Maybe someone with experience can verify this for you, but I think you will want less pressure on the back end of the system to help gasses move through the system.

Especially if you will be having such a constricted system.

maybe 4 inch exhaust would be good, and that also happens to be a real stove pipe size, so finding suitable material would be easy.

The only thing I wonder about by your description, and maybe my lack of knowledge is making me overly cautious, but if your stove doesnt move air through the system, and you try and start it up cold, so you got a cold stove, starting with a really cold burn and not much is happening, I just hope woodgas type of vapor does not accumulate in your tank while fire is smoldering/ going out. Then when you try and light the stove it causes a problem.

The lack of insulation and tall riser combined with your tiny system diameter it seems like there wont really be much physics to move things along in your system.

There does not seem to be much of a "heat pump" effect. Without the space in your tank being cooler than the air going up your riser, what will suck the air along?

The stove works on pressure differentials. I am not sure your system will have any.

maybe you can build a mock up without welding it first and see what happens.

Dont forget to add the exhaust in the mockup and see if gasses will move all the way through and out the chimney.

Dan Henn wrote:Hey all,

I figure it's worthwhile to experiment a little, maybe even use the heater in my fishing shanty until I burn it out.

Which, the heater, or the shanty?

: )

You mentioned it being a cool woodstove. Part of what keeps a woodstove "safe" is that the so much hot air is exiting the system that it carries with it any toxic/explosive gasses.

I have not built a stove yet, but I have been doing quite a lot of research, and the thing that concerns me most about trying alternative designs is that they have not been tested by anyone, so basically, I would be the guinnea pig.

Building an experiment that doesn't work will aid in your education. Building an experiment that injures you or someone else I suppose is a type of education too.

I saw a guy on youtube who built what appeared to be a functioning 3 inch heater in a tank of some kind, but he used a lot of the tested principles of RMH. He insulated the feed tube and burn chamber and riser really well. He mentioned that he initially planned on using just use steel pipe, but decided to use firebrick instead after so many people warned him away from the steel pipe.

His design also had a REALLY tall riser. Much taller than your propane tank would allow.

He also was following formulas from Ian Ianto's book pertaining to the areas of the different sections of the stove and how they should relate to each other in order to keep the stove functioning safely.

His stove APPEARED to meet the minimum requirements of functioning safely, but probably would not fare so well if tested for efficiency.

Since I am new to the math, and have no experience, I dont want to advise you on any part of your build. But definitely look in to exhaust ratios, you don't want a constricted airpath. Hopefully the worse case scenario is that your stove just wont work, or will function as a smoke generating device.

In any case, it doesn't hurt to do lots of research before you build anything.

You can learn a whole lot by researching before you even touch your tools.

Dale Hodgins wrote:   
    The plan is to build a U-shaped sheet metal slide which is set about 10° from vertical so that long lengths of firewood will lay straight and automatically feed into the rocket stove.

Hello Dale.

Did you ever try this?

What kinds of variables did you encounter?

Len Ovens wrote:

Do you have a specific application in mind? What size/weight are you designing for? I have thought of small scale rocket heaters, but have not had the materials/money/time to try my idea out. Personally I would try to include some mass in the form of soapstone or cast iron if nothing else. These are denser materials than cob or even fire brick and so would store more in less space.

A couple applications. One would be any type of livable vehicle from small to large motorhome.

Another would be anywhere one might set up temporary camp, such as fishing expedition, gold panning, etc. Small lean to situations. Occasionally there is a need to spend some time in a small one room cabin. etc.

One of the ideas I had was to fill an 8 foot, two inch diameter tube, laying horizontally, or slightly diagonal, with pellets/twigs and figure out how to make the fuel burn very slowly along the entire length of the tube, so that about only 1 inch of the metal would be getting hot at one time. The entire tube would either sit horizontally, or diagonally, and function as the feed tube, burn tunnel, and riser ALL IN ONE straight pipe. Or it could have an additional 3 foot riser on one end to create draw. One could bury this in dirt under a sleeping bag or cover with dirt on the side in a lean to. But now I understand rocket heaters better, so I know there would be no way to get hot enough burn to be efficient this way.

But that's just to give you an idea of how radical some of the directions I am exploring are.

Possibly if I could figure out how to burn the resulting woodgas at the end of the system with a small chamber it might work. Either way, I would not want it to accumulate wood gas and create a pipe bomb right next to me.

maybe I could start a fire at both ends of the 8 foot tube, and the at least the woodgas from one end would have to travel through the fire at the other end to exit the system.

Len Ovens wrote:
(Re)movable mass is another interesting idea.

on the 8 foot tube stove I had an idea to have semi-circular cement drain tile underneath the tube, then cover the tube on top with several pieces of the same material to complete a cement jacket around the entire burn tube. Because it would quickly get too hot, then the cement jacket would simultaneously block the extra heat, while storing it for later.

Or when the space got warm enough after initially starting the fire, I could cover with dirt and rocks.

