Rocket Stove project

Started by glenn-k, December 26, 2006, 02:49:51 AM

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I got started on the rocket stove today. Best is fire brick but I didn't have any so I did the heavy steel alternative.  Its experimental so we'll see what happens.  I'm building from the plans in Ianto Evans book, "Rocket Mass Heaters."  

Here is what I got  so far..

Feed tube, burn tunnel and heat riser.

This shows the bottom half of the tin retainer for the insulation.  Not good to use galvanized but I'll hold my breath for the first few burns. It's internal so should go outside anyway.  Behind it is the heat exchanger barrel.

I put it inside - now for insulation options - I think maybe clay and sawdust to make insulating clay -- we'll see what happens.  Gotta study more.


Don't remember if it's vermiculite or perlite (similar, expanded rock stuff).

Is that what is recommended for insulation?

Might have to--oh-my-gosh--buy some.

Be sure and get Mason's type, not Gardener's.  The latter, I'm told is open cell, and sogs up badly if you mix it with cob.

Those things go up to 1500 (but only F) degrees pretty easily.

Don't think you have to take as much in the way of care for earth ovens as the rocket stoves.

I want one.  And I'm glad to see you are motivated to build one.


I know this is one of your favorite projects.

Don't tease me about being a tightwad, Amanda. :)  You know I would never spend real money on a project.  I think I will buy some stove pipe instead of using well casing though.  Easier to change and I'm not ready to do the cob yet.  Trying to work this into the Roman bath as heat storage.

Perlite, vermiculite, wood ash but also clay and sawdust.  I have sawdust -- fine sawdust as they call for. :)  Also a bag of fire clay I bought several years ago.  The sawdust burns out leaving insulated clay - KiKo did some and said it was good until he rebuild.  I was hoping it would hold up against the fire in case the pipe burns out.  Also thinking of dropping a stainless steel liner in the riser as the pipe I used is oversized and would take it.  Experimenting is always fun --- sometimes it's fun anyway. :)


Lots of action over here. I go away for a couple of days and Glenn has built a rocket stove and made a movie.

(Glenn - I use Sony's Vegas Movies Studio - understand I'm no expert! - but it does allow you to do a narration track and replace the audio after you've assembled the video tracks. Might cut down the cacophony  :) a bit.)


This movie program will do a bit more too - It was the one that came with the computer and I never tried it before --- I hope to improve the next one -- maybe even get rid of the that automatically composed one - or de-composed one. :-/

I read the instructions after I did it and there are ways to improve things quite a bit.  Always time to do it over -never time to do it right.  (Or read the instructions ahead of time).  



You got me to start playing with video.  ::)

I took a few short videos with my Nikon Coolpix S3 while on a beach walk and produced this using Movie Studio to add titles. I found that reducing the quality (compressing) to about 25% of the normal WMV file gives decent download times and OK (for web) video. - this on my site. - the same video at Youtube

I've also uploaded another short video to Youtube (this was a straight upload from the camera - no edits):


See - I'm good for something -- got you to play with videos. :)

I didn't think about  You Tube -- the whole reason I went to the choppy edit program was because I was about 28 seconds too long for Photobucket.

Well - checking that out I still have a problem - It is to large a file - so start from scratch and do it right I guess.  Photobucket is 5 minute limit(or pay extra)  You Tube is 10 min -100 mb -I was over 100 mb


Can you save it in another format? I had the option of saving in WMV formats (varying compressions) but any of them reduce the file size considerably. Seems like Youtube isn't too fussy about the format you upload from. See if you can compress into a tighter format and then try to upload. (The beach-foam-256.wmv was 92 mb.)


I'll have to give that a try.  It will have to be with the edit program as my camera puts out Quicktime only.

Now - progress on rocket stove testing continues.  Here is a test of the sawdust- fire clay insulation mixture.  I used 10 fine sawdust 5 wood ashes w/ small pieces of charcoal as from the stove, 5 fireclay - mortar clay it is called -$5.08 for 50 lbs dry powder for masons.

