Rocket Stoves

My First Rocket Stove Fabrication!

Hello! I have completed my First Prototype Rocket Stove. It is capable of heating a 2000 square foot home and burns about 1.25 pounds of wood pellet fuel an hour. It is designed to run for hours at a time without fussing with it and requires no electricity to operate. It has unique feature that uses supersaturated steam to help with a cleaner burn and regulate the intense burn chamber temperature. The refractory acts as a mini thermal mass and stays warm for about 3 hours after the pellets burn off. The stove is started up in the morning using a propane torch and is shut down at nighttime. This stove has run for over two years now. I originally posted the build on Instructables for a metal build contest and it got 10k views in the month I had it on their site. I decided to take it down because I didn’t feel it was the appropriate site. I know that wood pellet isn’t exactly “salt of the earth “ homesteading but it is using a renewable resource. One would have to make a larger burn chamber for split wood.

That is pretty cool. Looks like something out of a steampunk novel.

Thank you! Pink is our favorite color!  The addition of steam morphed the design into a steam punk look.

Peter,    I agree with Daniel, totally steampunk!...I could see "Doc" from Back to the Future running it.  I would love to see the inner workings on this stove. Do you have it re-posted somewhere on permies?  Thanks Gerry

Here is a diagram of the “general idea “. Basically it is a standard J tube construction where there are ash trays below each 90 degree bend. The hardest part of the design is the delicate balance where embers fall to the secondary burn chamber ( the stainless steel pot and lid) Remember, you have to utilize the coals to add to the heat input so they become part of the heat equation. I originally called this rocket stove “Frankenstove” as the refractory was cast in a box holding it all together with threaded rod.

Impressed...

I would be very interested to know if you have the PDF of this when it was on instructables...

I have been trying to get an automatic feed for wood chips, but have not found a working design as of yet...

Thanks for sharing.

Thank you for the diagram Peter  
A few questions for you....

1) The Primary air cover...can't quite make out what it is made from? Is it adjustable for different burn rates?...or does the primary air come through those slits?
2) "It has unique feature that uses supersaturated steam to help with a cleaner burn and regulate the intense burn chamber temperature." Here's what I see: The brownish upside down bottle is the source of the liquid (water?) which keeps the stainless flask topped up. The liquid in the flask gets heated through the side contact with the stove. The steam produced travels through the copper pipe which then looks like it penetrates through the stove right after the burn chamber. So if any of this is correct, I'm at a loss as to how cooling down the fire with steam produces a cleaner burn? Could you elaborate?
3) What are you using as a burn pot?  (The cage that holds the pellets while their burning)

Hello Gerry,

I hope these answers satisfy your questions.

A1) The Primary cover is a repurposed glass pen holder with holes thru it that just so happen to give optimal burn and air mix that helps keep the burn chamber clear of ash (ash ends up being pushed down into the stainless steel burn pan directly under the primary burn chamber. There is no air adjustment per say, but I have found that sliding the pellet basket forward or backward in the burn chamber does allow for some heat output adjustment. Designing the pellet basket to allow for just the right size of ember to fall thru the grate down into the stainless burn pot.

A2) steam addition to a combustion chamber is common in industrial incinerators and where temperatures are high enough, the vapor dissociates into free radicals to react with unburined fuels facilitating a cleaner burn fort her down the burn chamber. Steam also displaces nitrogen in the secondary air which helps lowers the NOx  issues. I had a chemical engineer explain all this to me in fascinating detail but what I got out of it was; if the flame is hot enough superheated steam helps protect the burn chamber going in and further down the stream the water dissociates and reacts with all the unburined vapours. That is all I can explain, other than it works! Below are two versions of pellet baskets I have made. Each has a variation on it to allow burning of the pellets. The basket on the left is a fall  and the one on the right is used in winter.  It’s the difference in spacing of the grates in the picture below

A3). The burn pot is a square metal tube that slides into the burn chamber. Top is cut open to allow pellets to fall into it. The bottom has a series of metal rods to allow embers to fall down into the secondary burner / ash bucket. Making the burn pot is one of the trial and error things that anyone building a pellet stove will have trouble wrestle with.

Thank you for your explanations Peter.

Hello fellow Permies!  

