Can chimney be sealed in a RMH

I hope you can answer my question about a rocket stove I'm building. For the internal chimney, I have a 6" tube surrounded by an 8". I plan to fill the gap in between both tubes with perlite. My question is this, can once I pour in the perlite, can I seal off the top of the gap in between the tubes? It would be much easier to transport lying down, rather then worrying about the perlite falling out.

(The design shown will be surrounded by a old propane tank)

What are the tubes made of? Metal is going to burn out at the temperatures a rocket stove will reach.

Burra Maluca wrote:What are the tubes made of? Metal is going to burn out at the temperatures a rocket stove will reach.

Yep Shane, Burra is right. And failure can kill.

Shane Call : 1st, Welcome to Permies.com and our sister site Richsoil.com, and a Big Welcome to The Rocket and Wood Stoves Forum Threads.


I hope that you can see that 1) there are two ends to your tube that must be sealed if you are transporting your Heat Riser laying down. 2) with a 6''tube inside an 8''tube

you are only providing one inch of insulation at any one point between the 2 circumferences, Two inches would be much better . 3 ) with a full two inches to work with the

top can be sealed off by mixing in a mixture of clay slip and perlite to form a cap, Doing the same thing for the whole length of the insulate space is better. 4) A well made

Rocket Mass Heater RMH,, can easily be run hot enough to make the top of your barrel glow red, At this point any metal lining used for the interior wall of Your Heat Riser

is doomed to rapid failure . If you just had a loose pour of perlite the failure of that metal will ''liberate'' the perlite to flow down into the Burn Tunnel, and transport it willy

nilly through the whole system. This is why we 'go with' 2'' of perlite clay slip !

At the Apex of the Heat Riser where the hot exhaust gases slam into the underside of the barrel top The barrel acts as a big heat sink and radiator! There have only been a

few reports of failure of a metal tube -like ductwork- failing when used for that outer wall.

Location, location, location, can you share your location and climate type noting that at many locations your Elevation is a critical component. This makes it easier to help

determine the best size for your home .

Always remember that your RMH is a Space Heater just like any other Wood Stove, and can only be counted on to heat the core of your home, Retro-fitting a RMH into an

existing home is always a compromise. A sketch of your homes layout will help greatly. Remember that the RMH just like all other wood stoves needs clearances to walls

and other exposures. Also what materials does your floor consist of, you have a heat exposure there -And you must plan for the weight.

And last but not least please view any Rocket heater videos found on U-tube with a ton of salt there are huge boatload of stinking steaming crap videos of ''Flaming units of

Death'' passed off as RMHs !

You have come to the right place, everyday we start off a new batch of Future Rocketeers, and remember we all started off exactly where you are now. For the Good of

the Crafts Big AL !

So I can't use two metal pipes? They are both about a quarter inch thick. I don't think they'll melt/burn out, but please correct me if I'm wrong. There Is about 1 .75 inches of space in between and I was planning on just pouring vermiculite. If I were instead use that vermiculite and clay mix, would that work or is the whole mistake that I'm making that I can't use metal tubes that are that thick?

And to answer your question about where I live in my circumstances, I live at an altitude about a mile high I have a 3 inch thick chimney pipe for the duct and it reaches about 15 feet high. I have tile on the floor. I have a diagonal 8 inch feet or two with I put the wood and then a6 inch L shape with an 8 inch to around the vertical 6 inch tube right now I have orange Rockport in but obviously isn't working which is why I joined this thread I can get a flame burning and the rocket affect happens but all the heat goes out to him me and very little is radiated through the tanks I have around the farm. I cut two small propane tanks in half and welded them together which is surrounding the whole shape.

Shane Call : Irregardless of wall thickness of inside and outside pipes that you are using as forms -if the center pipe is 6 '', and the outside pipe is 8 '', Then you have

a 1'' gap all the way around pick any one point, it has 1 '' of insulation and 180º around the Heat Riser's diameter you have another spot with just 1'' of insulation !

With a inside 6'' pipe and an outside 10 '' pipe you have 10''- 6'' =4 inches /2 = 2'' !