Now before anybody asks, no I do not have a dog and no-- he was not eating asbestos.

The mixture held up remarkably well even under the torch flame.  This may not be the best insulation but it was free to cheap with the added clay.

Next a test of the partially assembled rocket stove - to start drying the insulation and for the fun of it- note that the barrel heat exchanger is not on and the heat riser is put temporarily into the stove pipe.

I'm sure you are wondering what the square hole is on the stove's bottom.  That's it's ash hole. :)


That is very similar to the model I used on my rocket stove hot tub heater.  You will see the same thing above but with an added feed tube for the wood to stand vertically in so they are a bit self feeding.  Fire is lit on the bottom.  Air going through the fire tunnel is preheated to be hot enough to burn even the higher temperature gasses that normally go up the chimney in a chunk stove.  Burning from the bottom prevents over-fueling.  Flames do not pass over the unburned chunk of wood forcing unburned gasses to escape. In place of the pot of beans is an inverted barrel when it is finished encouraging the stove to give up some of its heat to the barrel providing radiant heat.  Since you are forcing the heat down it requires a primer to start the draft up the chimney.  I'll get to that part soon after this part is well tested.

Note that when this thing is hot, the flue gases are nearly breathable similar to the heat coming off a drier although not as wet.  I added quite a bit of water to my insulation mix to get the clay to cover well so it will take a bit to burn efficiently - after things dry- but is burning remarkably clean right now even in it's unfinished state.


Insulation continues - mixed up about 6 wheel barrow loads - probably about 1/2 yard of sawdust or more.

Fire is progressing nicely and insulation is to the top of the riser now.  I made the outside of the insulation retainer from scrap flashing and welded spacers to the outside of the pipe riser.

The insulation is steaming and water in it is boiling now. Fire tunnel is mostly insulated now.

Sassy still doesn't think it is beautiful yet. :-/


It does seem to work - even without it having near the capability for heating it will have when it's finished.  It will look nice in the living room once we get the cob bench built - sorta like on the cover of Ianto Evans' Rocket Mass Heaters and the stairway behind it.  I'm happy we're getting to the "great room"  :)

Image from


Work continues on the rocket stove.  

I made a tin collector for the heat from the barrel heat exchanger.  Hot gasses flow through the heat riser in the center of the insulation then down the outside of the insulation between it and the barrel.  From there they are pushed and pulled through the collector and go into the decking channels under the cob bench.  They then go past the primer and up the chimney to the outside.  

Today we cooked a garbage can lid oven pizza.  I burned it of course but we did it.  Temperature in the center got to over 450 degrees.  Note that you can see the deep decking I used for a heat channel, going to the stovepipe.  I used 2 inches of crushed rock covered with a piece of corrugated metal under the decking to give a thermal break from the ground.

We got a start on the cob bench tonight -got it to about 10 inches high -- I smoothed it out in case I don't get back to it for a bit.  Note the water distilling from the insulation dripping on the bank behind the stove pipe.  Even with all the moisture and cold mud there was nearly no smoke.


With everything cold and wet it took a bit of coaxing to get the rocket going yesterday.  I have also noticed that high pressure build up on the side of the house causes wind to want to go down the chimney sometimes.  I guess I'll have to get it extended and get a directional weather cap that changes with the wind to help that out.  

Since heat is absorbed by the bench, exit temps are a bit low to get it started easily but that should improve as it heats up the mass.

I'll give it a try today and see if conditions have improved. :)


Talk about a difference! :o

This morning the heat exchanger barrel and part of the cob was still warm.  A lit piece of paper indicated a slight draft at the feed tube, so I lit some more paper and kindling and it started up without even priming.  

This also tells me that a bit of the stored heat was going up the chimney, but the cap I saved for the feed tube can should stop that problem.  It will be put on after a burn and heat is stored in the bench. :)


Most interesting Glenn. A neat invention.