Well, it is bittersweet feelings that I add to my post on Rocket Stoves.

As I mentioned in the beginning of this thread, my prototype Rocket Stove / Thermal Mass Heater powered by sustainable wood pellets has finally outlived its purpose and has been scrapped.

There is a sadness to see the decommissioning of the Rocket Stove that I will always fondly refer to as “Frankenstove”. However, this stove had taught me a lot about Rocket Stove operation and I will use some parts to manufacture a “Mark 2” version that will resolve some of the issues of the prototype “Frankenstove”.

So without further wording, here is the “gently demolished” remains of “Frankenstove”.

So, my fellow Permies, you can see the above photo shows my attempt to take apart a cast cell of refractory material. You should note that there is a clean fault at the back of the cube. This crack happened the first time I fired up the stove. It takes time for refractory cement to cure and unfortunately, I did not wait long enough. I think I waited less than a week before I grew impatient and fired it up. I used high heat silicone to seal the crack after I noticed and it held for the entire three years of operation so I’m impressed with the red silicone.
The next thing I would like to comment on was the extreme heat that the 3” riser had to endure. The riser was fabricated from automotive muffler pipe and was held in place with “L” brackets that were also welded to the metal pipe that held the form for the refractory cube of the burn chamber / thermal mass. The pipe riser became very brittle and you can see the jagged break on the bell housing. Which was also welded to the riser.  I think the continuous expansion and contraction of the cold to hot back to cold cycle every day for three fall, winter and spring cycles were just too much for Frankenstove. It could also be that  the part of the metal riser in the top bell housing was not insulated and that caused there to be heat stresswhere the bell housing was welded to the riser. Not exactly sure, just an observation.
I would also like to point out that the thermometer usually read an operating temp of 600 degrees Fahrenheit so I think that there was insufficient bell surface area to dissipate the heat from the riser.
Ok, so these are my observations on a Rocket Stove that I built which ran well and kept our 2000 square foot two story house cosy at roughly one pound of wood pellets per hour running 10 to 12 hours during the day and shut down at bedtime. I am very happy with the performance of this design and will consider a few modifications for a second design. I will capitalize on using the proven pellet burner and secondary ash burner but will discard the glass ash jar immediately below the riser. I will let you know how that turns out. I will also continue to pursue the steam addition as I think the temps are high enough to allow steam reformation to react with any free carbon in the combustion stream. I know that many will argue the effectiveness of steam addition but it appears to work with my design.

I loves me a good postmortem, Peter. Eagerly awaiting the next build and I hope you post early and often as it happens.

Where in the world are you? How cold are your winters?

Hello, thank you for your interest!

I live on an island on the West coast of Canada. Our weather here is much like that of Seattle, Washington. Basically it is damp, rainy and cold from October until April every year with light snowfalls December and January.

The pellet stove runs about 10 to 12 hours a day and the thermal mass still generates heat for a couple of hours afterwards. The refractory block is still slightly warm to the touch in the morning though.

The pellet feed and burner assembly work very well and can go hours without flaming out or pellet feed jams so when it comes time to make the “Mark 2”, I will probably salvage the burner assembly from the prototype. That way it can serve as a benchmark.

Changes I see for the “Mark 2” will include:
1) a more sleek look for the refractory burner and heat bell.
2) a better secondary ash / secondary air system
3) a way to “change out” burner tubes
4) a better stand where ash clean up is easier
5) a vertical pellet feed
6) a larger pellet hopper
7) better aesthetics
8) addition of a Pink Salt Block as a decorative thermal mass ( a want, not a need)

The steam idea might work, as things like cavitation only work with liquids.

So sorry your Frankenstove died ... Nice autopsy and analysis :) I look forward to seeing your mark 2! :D

Hello Ted,

I have carefully been monitoring the burn characteristics of this pellet fed Rocket Stove and I have found that it needs the additional moisture to burn clean. I have found that if I let the stainless steel water bottle boil dry the flue gas has wisps of black smoke which is in burned fuel. It also has a hint of an acrid smell. When I fill the container and reservoir and it reaches the slow boil to feed the secondary air behind the flame, the smoke and smell cease and the clear exhaust returns.

