosity perhaps?  I have read posts from others who had considered a slightly inverted conical configuration but I know not if they actually attempted it.

I think you are right about abandoning the barrel and going for a masonry bell.  This idea feels a lot safer, though it does put me back at the drawing board somewhat as I do not yet have a solid understanding of masonry bells and how to build them.  Earlier in my career, I had the experience of a couple years of service to a stone mason who engaged in some very artistic stone work.  We didn't lay bricks much at all, just natural stone, so if I could, I think I'd like to incorporate some of that into my bell.

Thank you again.  It's a great honor to hear from you.


 
Glenn Herbert
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If you have experience laying stone, you will most likely find brick easier and simpler. For "masonry heater" use, per international building code, you must use refractory cement mortar (generally mixed like thin pancake batter, and bricks dipped in and placed immediately). Regular brick mortar is not acceptable as it deteriorates from flue gases. Clay mortar should work, but is not strong and does not meet code. In the US, masonry heater bells are required to be double skinned, with two layers of masonry. This is just for your information; you can follow it or not unless your installation will be inspected. The inner layer is supposed to be firebrick; for safety, you should use firebrick for at least the top third where it will be hottest.
 
Peter van den Berg
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Thomas Tipton wrote:... so while I have your attention perhaps you might indulge me on why you attempted the twin riser design.  Were you attempting to lower the riser height requirements as I am doing, or was it for another reason?  Just curiosity perhaps?


I tried to build one heater which was less deep back to front. Both risers were built exactly the same but one can't regulate uneven rising of temaperature.

Thomas Tipton wrote:Thank you again.  It's a great honor to hear from you.


*Blush*
 
Glenn Herbert
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A masonry bell is about as simple as you can get, just a hollow box of masonry with entrance and exit (especially exit) as low as practical. There are guidelines to sizing a bell, based on internal surface area, not volume, which can be found at batchrocket.eu.

The only potentially tricky part is closing off the top, which can be done various ways, like a cast slab or corbelling in the brickwork.
 
Thomas Tipton
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Glenn Herbert and Peter Van Den Berg have given me much to consider.  I am thinking the reason the multiple risers fail to function properly is because there is no way to prevent air from descending one or more of the other risers in such a system.  Perhaps if they were all fully preheated and then allowed to draw from the fire, they might operate as intended.  That being said, I am sold on the masonry bell.  I was wondering if the constant blast of hot gasses cause deterioration in the bell ceiling.  I thought perhaps it might be beneficial to position a piece of calcium silicate ceramic sheet between the top of the riser and the ceiling, something to take the brunt of the punishment so as to spare the roof of the structure from excess stress.  Just a thought.

I had a rather good size daybed/bench in mind, and I'm not certain how this will match up to the batch rocket bell I end up building, so I'm going to take some time to draw up a sketch of what I had in mind for the bench and where the Rocket Bell will go.
Perhaps you fellows will continue to help me refine my ideas and keep on a workable path. I know every build site presents different opportunities and challenges and I want to exploit my situation for the maximum advantage.

Bottom line, I'd very much like to only have to build this once, have it work well, and have it last for a very long time.  And, might I add, maybe even contribute something to the craft.
 
Glenn Herbert
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I would suggest that you may want to make the tall bell only as large as it needs to be for proper function, and add a bench-style bell to absorb the rest of the heat before sending the exhaust to the chimney.

Batchrocket.eu has some information about bell sizing in relation to core size/power, and there is more elsewhere on the internet.

I think that Peter's observation that his two risers were randomly uneven in output means it is an inherent difficulty. Friction is a minor factor in this case, so there would be nothing inhibiting whichever riser gets a head start from getting more and more hot and rockety, stealing draft from the other one. Long runs of parallel ductwork would have less of this issue, as friction would increase dramatically with increased flow, tending to equalize the competing paths.
 
Thomas Tipton
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I would suggest that you may want to make the tall bell only as large as it needs to be for proper function, and add a bench-style bell to absorb the rest of the heat before sending the exhaust to the chimney. 



I totally agree.  I am thinking my defunct fireplace cavity, since it will be the next highest elevation from the bell, will provide a place for hotter gasses to accumulate and heat the flue liner which will extend very nearly to the floor of the bench bell.  In this way the flue will always be exposed to the hottest temperatures in the bench to help ensure draft.