Would you like me to draw up a diagram showing how it was built? We could post it here or as a PDF download.

If you could send me some sketches we could work this up into a usable plan what with your photos.

Then, after a year or so, we can update it with what you would do different if you were to build it again. ;D


I'll try to sketch something up in Paint and send it to you soon.  

Working great all day today- cob is drying and shrinking - I may make it higher for seating height then let it dry again and crack then grout the cracks.  

I went to the store today and bought 2 pizza's - frozen - room for one in the freezer so had to cook the other.  Raised the garbage can lid about 4 Inches with a sheet metal strip on edge curved to fit the radius.  Put a stone pizza pan on the top to spread the heat - temp was around 325 to 350 at the edge.  About 22 minutes and the pizza was done - a bit on the crisp side but I'm just learning.  I'd say it could be considered a success this time.  3 pieces of pizza and I am getting quite bloated. :-/  

The sheet metal ducting is my own variation of a cob duct - easy and fast then can be cobbed over without worry of blocking pathways.


I'd really like to see those plans, you guys.  Glenn, thank you for all the experimenting you do, and then for telling us all about it!  I'd sure like to actually see the stove -- haven't quite figured out how you are cooking on (or in?) it.  I have been thinking about building something like that in our attached garage for emergency heat, since Grandma doesn't want a wood stove in the house.



The model in the cover picture on the book is sitting on the earthen (flue) portion of the heater.  In front of her there is the rocket stove part.  Looks like the lower right is the feed tube and burn tunnel, and the heat riser is the 55 gallon drum.  That part should get very hot indeed when it fires.  

Links are most likely to be under "heated cob bench" or "rocket stove".   "Rocket Mass Heater" is the most current description, as well as the name of the expanded version of the book.

This violates a lot of principles of making/installing/using a wood stove.  The hot air goes down around the heat riser and flows up to 25 feet horizontally.  Then it goes up.  Because you burn your wood both hot and fast, the wood (and the wood-gas) is much more likely to burn completely (to nearly all CO2 and water) than most stove/fireplaces.  So, I'm told you only feel slightly warm, damp but clean-smelling air coming out of the chimney if you stand above it while it is firing.

Ocean, the reviewer quoted at, who has worked on a handful of these, believes that this really is a "study the book" before you try it.  But since everyone who hears about them loves them, I would think there are at least a hundred of them now.  Probably some have failed to work properly.  One of Ianto's didn't have enough cob on top so it singed the cushions there.

(I put a link on the small house heater thread--been there for a long time  ;) )

Here's a link I just found--a handful of pictures, including a rather squished looking one of Ianto Evans lounging on his, elbow on the big drum.


The top center of the barrel is where the heat riser hits, as Amanda mentioned.  We measured over 500 degrees with the oven thermometer laying flat.  Cook on top the drum just like you would a wood stove burner - top center is hottest.  I put a round stone pizza plate on top - made a 4" high shield and put a cover -(garbage can lid) over it to make a little oven.  This cooked a pizza in less than the recommended time so it was not quite browned enough on top.  I think a stone cover on top heated properly before cooking would do a little better-- maybe slate on the lid?

I made toast on it this morning -- faster than a toaster.


Thanks for the links and comments. Thanks for the sketch Glenn. I will send it back with some comments and questions.


Another thing I find about this -- it does need attention every 1/2 to 3 hours depending on what you are burning.  Great for a pyromaniac who likes to fiddle with the fire - for less fiddling, smooth hardwood a couple feet long is good.  A hang up with softwood may put a bit of smoke into the room as fire starts to creep up the vertical wood- especially under low fire conditions, but not often.  A brick in the wood container that can be moved can limit the opening to make plenty of draft to suck the fire down the opening but still be adjustable.


I hope nobody minds, but I broke this off from the underground house thread and started a new "Rocket Stove" topic for this thread.



I think you have a pretty good handle on it, Amanda. :)