According to the label on the wood pellets I purchased, the moisture guarantee is around 6%. Now,  I have no way to accurately figure out if this is true but I have researched that wood that is too dry is also problematic for combustion. If you go to the website woodheat.org, the ideal moisture content for wood is 15 to 20%. This gives me reason to believe that moisture is actually necessary to feed the secondary air at atmospheric pressure.

Remember that this stove operates without any electric input ( the circulation fan is not necessary) so the stove draws only what it needs by draft alone.

Again, these are observations from my prototype stove of my own design where I have combined many designs off the internet and then “bastardized them” with my own ideas and experiences. I would like to encourage anyone that decides to build such a stove to glean what I have figured but I am certainly not interested in arguing the theories.
What I have built, this particular design, burns just slightly over a pound of wood pellets an hour and heats a 2,000 square foot home without having to babysit it. We originally had electrical heat and according to our money grubbing electricity utilities company, we have documented our power consumption is down 33% compared to when we didn’t have the stove.
I have put these ideas on this site to help those that want to pursue an off-grid lifestyle and whose mindset is open to unconventional thinking. I have had the fortune of working around engineers whose sole occupation was to figure out how to incinerate nasty chemicals so they wouldn’t draw attention of the neighbors with sensitive noses!

hello Fellow Permies!

I spent some time over the past few days reading some posts on this site and I have to admit that I actually have a design flaw in my Rocket Stove.  That flaw is a Metal Riser. Now, I did not know that a metal riser was technically a "no-no" in the design and I did so out out of ignorance! That being said, my observations of the compromised riser in the autopsy of  Frankenstove. This does bear witness to the extreme heat generated in a Rocket Stoves and the need for high heat resistant materials to be used to prevent this from happening to your creation is necessary. So, if you are contemplating building something along the lines that I have, use something non metal or after about three years of operation, you will have the same issues I have had.  Now, here is the kicker,  I have made reference to a "Mark 2" design  with a list of things I would like to change from the original Frankenstove prototype.  Well, that stove does exist and it is currently heating our home and actually doing a better job than the original design.  The problem is, I overlooked the issue that this design utilizes a metal riser like the original; which does not qualify it to be a Rocket Stove by definition.

So, basically I have a pellet fed wood stove that makes a rocket noise as it burns but it is not a Rocket Stove. For that error in ignorance, I apologize and I ask for forgiveness among the permies community. I do, however, maintain that there is merit in the design of my stove and the next variant will incorporate a cast refractory riser to satisfy the full definition of a Rocket Stove. As we are all here to help each other to lessen our need of the "Conventional and Convenient Grid" it is also important to notify each other when we have overlooked a design parameter that may cause someone else grief { or catastrophic failure } down the road.  For those of you that have expressed a wish to see the Mark 2 please standby as I am preparing a write up with an instruction to use a refractory riser as opposed to a metal one. If we do not learn from History, we are destined to repeat it!

Thank you for reading my post

Perhaps you might want to consider a ceramic fibre riser, more efficient in many ways and easier to build than a refractory riser.
They are commonly termed “5 minute risers” due to ease of construction.

OK then, all is not lost.  Where do I find this "Ceramic Fibre Riser " material.... Does it come in a can? Can I apply it to the inside diameter of my riser tube in situ? Please help!

Hey Peter;
Here is a link to my post on ceramic blanket. https://permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket
Truly a pleasure to work with and it works outstanding!

There are many types of ceramic fibre blanket available but Morgan seems to be the only manufacturer that claims to have low biopersistence.  This means that if you breathe the fibres into your lungs they eventually dissolve.  Whereas persistent ceramic fibres just stay in the lung and potentially cause inflammatory lung disease.  There's no good evidence yet that any of them act like asbestos fibers though caution is advised.