Regarding the normal 4.5 to 6 inch RHM with small mass such as over an existing fireplace hearth. For portability I wanted to try 5 gallon cans filled with water, stacked very close to the barrel and vertical exhaust. I figured for a small space they would block some of the initial heat that would be too hot for small space, but would absorb plenty. I could get 10 5 gallon cans stacked two high around a 20 gallon barrel and vertical exhaust leaving the front of the barrel exposed to direct heat. The cans could simply be emptied of water, and with lids off they would stack and be very light and take up very little space. I would either leave holes in top to vent if they got to hot ( which I doubt they would, or create a system of tubing to vent outside if it was creating a steam problem in the space.

Len Ovens wrote:
I was going to suggest using a gasifier set up next... it turns out that is what the Kimberly is

I am starting to think this is the only way to achieve a very small volume fire, and at very slow speeds, and have complete burn.

That is another one of my objectives to meter the fuel out VERY SLOWLY. It does not take much heat at one time to take the edge off in a very small space. And metering out heat very slowly is contradictory to a complete, efficient burn rocket style. But since gasifying works by INTENTIONALLY making a very slow, cold burn, one can achieve a much smaller btu per hour, while simultaneously extending burn time.

Which for small spaces is perfect. Because if one does not have the MASS to accumulate the energy from a very fast, hot burn, the only other option is to slow down the burn to achieve a the goal of not having to constantly put fuel in.

Unfortunately the gasifier setup seems a lot more scientifically complex. Where as I got my mind around the physics of a rocket heater very quickly, I am not certain what is required to build a kimberly type of gasifier.

The guy in the video with the motorhome did mention that at the top of the stove was a "honey comb" thingy, which I can only assume is a catalytic converter, which would make sense, seeing as how the fire is being metered out so slowly and with such a cold burn, that the exhaust is mostly wood gas.

I have to admit, that playing around with woodgas in chambers, seems a lot more technical, and critical, than achieving a quick clean burn based on the simple physics in a conventional RMH.

But there cant be that much to how the kimberly stoves work, and at $4,000 there will probably be a lot of motivated people trying to figure out how to build a reasonable facsimile.

Len Ovens wrote:
so the highly efficient iron stove in the lab becomes another 10%er in the home where it is run at an idle/smolder.

Len Ovens wrote: and cut the air flow till it smolders along nicely. Six months later when they have a chimney fire

My question specifically eliminated that eventuality. To make sure I am clear, having a "smoldering" fire is not my intention. I indicated that I completely understand why incomplete combustion would result in creosote, and in turn, flue fires.

"To clarify, if the riser is the same heat, and the barrel is the same heat, and exhaust leaves the house at the same temperature"

What I meant by that was the the fire would NOT be smoldering. The main thing which attracted me to rocket heaters in the first place was the complete burn, and videos of people showing how only steam and C02 was exiting the system. So for me, if I am not getting complete or near complete combustion, I am not interested.

Len Ovens wrote: you take away the mass and you have an iron box that can not (except for testing) be run at full efficiency without over heating the room.

If a smaller volume of fuel was on fire, then the fuel could be burned at full efficiency, and the room would not overheat,

Len Ovens wrote:
So just remember not to call it a rocket anything so those who are working on the RMH don't get blamed for problems with the iron box stove that gets built.

Do I have your permission to at least call it a "Pet Rock Stove", or is that too close to "Rocket"? How about "Racquel Squelch"? ( cold burn, not-rocket sound ) : )

Len Ovens wrote:It is not that design is flawed and won't work without mass, just that it is not appropriate for home heating

My interest in non-mass rocket stoves is not for heating a standard home. In a standard home situation I would MUCH rather have a large thermal mass radiating all the time.

In small spaces however, mass, starts to become a problem. Either because of space, or because of weight.

My nature is to try and figure out new solutions, and create new things. Sometimes I succeed. Sometimes there is no room for improvement. Occasionally dogma bares it's fangs.

Have you seen Kimberly Stoves? I saw a video of a guy who had one in his motorhome. Looks awesome. The webpage says it burns for EIGHT hours on one load. Cool, where do I sign up? I would just get one or two of those, except...they are almost 4,000 dollars! Yikes.

The Kimberly stoves I have seen have no mass. And to complicate matters, it is also an Iron Box, well more like a Steel Cylinder. Both descriptions would be equally suitable as 80's metal bands, or porn titles, capitalization was an after thought.

Either way, the stove has no mass, and does not appear to be prone to flue fires. And I am not naive enough to believe that advertisements are real life, but the people enjoying the stoves don't look "edgy" at all, in fact, they look quite comfy. : )

So I was just trying to figure out specifically what aspect of massless designs would cause flue fires. I was not sure if the point was that the exhaust came out a lot hotter, or that the mass itself could tolerate an internal creosote fire, or both.