Do a google Search for " High Temperature Hydrogen Attack '' ''And ''Steam embrittlement'', No-one expects you to write a Term paper on it just believe this is a real

world thing that will bite you in the ass if you try to ignore it ! For the good of the crafts ! Big AL

Heavy steel pipe for the riser liner will last longer than thin sheetmetal, but it will still corrode and fail. If you are getting it hot enough inside for complete combustion, it will likely fail within one heating season. If it lasts, it will be because the steel conducts so much heat away that the combustion zone can't get hot enough.

And if you have loose vermiculite or perlite insulation, when it fails you will lose all the insulation and maybe have a dangerous situation... and of course this will happen on a cold day when you are burning hard.

If you can't make the internal chimney with metal pipe, what should it be made with then? All videos I've seen on how to make all hace metal chimney's. I'm trying to make this type:https://youtu.be/UBWSNjvO4cc

Shane Call : - I am getting a ''Video unavailable message !''

I am providing a link to a simple rebuild / upgrade for a Rocket Mass Heater RMH, that was previously in use for 5 years . The unit was used in a very warm region

in the Southwest (I Think) And was located in an Adobe / Cob house with little to no insulation, note that the uninsulated ceiling is also the roof ! // Link below :


https://permies.com/t/52658/rocket-stoves/video-Kirk

In this location the heat off of the barrel was sufficient for that homes entire heating season. No additional heat was needed and the original RMH was never used

near full capacity ! As a result the interior piece of metal ducting was not exposed to the Freaky high temperatures leading to its distraction as was expected.


When stove pipe / ducting are used for the interior form for a heat riser made of Perlite and Clay Slip it is with the full expectation that that piece is a sacrificial form

that will fail when used anywhere in the Northern 1/2 of the U.S. It is possible that the video you referenced was also built and used in a manner that never challenged

its physical integrity .


So - Your location, your installation location, and floor type, these will help us more accurately answer your questions For the Good of the Crafts ! Big AL

For longevity and efficiency, you need refractory ceramic materials facing the fire path. They also need to be very well insulated, up to the point where combustion is complete; then you go through radiator or mass storage zones to put the heat to use. A major problem with most woodstoves is that heat is drawn off immediately from the firebox, lowering the combustion temperature and efficiency.

Firebrick available at building supply stores will work for all of the internals and is easy to use, with insulation added around it. For greatest efficiency there are insulated castable ceramic materials; these are often more expensive and require some skill to build with.

Try this link
https://youtu.be/UBWSNjvO4cc

Allen - the video is of a shop heater in England, very small, with possibly 6" x 6" steel feed tube, but the burn tunnel may be lined with 1/2"-1" of refractory. There is zero information about the internals. I think the exhaust goes straight from the base of the propane tank barrel to the chimney. The poster or other commenters mention the idea of adding mass after the barrel.

But all in all, this is not a rocket mass heater example to follow. Something that looks like it could work to heat a small shop in a mild climate.

Most youtube videos seem to be of experimental metal builds, but few of them include followup videos showing how they worked or more likely failed after a season's hard use. There have been a few honest and thorough videos showing the metal internals destroyed from heat and atmosphere.

So in Kurt's video he has steel iner pipe I surrounded by about 2 inches maybe 3 of inflation which of those I'm guessing is like that Clay slip with vermiculite or perlite and then he has another pipe around it. that is the same as what I was trying to do, but you're saying not to. I think what you're trying to say that it's better just to have the insulation alone and not have a pipe on the inside or the outside? In order to do so, what you use chimney duct pipe say 6 inch and 10 inch and then fill in the in between with the vermiculite or perlite clay and then once it's hardened remove the metal piping and just use the hardened insulation as the chimney/ heat riser?

Shane : Close, Removing the inner lining without damaging the Ridgidized Heat Riser is often a major problem. Some solutions is to simply use the lightest and cheapest

grade of cold air ductwork and expect it to burn up in place. Also popular is 6'' heavy duty Cardboard ''Sonotube''® Like you get at Home Depot as a form to fill with pored

concrete to make a pier ! The outside can still be metal -though to get the thickness of insulating Heat Riser you will need to measure the actual space between the sonotube

and 10'' stove pipe.