Their HT product is rated to 1300 C.

http://www.morganthermalceramics.com/media/1141/english-superwool_brochure_single_pages.pdf

Thank you Graham,

I will keep the ceramic blanket route for another stove. Sadly my current "Mark 2" has the same 3' riser so I am investigating a ceramic refractory spray like the kind that is used in Automotive Manifolds to prevent the metal muffler pipe from becoming ruined.  Currently the Mark 2 is operating better than expected with lower bell temperatures due to the more than double surface area and the consumption of pellets is just around a pound per hour. So at $8 / bag for a 40 pound bag, I am heating my home for less than $0.20 an hour or less than $2.50 a day. That's still a win in my pocketbook

frankenstove sounds vaguely familliar somehow.  cool build, very steampunk indeed.  i've never seen a 3" riser before!  although i do like the exhaust pipe riser.  one of my recent stoves uses a 5" diesel stack lol.  3" seems kinda small for a rocket riser but if it heats 2000 sq ft then something is working!  my first RMH hybrid burns stick but also pellets unassisted.  it's nice to be able to burn pellets if you dont want to split the wood, or other way around if you dont want to buy pellets :)  im in ontario canada :D

Greetings John,

The 3” riser was used to maintain a continuous cross sectional area from the exit of the burner tube to the top of the riser. If you take a closer look at the refractory autopsy you will see an intense heat discoloration at the 90* bend where the combustion gas changes direction. I have spent a fair amount of time and lost a bit of sleep trying to figure out a way to prevent this from happening in my ‘Mark II” version. I know that it is necessary to have a 90* bend for a Rocket Stove to operate but having this localized heat probably lead to the demise of “Frankenstove “  by  constant thermal cycling every startup and shutdown every day for the entire heating season here on Vancouver Island ( October to April) ish.  I know I have alluded to this “Mark II” Rocket Stove quite a bit and shake down trials are very favorable. However, my ideas stray from conventional design parameters ( metal risers with ceramics coatings) I and lead to reply’s that doubt the actual effectiveness of my ingenuity.

What I will tell you is that the “Mark II” heats out 2k square foot two story home for about $175 a month, all things considered and that is a number I quote from my wife ( a very smart) number cruncher.

Another interesting tidbit is the stove is being operated independent of my tinkering by said wife and her 21 year old son as I have a traveling gig at the moment. The fact that they can run this stove without me buzzing over and around them is testament to the ease of operation of the new design. That counts because I know they would not use it if it was problematic to operate.

Finally, I will add that I did the “Mark II” build without photo documentation until the very end so there are some internals that you will have to take my word.

Patience, all will be revealed. Stay tuned!

Hello Permies!

As promised, I am posting the pics of My "Mark 2 Modified Rocket Stove" or "Mark 2" for short. The most casual observer will note that the burn chamber. Hopper and Steam Injection have been cannibalized from the now decommissioned "Frankenstove". My apologies for the wait as it has been going thru shakedown trials and evaluation by my family for ease of operation and basic reliability feedback as test subjects for lighting and maintaining. So. without further audieu, here is a pic of the final build.  I will continue with a more detailed explanation in following posts as I am still working on how to make this flow better. I do not know how to rotate the photo. Help me

will someone help me rotate this photo please

Hi Peter,  Thanks for the update. Look forwards to hearing about the details.
As for the photo turn, I bring it to a photo editor first where the rotating is done then upload it....also helps to keep the pellets from spilling out this way don't you think? :)

Thanks Gerry! I didn't know it was on its side when it posted because I couldn't get the preview button to work.... besides it does hamper the gravity feed too!

Ok, so I will try to discuss the innovations from the "Frankenstove" to the Mark 2 in a photo by photo discourse in an order that will follow the flow of the combustion flow as this makes the most sense to me and may help you build an even better version for yourself.

Here goes!

Below the 90 degree burn chamber I have installed a 1.25" threaded pipe that allows two things to occur; secondary air inlet and secondary ash fallout.  The 1.25" pipe has a threaded Tee attached to it with the 90 blanked off and the bottom reduced to a 3/8 x 1.25" reducer screwed to it.  the 3/8" reducer allows enough secondary air to aid combustion yet allow the secondary ash to fall down into the base of my stove which serves as a large ash bucket.  This is an improvement over "Frankenstove" as I had to empty the ash from the glass jar every two weeks or so. Now I just vacuum out the ash before start up and have only done it once since I started shakedown trials.  I purchased reducers from 1/4" all the way to 3/4" and found 3/8 allowed the best burn. So mark one up for trial and error!  