I have several designs for wood and pellet burning stoves I am working on, a couple don't even resemble RMH in the least. And I am DEFINITELY planning on building one or more RMH stoves in the mean time which follow the known and trusted methods as close to the letter as I can. The current design is brilliant, and brilliantly simple, and I definitely want to know how to build the current design as a LIFE SKILL, and as something I can always fall back on, which I know from other people's experience, will definitely function and function safely. And I am gathering as much information as possible to help with all of these processes.

Constructive input is much appreciated!

: )

allen lumley wrote: so there is little room for gain!

This is what I wondered about. It seems that the conventional RMH is very efficient as it is regarding the maximising of fuel burn.

Also, the inner dimensions would need to be completely altered to accommodate the extra drag from the converter, so clearly it can't be tried on existing functioning rmh's.

The other downside would be that one of the most valuable aspects of current RMH design is their simplicity. Designing an RMH to accommodate a catalytic converter would ADD variables. And my general approach is that it's better to go with solutions which have fewer variables. Also the converter would wear out, so it would also be a part that needs replacing, and a part that is not easily manufactured DIY.

But still it sparks my curiosity just because it's a different WAY to burn the leftover gasses, so I wondered what would happen when combining two different methods to process the same effluent.

But it's probably a pull yourself up by your own bootstraps type of fallacy. It would probably alter the flow to such a degree that what happened at the barrel would no longer be as efficient, thus countering any gains achieved by the converter.

Satamax Antone wrote:It would burn!

What if it was high up enough to where the flame didnt touch it directly?

What would happen if a catalytic converter was placed at the top of the riser, before the barrel?

would this burn more off the unburned gasses?

Any thoughts on this?

I was wondering if anyone has ever experimented with something like a basket ball size sphere inline about half way up a 6 inch riser configuration, then closing back down to the 6 inch riser diameter for the rest of the riser.

or possibly more than one sphere in a line in the riser.

Or possible different shape like a toroid.

I was wondering if this would create a low pressure zone(s) that would cause the unburned gasses to slow down and mix around in the chamber, and also a super heated flame would be shooting right into the slower gasses possibily helping to combust them?

I understand this is essentially the effect the top of the barrel has. I was just curious if closing the diameter back down again, with a ratio of dimensions that would still keep the airstream moving through the riser would have any kind of interesting effect.

Anyone have any thoughts on this?

Ok. Thanks cindy and charles, I understand now. : )

Cindy Mathieu wrote:I second what Bill Dubiya said. The refractory material has different components which will stratify in the bag. So, if you don't mix up the whole bag, you won't have the full complement. We learned this the hard way...trust me.

So, mix up the whole bag and then take what you need for the job from the mixture and save the rest.

I thought the refractory material was homogenous?

When you say "save the rest", you mean ix up the bag wet, then save the wet material?

can I just "stir" the bag or mix it around dry until everything is evenly dispersed then take part of it?

I am confused now.

Adam and Steve...

Lol. Thanks that is what I was hoping to hear. I figured it was probably profit driven.

For the life of me I could not figure out what the hell they were on about.

I could see if it is a really humid area, ore people just leave the bags open after they use it.

I mean if worse came to worse you could suck the air out with a vacuum pump before you close it. But waste the remaining contents of the bag???

xD

Thanks.

Chris...

F'ING WOW.

I love the small footprint and yet there is sill at least some thermal mass to help moderate temps.

What are the external dimensions of the barrel? And how many gallons is it?

Also, how much space between the riser and inside of barrel? maybe you listed that in your dimensions but I didn't understand which stat represented that value?

What do you mean you kept the exhaust "above the negative pressure plane"?

When you fill the feed tube, and pellet feeder, how long does it on on a burn?

I am assuming this system can be scaled up with no problem.

Why do the instructions on cement say "never mix less than a full bag"

So if I buy really expensive refractory cement, I cant seal up half of it and use it later?

Is the bag ruined by moisture once you open it?

I have been searching the internet and I cant find anyone who has commented on this instruction.

Wow. REALLY cool. That link to indoor RMH is exactly how I am trying to set up my friends hearth/unused fireplace. On the indoor RMH, how long does that stack of pellets burn for?

Back to the 7 gallon stove....

Chris Burge wrote:
This little stove just has a chimney, not a heat riser, and insulating it would defeat the purpose since a lot of radiant heat is emitted from the chimney itself.

I was thinking that one could put an external barrel over the riser to essentially trap more heat because the exhaust gasses would ultimately be cooler because more heat had slowed down for radiating to the persons standing around the barrel/bell.

Chris Burge wrote:
I would not consider ones of these stoves to be as efficient or hold more wood than a "conventional" RMH, but they do produce high enough temps to result in a very clean burn.

I could see where you would say it is not as efficient because the proportions/draw etc of existing RMH design is already close to perfect. But it doesn't hold more wood?

How long is the effective length of the feeder tube? It looks to be at least twice as long as the feed tube on any standard j tube. Am I looking at it wrong? How could more/longer wood not fit in to a longer tube?