Long runs of parallel ductwork would have less of this issue, as friction would increase dramatically with increased flow, tending to equalize the competing paths.



Not sure if you are trying to encourage me or not. 

I still think it can be done, and I will probably try.  But I think it's going to be a while before I get around to that riser complex.  Speaking of risers, has anyone ever used shredded mineral wool, spritzed with rigidizer, and gently tamped into a mold to form a riser?  I am thinking of trying this and then coating the inside smooth with a 3000 degree type refractory clay.  I really want this thing to last and I'm pretty sure the jury is still out in terms of a decisive ruling regarding riser efficiency v.s. longevity.  How to get both?

Thoughts?

 
Thomas Tipton
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attempting to upload a file here.
 
Thomas Tipton
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Here's another try at uploading an image.
Filename: Scanned-at-3-12-2018-18-11-PM.bmp
File size: 25 megabytes
 
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Thomas, you will need to convert that image from bitmap to something compressed, such as jpeg or png, to have it show up in a web context. 25 MB is massive.
 
Thomas Tipton
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I was afraid that was the case.  Stay tuned, I'll see what I can do with it tomorrow.  Thanks for the heads up.
 
Thomas Tipton
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How's this?
Filename: BBR-Project.bmp
File size: 2 megabytes
 
Glenn Herbert
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I saved the file as a jpeg and reloaded so it will show on the page:
BBR-Project.jpg
[Thumbnail for BBR-Project.jpg]
 
Thomas Tipton
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Thanks Glenn,
Graphics is not my thing, but  you probably already know that.  I spent some time today working with sketchup so I hope to produce something a little better in the future.  What you can see from my drawing is my basement space with 7 foot ceiling.  The BBR, with insulated top and heat shielding.  Bell Bench leading out of the BBR from the right, where the hottest gasses are allowed to collect up inside the fireplace flue cavity.  This should allow the flue liner to be warmed early on, and, because it is the highest part of the bell, the last area to cool.  The bench continues past the fireplace where it turns 90 degrees to the right and follows the adjacent wall for another ten feet.  I am thinking of constructing the bench by stacking cinder block 2 high, in two rows, with another dividing row of block or brick in the center, filling the cavities with sand.  The center row will allow me to cap it off with patio paving stones and cob, with a top layer of sandstone slabs that I can get as reclaimed old city sidewalk.  Everything, front and back, sealed with cob.  The center row of bricks serves not only as a bridge, but as a channel to keep the gases flowing in one direction.  I have considered using cob to create a slope from the BBR opening, where it is the highest, to where the gases return to the flue liner, that sits as low to the ground as possible.  This slope should ensure that the coldest gases do not accumulate at the beginning of the run, but follow along to the flue.  This might help to insulate the interior of the bench from the cement floor somewhat. 

This drawing is not quite accurate to scale, as the BBR is about three feet closer to the fireplace. Other aspects are pretty close.

I have confidence in Peter's assessment that my ceiling will be safe with a masonry construction.  In the meantime, I'll be confiscating the boy's Legos to figure out how to put it together.

 
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Erica Wisner wrote:  I have never seen one that was an improvement on the brick J-tube, for smokeless performance.  [Edit: As of 2018, I have now seen 2 batch box models built and operated by Peter van den Burg, that did burn amazingly clean.  However, I've also seen other people have major smoke issues using the identical batch box stoves without Peter's meticulous procedural care.]

-Erica



you have not seen my system. My Rocket Mag Heater burns clean, complete and holds hours of wood. I even have videos of my Rocket Mag Heater burning a 10" log completely. my Rocket Mag Heater seems to first vaporize the wood as it burns at the bottom of the magazine next to the burn tunnel as it turns the wood into charcoal. the vaporized gases burn super hot 2000 plus degrees as the fire brick glows orange and then when all the gases burn at super high temps it gets a boost when the charcoal then ignites and burns like a forge as it collapses into the bottom at the end in a complete burn. no smoke, complete burn, large wood magazine that holds large logs. mega improvement. i have a super insulated and decent size heat riser and bell that may make this system suck the warts off a toad. the secret is the air tight top lid and bottom air intake.
 
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Cool
 
Danger, 10,000 volts, very electric .... tiny ad:
One million tiny ads for $25
https://permies.com/t/94684/million-tiny-ads
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