This large area will also act as an outside air vent when I get a chance to convince my wife I need to drill another hole in my outside wall of our house to "allow cold air in the house to have a warmer house" by not using  the air in the house.  I am struggling with that concept at the moment! lol

By allowing this small amount of secondary ash to precipitate down from the 90 section of the burn channel, I suspect that there is less ash being drawn up by the convection and draft of the riser and thus a cleaner burn. Also, the secondary air is preheated as it rises into the 90 part of the burn chamber and then aiding in combustion.

The secondary ash falls into the metal base and is vacuumed out prior to startup to ensure no hot embers enter your dyson and turning it into a plastic eating afterburner, followed by a visit from the Fire Chief of your local town who will question the certification of your rocket stove and all the grief that goes with that! not to mention the smell of burning plastic! yuck

As the stove is running well as designed I will refrain from more steam injection thru the installed Tee but I may experiment with that another day.

Stay tuned for more insights and photos of Mark 2 and the adventures of heating your home while being kind to your pocketbook

Peace, Health, Happiness

P.S. Oh yeah, Gerry, can you rotate this picture too so the ash can fall out with gravity. Thanks, much appreciated

Continuing with the Mark 2 build explanation, I will take some time to explain the innovation in the 90* burn chamber.  Once again, I would like to bring your attention to the Frankenstove Autopsy and have a look at the 90* bend where the flame from the horizontal flame continuously contacts the refractory.  I know that this contact is necessary to cause turbulence to slow down the flame speed and deposit energy into the rocket stove, but once again, I challenge conventional thinking and threw in three design modifications and will now share them with you.  

I know what you're thinking... why are you changing so many things at once? Well, the short answer is, that's what I do. I look at each piece of the rocket stove and ask myself, "What can I change to make this work better?"  I know that refractory has resistance to heat but blasting it directly into a flat wall to create turbulent flow and mixing seems a bit harsh. Again, learning from the massive crack in my refractory and watching it grow over the three years of operation got me to thinking...... What if we could induce turbulence flame to get the "swirl flame" with a gentler approach. So the next innovation was to cast a "Spherical Burn Chamber".  This flash of inspiration got me running down to the craft store to find a 5" styrofoam ball.  The thinking with this was to attach the horizontal burn chamber to the bottom part of the styrofoam sphere such that the flames would be forced into a "loop" and have a more efficient "swirl" as it would then be better oriented to travel up the riser.  Of course this was all in my mind's theory but it did make sense as any kid playing with a marble in a bowl can attest; it i s easy to initiate a swirling motion. Then, I addressed the secondary ash issue and thought it would be novel to have a conical drain to the bottom of the riser much like industrial dust collectors .... so I ran back to the dollar store to buy a large plastic funnel that would fit into my 1.25" pipe and got my glue gun and started aligning everything into position.

I know what you are thinking y'all ..... You are way overthinking this rocket stove thing! My simple answer is; I had some time (I was driving trucks across the Prairies! lol)  If you invest time in thinking about how to build something you save a bunch of time, energy, materials if you build it right the first time!

But I didn't stop there. Rather than the flames still contacting the refractory directly, I shaved a small bit of the styrofoam ball so I could attach a 12' cast iron frying pan to it dissipate the heat to more of the refractory. The cast refractory will hold all this in place until it cured and I gave it at least 3 weeks to cure in summer heat before I finally had the courage to drill holes in the styrofoam and burn it out with a torch.....  the styrofoam acted as it's own fuel.  Do it at night so your neighbours don't see the black cloud of smoke. Then beg forgiveness from Mother Earth for a momentary sin of burning plastic. Short term pain for long term gain.

Unfortunately, I didn't take a picture of this part of the build, so you will have to take my word for it.  So that's guts of my Mark 2. Works great, my family can light it up every morning when I am on the road. The house is warm, our electrical bill is way down, as that is what our home ran on prior to Frankenstove.

Finally, as a decorative touch I added a large Himalayan Salt Lamp on top of my Mark 2.  I took advantage of the 3/4" threaded pipe fitting which has a 6" nipple and cap attached. The salt lamp fits nicely over this fitting

Here is a picture of the swirl pattern looking up thru the Secondary Air Inlet / Secondary Ash Sorry about every pic being 90* out of horizontal.