Cindy Mathieu wrote:Jacob,

I'm in the Class A chimney pipe camp. I know it costs a lot more than venting a gas heater, but you are risking your house if anything goes wrong.

Of course. Safety is a prerequisite. Cost is a luxury. Especially if the entire stove can be built really inexpensively, and especially if by eliminating the mass, there is more risk of a flue fire, then it makes perfect sense to use an exhaust system that is as close to foolproof as one can get. It really makes perfect sense regardless, to use high quality components.

I still am unclear on how the "massless" aspect of alternative RMH designs is more likely to create a flue fire, assuming that the burn chamber, the riser, and temperature leaving the structure is the same as an identical RMH that features a giant thermal bench.

I understand how a system with random dimensions and random inventions thrown in on a whim could create a flue fire. I understand that any variables which result in incomplete combustion could build up creosote and are more likely to create a flue fire.

Such as insufficient riser temperature, wet wood, incorrect fuel to air mix etc.

Cindy Mathieu wrote:
Matt Walker has done a lot of work with Peter's batch box; Satamax Antone is also a big fan. Matt entered the batch box in the Woodstove Decathalon last Nov. in DC. It was not the most efficient unit there.

I just looked at that and it brought me to walker stoves. Very impressive.

So that is the same Mathew who was so helpful, and listed out a series of concise answers to my questions? Thanks Mathew. : ) your stove looks excellent!

Actually he was the one who helped me with the important puzzle piece of how the insulated aspect of the stoves are so important, and it finally clicked for me how the process worked.

Prior to that I thought one wanted dense fire brick in the airpath because the brick would heat up so much, it would ignite the unburned gasses higher in the riser

xD

So you say the batch box design was not the "most" efficient. Was it in the running? Sometimes a little efficiency can be sacrificed to acquire a different KIND of efficiency.

One thing that a lot of people tend to overlook when thinking green, is that TIME is ALSO a valuable resource. The more free time people have, the more gardens can be tended to, or bottles recycled, or soup kitchens volunteered at, etc.

I am glad people like Peter Berg and Mathew Walker are trying to go beyond the confines of the box, literally and figuratively, to strive for new heights. Innovation does not happen, from not trying. That I know very well.

Innovation is something I am an expert at, even though I know very little about rocket stoves/heaters etc.

The cold hard reality is, a LOT of people in the mainstream will simply not be interested in wood as a fuel source, if the process requires constant effort. I think it is GOOD to try and come up with new ways of achieving this. And if it fails, try again. And if it fails, try again. And if it fails try again.

That's my philosophy any way.

Hello Erica. first I would like to say thanks to you and Ernie for helping to pioneer this field and bring people everywhere a very inexpensive and simple means of using twigs and windfall to keep themselves warm.

I have a couple questions regarding this topic.

Erica Wisner wrote:

a) - and why I get edgy whenever someone wants to do a 'rocket mass heater without the mass.'

I don't understand the problem here. Are you saying that without the mass, then the exhaust gasses will be hotter, and more likely to start a flue fire?

What if one has an identical system with no thermal bench, but has run horizontal pipes which release most of the heat into the space on demand, but the only difference is the heat is not being stored in a thermal mass. does this matter? Or what if in the same exhaust run where the cob bench would be, someone has cooled the exhaust with a water coil to the same temperature it would have been with a thermal bench, will this make a difference?

To clarify, if the riser is the same heat, and the barrel is the same heat, and exhaust leaves the house at the same temperature, does it sill matter if it goes through a huge mound of clay or not?

Or are you saying that the huge mound of clay would easily mitigate a creosote fire inside, as opposed to a piece of stove pipe which would obviously fare worse?

In which case I would understand the logic there.

Also to clarify a seperate issue, is it not okay to run exhaust straight out the house gas heater style? I saw a video where people had done that either in one of your workshops or one where paul wheaton was involved. And they were marveling at the low heat exiting the side of the building and just steam and c02.

Is this bad? Is the urpose of running the chimeny vertical after it leaves the exterior wall and raising it up above the roof in case there is a flue fire, so that is the safest place to have a fire exit the system? rather than down under the eaves?

I thought the different lengths of chimneys I had seen was just to get more draft if it was needed, but if there is also a safety reason for running the chimney up past the roof line, I will do it.

I guess my understanding was that if the burn chamber and riser and barrel were functioning properly, creosote is not an issue so the only difference between having mass downstream or not would be how much heat do you want to recapture before it leaves the building, and also that the mass creates a more stable temperature.

Erica Wisner wrote:

1) AVOID: Swap a big ol' woodstove box for that dinky little weird J-tube burn chamber
2) AVOID: Add extra air feeds, pre-heat the air, enrich the fuel, burn painted or soggy-wet wood, or add fans or other draft boosters to monkey with the mix and

I can understand why one would not want to do this willy nilly, but what do you think of Peter Berg's batch box?