The Salt Lamp on Top of Mark 2

Peter Chauffeur wrote:This large area will also act as an outside air vent when I get a chance to convince my wife I need to drill another hole in my outside wall of our house to "allow cold air in the house to have a warmer house" by not using  the air in the house.  I am struggling with that concept at the moment! lol

Not sure if you've seen this video Peter: External air  or these discussions : External air   and   external air 2. There may be more too....
Just some info to consider but every situation is different so always go with your own method of madness and get your own experience.

i like the salt rock, as it's massive.  it's mass :)  i also like the small footprint , it doesn't take up much floor space.  This reminds me of a combination of something i would build, and a combination of something F-Styles would build

Thanks Gerry!  I watched the videos and what Paul explained with respect to outside air supply.  Our house has enough tiny drafts to feed the stove.  That saves me from drilling another hole in my house which saves me time and energy.   The thought of getting rid of stale air is a good thing!

The Pink Salt Lamp carries the pink theme from Frankenstove and Yes! It does add mass to the stove. The Salt is very warm to the touch which turbo charges the ionizing effect of salt lamps that normally have the 25 watt light bulb in them so I guess there is now an added holistic benefit to my new design too!😎👍.  Another friend finally returned my laser infra red thermometer so expect to see some digital temperatures from all over the stove so y’all out there can get a better idea how the Mark 2’s design manages to dissipate all that heat from a tiny combustion chamber and low pellet usage.
Thank you, John for noticing the low square footage “footprint” of this stove! Again I drew on the original design with the added advantage that the 100 pounder propane cylinder is it’s own form for the refractory. Furthermore, heat is transferred from the refractory burn chamber to the metal shell and adds to the warmth of the surrounding area.
Also important to note is the increase in the surface area of the metal cylinder ( as opposed to the 30 pounder on Frankenstove) lowers the temperature of the bell. This makes it (a little safer to the touch) I still maintain that one should be wary around the area where the pellet feed entered the burn chamber (that area is VERY HOT!)).  It will be something I address when I replace the burner assembly that I alluded to in a previous post.
I do have a colleague that has access to a Thermal Imaging Camera and hopefully I will be able to borrow it to also aid in further demonstration of how this design works. For that, I ask for your patience!  
For those of you that read this whole post, I hope that it helps you build an even better version of what I have done here with my modest skills and imagination. Rocket stoves are amazing machines and if you enjoy being warm and spending pennies to do it, not to mention using a fraction of the you would normally use - build one. Buy the 4 part DVD on this site, surf the web, study some combustion theory and have some fun doing it. Be safe and your reward will be warmth and comfort that everyone will appreciate. Above all do not forget to install a smoke detector along with a carbo monoxide detector and test them regularly. As much as I am a self admitted closet pyromaniac; I am also a nationally certified firefighter that has witnessed the foolishness of souls not practicing safety where any form of combustion is concerned. That my friends, is the you have been warned disclaimers!
Be warm

yes i think a 4" or 6" version would be awesome.  i don't think i've ever seen the original 30lb stove...

Hi John. The original stove was the one I started this thread with, that is, or more appropriately was Frankenstove. The 30 pound heat bell ran quite hot and drove temperatures in the bell at the temperature gauge to 500 and 600 *F.  To me, that represents the 30 pound bell could not dissipate the heat fast enough which in turn caused the whole stove to decommission itself prematurely due to heat stress in the riser. You will also notice I placed a second thermometer right in the path of the riser outlet to the bell. When the Mark 2 is running full out that temp is near 700*F while the front facing thermometer is topping out T 375 to 400*F. I’m interpreting that as there is a good heat sink in the 100 pounder cylinder bell. Once again it would be nice to have a thermal imaging camera to see this heat transmission. Stay tuned for more ramblings and thoughts. By the way, that pink chunk of salt on top of the stove weighs close to 30 pounds and gets hot enough where you cannot remove it with your bare hands ( I foolishly tried). Since the body of the stove is operating well, I will continue to work on some of the other things mentioned on my “wishlist” in an earlier post, namely: a larger hopper, vertical feed, multi fuel port ( twigs and pieces of other wood off cuts)
A better steam injection thru the lower Tee where the secondary air / ash removal is located and finally a better ash bucket. Stay tuned, my friends