Do you think it is possible if done scientifically, there is a method to add a larger fuel capacity while still meeting the requirements of an efficient burn using a "rocket" type riser, and external barrel?

Because I was hoping to copy his design. Do you think his batch box design is a creosote hazard that the conventional RMHs do not pose?

Since peter berg's batch box gives people the option of not having to babysit their feed tube constantly, it seems like a worthwhile pursuit.

Lastly, if one does fore go the mass and instead just has stove pipe exposed to open air inside the dwelling, would simply checking for creosote often solve the whole flue fire conundrum?

I really thought that the complete burning of the wood eliminated the creosote issue.

Thanks for any feedback.

I want to make a few 5 gallon cooking rockets with simple homogenous pour in to 5 gallon buckets, metal will be left around core.

I plan on lightening up the cement with perlite/vermiculite etc to make the burn tunnel and riser hotter, and to make the stove lighter.

I have seen a lot of people make these, I am wondering do the glass particles added to the cement get scraped of the burn tunnel when putting fuel in, then get rocketed up in to peoples lungs and in to the food they are cooking.

How about the riser? Does it heat up and make a solid layer of glass after the first burn? Or can forge like flames shooting up the riser break down the delicate mix over time and also be rocketing freshly exposed glass to rocket into the airstream.

Does anyone know anything about this?

Chris Burge wrote:Just made a pellet feeder for the pocket rocket!

Cool.

How does the bottom of the pellet feeder work? What keeps the pellet feeder from becoming a chimney? What kind of barrel is that and where did you get it?

Also, with or without pellet feeder, how long would one expect a system like this to last before the metal falls apart?

Could such a system be improved by insulating the air path, also increasing longevity of the metal?

If burn tunnel and riser were insulated, would this system compete in efficiency with a well built RMH? What if it was insulated AND added a barrel around heat riser.

I am interested in this shape mainly because it holds WAY more wood than conventional rmh. Resuting in less tending. Especially if you can use pellets to feed it.

Thanks again for everything, I am ready to build now. I am definately going to incorporate some of the things I learned from you in to my designs.

Incidentally have you seen this guy, he has come up with an impressively simple way to build small rocket heater. Only drawback is horizontal feed which doesn't allow putting really long vertical sticks in to reduce fire tending. But he got amazing results using portland, and no insulation on the heat riser. He has 140 degrees coming out the exhaust to outside with no visible smoke.

His design started as a rocket cook stove, and morphed in to a rocket heater. The build is insanely simple, and doesnt require a manifold:

here he starts as a cook stove:






and here when he turns it in to an indoor heater:





I think I will build a similar design, except I will use your fireclay mixture, and your insulated heat riser. Or possibly I will use vermiculite board or thermal blanket with hardener since you inspired me to go insulated to get the air as hot as possible. Also, one HUGE flaw in his design is that the build itself is rather delicate. If it got heat stressed, or weight stressed, or knocked into with a heavy solid object, it could easily collapse/crack through the entire shell of the burn core, and all that heat and fire spilling in to the living space would be a huge problem. Pouring in a metal tub would solve this as would several other methods.

If you have any cautions about the build and vent configuration I provided a link to as regards to C0, or woodgas explosions, let me know. Other than that I dont need you to answer any more questions as I fell ready now to build.

I don't mind if I build an experiment that doesnt work, I just dont want to build an experiment that endangers me or someone else.

I will be using a C0 moniter btw at all times indoors, I was just trying to get any helpful info to avoid a pointless build if I could.

Thanks again for everything, you have been really helpful.

: )

Oh, as pertaining to this comment:

"You won't get long heating cycles without mass"

Check out my recent topic regarding "device to retrofit standard j tube to burn pellets resulting in less fire tending"

I am working on the problem of short burn cycles with no thermal mass. : )

Someone really needs to figure out how to burn 6-8 foot fuel in the feed tubes to reduce the amount of refills during the day. I present several ideas in that thread. Maybe you will think of something to add.

thanks again.

Matt Walker wrote: Here's an old heater I built that was poorly insulated and worked just great. And, it's in an old metal tub, just like you are discussing.

http://www.permsteading.com/viewtopic.php?f=6&t=8

Cool thanks. That is exactly what I had in mind.

What did you make the heat riser from? and what was the diameter of the riser, burn tunnel, and exhaust?

I am trying to make a 4 inch system, and from everything I have read, the smaller you go, the more insulated and efficient things need to work to get proper drafting.

Also, what about using super insulated burn chamber and riser, but building mass around the OUTSIDE of the external barrel?

Would cooling the outside of the external barrrel mess up the functioning of the stove in any way? I was thinking of just leaving the top of a 17 gallon barrel exposed for immediate heat, and building a layer of cob or equivilant AROUND the barrel itself.

The reason for this is I am trying to keep the footprint very small, and eliminate te long external run through a cob mass.

Also, without the external exhaust run, I am hoping this will help the smaller diameter system draft more effectively, and save on exhaust pipe costs.

One more REALLY IMPORTANT question.

In your single more you mentioned lots of cracks, but said that would be ok because it will be covered with cob.

If those cracks were left exposed and not covered with cob, would that present a carbon monoxide hazard? or would the cracks in the burn core suck air rather than emit it?

I am assuming most C0 problems come from the exhaust portion of the stove leaking back in to the environment. Also the stove inlet is a HUGE hole by comparison to the cracks so if carbon monoxide wanted to escape it could just go through there during times when burn was winding down.

Thanks for all your help, your comments and contributions have been excellent!

: )

Matt Walker wrote:

mass in the burn area actually cools the gasses as the heat is drawn in to heat the high mass of the bricks. In this way any mass in the riser will actually rob heat from the burn for a while to achieve that hot effect you are describing,

One of the ideas for mini RMH to use in small space I was toying with is to do a single pour in to a small metal tub. In this case I was thinking of using the tub the core sits in as some amount of thermal mass. This would involve using a material that attracts heat, and radiates it back out. For a small space this would be effective because no external thermal mass would be necessary for the exhaust run, thus keeping the footprint as small as possible and still having at least SOME amount of thermal mass that radiates after the fire goes out.

If using this method, the burn chamber and mass surrounding it would absorb heat from the burn, not repel it.

In this scenario, would an efficient burn at least be possible AFTER the unit warmed up? I realize it would not come up to efficiency nearly as fast as what you are doing, but if I could get it up to efficiency after say a half hour, I would be fine with that.

Also I understand the purpose of insulating the riser, but I thought the CORE of the riser was ideally firebrick (dense material that absorbs/radiates heat ), then wrapped by an insulating layer which in turn is contained by metal shell, so that in essence the entire riser is insulated from the external barrel chamber, but that the CORE of the riser itself did not refract heat, but rather GOT HOT, to enhance chimney effect, and burn gasses?

I thought I had read Ericka and others said using the dense firebrick in the core of the riser (heat absorbing) then sandwiched by insulating material to insulate the riser assembly from the external barrel airspace was ideal.

I thought the purpose of YOUR riser design was simply to split the difference between insulative and heat absorbing, so that the riser would not need to be constructed of multiple sandwiched layers, thus making the design and build simpler.

Just to clarify, your riser design is simply ONE homogenous material enclosed by the 17 gallon drum to hold it together, is this correct? Then you simply place your external heat barrel over that assembly to complete the stove, correct?

Also if I use material in the metal tub that works as a heat sink, would it then be NOT a good idea to use your riser mixture which repels heat? And instead it would be good to stick with a riser core that gets real hot, sandwiched by insulation as per old designs?

My concern would be if I create a thermal heatsink in the tub that holds the burn chamber, eventually it would get hot, and if the core material in the riser does not also get hot, but instead repels heat, ( as per your design) that the burn chamber would be hotter than the riser, thus impeding and or cancelling the chimney effect.

Any thoughts on this?

I am wondering if cracks in a precast core, or the cob, or both could allow gasses into the living environment. I am thinking about variations on current design, one would be precast core elevated on firebricks in a metal tub, and simply filled all around the core with pebbles/rocks sand etc for thermal mass. This would make for easy disassembly.

In this design however there is a lot more potential for cracks in the core/burn chamber to allow gasses out in to the living environment.

Is there any extra carbon monoxide risk, if tiny cracks exist that link the inside of the burn chamber with the outside of the RHM surface?

Or would the cracks simply be extra place for the air to get sucked IN to the burn process. thus keeping air traveling in not gasses traveling out.

Also, if experimenting with water heating designs, or heating large pots of water...If a LARGE amount of water should ever fall on/in to a cracked core, or thermal cob surrounding the barrel, is there any chance that this could trap moisture that would make the core or cob mass EXPLODE? Or would the h20 simply find it's way back out in the same cracks it found it's way in?

Thanks for any thoughts.

Matt Walker wrote:Troy, I describe my mix in the video linked above. Furnace cement as an additive is all you'll need.

Hi matt. Excellent videos.

I had two questions.

1. Is your heat riser a different formula than your core? I did not notice you add any fiberglass to the heat riser but you did to your core.

2. Your heat riser mix looked like it had more perlite, so this has MORE insulative properties than firebrick correct? I thought the goal with the heat riser was to use material that got hot, not reflected heat, to help create chimney effect and keep air moving through the system, and more importantly to burn more of the leftover smoke to increase efficiency and reduce smoke out the chimney cap.

I am a little confused that your heat riser mix seems to be even more insulative than your core mix.

I notice however that you don't need a separate insulating layer for your heat riser which keeps construction less complicated. Have you simply struck a balance between insulative and heat sink qualities that works ok for a heat riser? If so, do you think there is any, even slight advantage with hotter heat risers made from firebrick core wrapped in insulation, that heat the air in the riser to higher temperature than your design?

Cindy Mathieu wrote:Pellets, by their nature, have a lot more ash than cord wood. Moreover, they have some amount of embedded energy because a machine somewhere else had to make them. They are very uniform, so what happens to all the material (such as bark) which doesn't match the color the pellets are supposed to be?

Pellets can be made from a wide variety of pulpy biomass. They are not even difficult to manufacture at home. They use up the last of the scraps from manufacturing and construction process, woodworking, etc. I am not concerned at all about the color of my fuel pellets, so I have no aversion to bark and all being used to make pellets as long as it burns.

Cindy Mathieu wrote:
Also, the point of the mass in a rocket mass heater is the same as the masonry materials in a masonry heater. You burn fires to heat up the mass of cob or the mass of fire clay bricks and then you don't have to tend the fire all the time because the mass is still giving off the heat.

If one had an easy method to automatically feed a small combustion device with fuel creating on demand heat for 8 hours, the entire design and mass of a RMH would hardly be needed. All of the clay, cob, brick, and barrel could be eliminated creating a totally different animal, with different functionality.

Keep in mind that RMH with huge thermal mass cob bench is not practical in tiny spaces, for camping, for trailers, or for moving from the living room into the workshop, etc.

This addresses a different function than the ideal way to heat a standard home. For a standard home application, I would rather have a large thermal mass always radiating.

But even in that situation, some kind of 6 foot fuel that fed itself into the burn chamber so the fire could be unattended for 8 hours would be nice. What if all you have is a bunch of 1 foot twigs? How often would you need to be replacing them? Surely a 6 foot piece/pieces of fuel that fed itself in automatically would require less fuel tending.

also if the fuel was always occupied a uniform space in the inlet tube, would that allow for new modifications to RMHs which precisely dial in air intake and exhaust ratio.

One thing that RMH/rocket stoves do not currently have is any kind of "dampening" function.

The air intake and exhaust is always the same opening, the only thing which alters flow in an RMH/rocket stove is how much fuel are you burning at one time, and or how packed with sticks in the intake opening. There really is no way to adjust the air fuel mixture to slow down a burn, to lean the mixture, or to conserve fuel etc.

I think the current design and functioning of RMHs are excellent, these things I am proposing simply add to the current functionality that already exists, they do not supplant it.

thomas rubino wrote:add as much dry refractory as you can afford.

Thank you very much for the specific examples and the prices too are helpful so I know what to look for and where to look.

I did not understand the refractory comment.

Is 'dry refractory" a material separate from the the fireclay and perlite? Or is the dry refractory the fireclay itself?

Also as I understand it, if the core gets to hot it will compete with the chimney effect, so is this where the "dry refractory" becomes important? To add insulative qualities to the core rather than heat-sink qualities?

Troy Fairclough wrote:

your burn tunnel needs to be the tightest of all areas.

all areas past the burn tunnel need to be at least as large as the overall area of the burn tunnel if not bigger.

make sure that where you connect your exhaust to the burning unit that you do not cramp the space at that joinder.

if any areas are smaller in surface space than the burn tunnel the unit will not draft correctly from what i understand.

as far as having efficient burn that all comes down to the height of your heat riser.

Excellent thank you. These type of concise "rules"/comments are exactly what I am looking for. I understood them and their significance immediately. With enough of these types of rules, I simply make sure not to break them and I can design within those parameters.

One question on the "burn tunnel". Do you mean inlet where the sticks go? Or a litle further in where the combustion occurs? and what do you mean tightest of all areas? Should this diameter be smaller than the heat riser?

For simplicity's sake I was planning on having the entire inlet, combustion area, and heat riser be the same diameter. UNLESS people here tell me the burning will be more efficient with different diameters in each of the sections.

I watched some videos with an excellent rmh design that used a self feeding pellet chamber to "automate" the feeding of pellet fuel in to a rocket mass heater.

To accomplish this, the designer built a permanent trough for the feeding of pellets.

This got me thinking. Has anyone ever tried just filling a metal tube (8 foot downspout for example) with pellets and putting one end down in to the j tube of a conventional RMH?

One would not want the pellet tube to become a chimney, so the portion of the pellet tube which feeds in to the j tube would need to be equipped with some kind of mesh or perforated material that would allow air flow at the opening of the j tube, thus negating any/much heat trying to go up the pellet tube. Built in to the bottom would probably have to be an attachment which provided a grate for pellet ash to fall through.

This idea seems more ideal with RMH that have a diagonal feed tube rather than a vertical one, but who knows.

Also this idea would really open up the functional possibilities of smaller rocket heaters possibly made entirely from metal, thus eliminating the need for thermal mass to keep producing warmth after the fire goes out.

What would be cool is to figure out how to make a 2 inch steel gutter filled with pellets feed a small portable stove in a small space for 8 hours of sleep time.

Also what would be cool is refueling the stove only THREE times in a 24 hour period. Thus being able to leave for 8 hours at a time with out the fire going out.

I have only seen a couple RMH that incorporate pellets, and the entire inlet for the RMH seems to have to be designed to incorporate the burning of pellets.

There must be a simple retrofit for people who already have a conventional RMH but want to burn pellets.

This idea would be convertible and would allow easy switching between burning pellets, or the standard twigs/ sticks fuel etc.

Any ideas?

Comments?

Haters?

: )

P.S. this general concept applies to burning normal twigs sticks as well, however more variables come in to play seeing as how the fuel is random and not uniform.

I have never seen anyone discuss how LONG the sticks feeding in to the RMH are. Obviously a stick which is twice as long will need to be replaced half as often as a stick half the size. And a stick FOUR times as long will burn FOUR times longer before being replaced.

There may be a way to load sticks/twigs in to a similar device, but it does seem that various elbows/snags in the natural sticks and twigs could get hung up while falling down the tube.

And another variant that comes to mind would be to DESIGN/manufacture "perfect" single long runs of compressed fuel to feed in to the j-tube.

How about a SINGLE 6 foot by 3 inch compressed fiber fuel pellet that will self feed in to ANY standard RMH?

This also opens up the possibility of using even SMALLER biomass such as grasses etc. It probably would not be to hard to figure out how to compress and extrude a single long fiber log that is optimized for feeding in to RMH systems.

Any ideas and comments are welcome.

Hello.

I have been doing a lot of research on rocket mass heaters. I am quite certain I understand all of the principles to build rocket mass heaters as they have been built for the purpose of heating larger homes/spaces.

I have a few questions on materials and theory so that I may experiment with some variations on the theme. Hopefully they will spark some new dialogue and possibly forays in to new territory.

First question has probably been covered before but I have not found a definitive answer.

1. Is there a simple tried and true formula/ratio to create ideal refractory material for a chimney/heat riser? Ideal meaning high temperature, resist shrinking/cracking etc. Is this formula also the same if one wants to cast the entire core and heat riser from the same material? (different sections though) And if I pour a j-tube, is there a formula that will not require waiting 4 weeks before the unit can be used?

I would love to figure out a homogenous material I could cast the core from, I plan on casting it in a small galvanized feed type tub. For simplicity sake it would be great to come up with a simple j-tube form to hang in the tub and do a SINGLE simple pour in to the tub, rather than all of the multiple stages/materials that are used to currently build the cores.

I keep seeing people build these 2000 degree furnaces in a wood box. Metal seems like a much more suitable material to contain fire. Wat up wit' dat?

I have seen all manners of materials and ratios for heat riser. I understand firebrick wrapped with stovepipe and stuffed with insulation works best, but I am experimenting with MUCH SMALLER units, and the firebrick method requires too much space basically forcing you to use a huge drum as the final shell which creates a huge footprint.

I like the idea of perlite and clay mixture, that seems the most "rock like".

2. What kind of clay? where do I get it? I need to be able to purchase these materials, I can not dig any up.

3. if I decide to use a sacrificial metal liner for the riser, that burns off, leaving the refractory material, what happens to that material? do any toxic compounds become embedded in the refractory material to be slowly released in exhaust over time? The reason I ask is because one of the applications will be exhausted very close to a chicken coop. I do not want zinc or other metals being exhausted in to their environment/runs.

I also wondered about perlite in the heat riser. Does the surface of the riser release particles of volcanic glass in the exhaust? Are exposed bits of perlite in the heat riser and exhaust run a health issue for any humans who may be very close to the exhaust flue? This question also applies to COOKING rocket stoves people pour in 5 gallon buckets. Are they breathing in small particles of volcanic glass rocketing up in to their food and lungs?

4. Is there an ideal ratio for the inner dimensions of the core/heat riser/exhaust? Are there any rules dimension-wise which are especially critical while there is room for experimentation elsewhere? In particular, is the diameter of the airflow path the SAME ALL THE WAY THOUGH the system? Or do you need the exhaust portion of the path to have larger diameter than the core/riser to create lower pressure on the back end, to let the gasses slow down and get burned rather than being rocketed out the chimney cap before getting burned.

All of my designs will incorporate very tall heat riser to amplify the front end updraft as much as possible.

For smaller applications I may or may not be that concerned with a long exhaust run/large thermal bench. I will be trying to use a few horizontal runs made from metal to release the heat inside the dwelling rather than a huge slab of permanent cob using up all the floor space. I completely understand that less thermal mass means more fluctuation and fire tending. This is fine for my applications. Plus I have an idea to keep the fire burning unattended with zero thermal mass in the system which I will present as it's own topic.

Thanks for any help, and thanks to all who have pioneered these devices. The efficiency and ability to use scrap twigs and branches that are useless in conventional stoves is truly an evolution in heating technology.

Huge chunky cord wood trees could be put to much better use than belching heat and smoke out in to the cold night air!

: )