G'luck Big AL

Advice about how to build needs to be informed by a sense of your intended use. You said "rocket stove"; are you intending a fast heat unit with cooking ability, or are you trying to get long-term even heating of a house? And how big a space are you heating, and what is your climate?

Shane, for portable units. Since you seem to want that.

You can use rigidized superwool. Hard to find, and not cheap. Usualy you can get it at foundry supplieers. It's light enough to be transportable.

Or vermiculite board. Not as durable, and your heat riser has to be square.

For the core of the rocket. Feed tube burn tunnel and begining of the heat riser, you can use insulated firebricks encased in metal. That should be light enough, and portable enough.

As for metal, please read all this. https://permies.com/t/52544/rocket-stoves/metal-burn-tunnel-heater-riser

Yes, I'm trying to heat a small 2 bedroom home, no cooking, and no mass around the exhaust. I live in the sw USA. Fire brick would not work because the propane tank that surrounds the chimney is only 14 in.

No mass? That is a big mistake. It means in mild weather you can't keep the fire running flat out but will have to damp it down to keep from roasting your space, and that will destroy the complete combustion and possibly allow creosote formation. Then the next time you run it hard the creosote catches fire and you have fire where it is not designed for it.

A rocket mass heater needs mass so the fire can be run hot for a relatively short time, give its heat to the mass, and the mass releases the heat over a period of hours after the fire is out (overnight). You need considerable clearances from a stove to walls etc. anyway; if you put mass in that space, you help yourself in many ways.

Since you plan to use a propane tank for your "barrel", you will need to do the riser construction method of two thin shells, inner and outer, with perlite/clay between them. As Al said, the inner shell can be either cheap thin sheetmetal, or cardboard if you can find a tube the right size, or even thin wood strips tapered and bound together into a circle, then burned out with the first fire.

Shane : On this side of the pond we have many ''Jiffy Lube"/ 15 minute oil change shops, where you can get an oil change and all your undercarriage grease 'fitments'

Lubed ! This lube comes in 18 (U.S.) gallon drums that can be suitable for a barrel !


Counter-intuitively With two Identical working RMHs one with a large 55 (U.S.) gallon Barrel/drum, and one in the 18 , or 30 gallon drum- Both have to radiate off the

same amount of Heat Energy. The smaller drum will Always radiate that heat off at a Higher Temperature !


Under the category of information that should have been passed on, but wasn't because ''Every one knows that " refer back to the Video link above -where the top of

'Kirks ' Heat Riser was slanted at its top to reduce the amount of ashes that can build up there ! This almost universal tip sometimes does not get passed on ! Big AL

Ok, so from all the info you given me (which is awesome), I've drawn a design on how I think best to build it. Please take a look.

I've written questions on it, I'd love for you to answer. Anything that looks wrong or improvable, please let me know.

This is one of the coolest forums I've ever joined. So glad so many are willing to help!

What diameters should the heat riser be? 4" inside/8" out, 4 in/10 out, 6/10, or 6/12?

How do I make the vermiculite/perlite clay slip? Can someone tell me ratios and ingredients? Is perlite or vermiculite better? It still seems odd to me that it is going to be solid enough to withstand that much heat and not burn/melt/crumble.

The riser should be about the same cross section as the rest of the system; in your case 6" diameter should be good.

Your burn tunnel must be protected inside the steel, or it will burn out probably within the first heating season. If you are set on steel forms, make it about 8" x 8" and line it with firebrick splits (4 1/2" x 9" x 1 1/4") cut to size with a masonry or tile saw. You can alter the shape to make fitting firebricks easier, but if not square, it has been found to work better to be tall rather than wide. A 4 1/2" wide x 7 3/4" high (interior) burn tunnel would require very little brick cutting, though I don't know if that is pushing the shape too far.

The method I like for making perlite-clay is to dampen the perlite (in a tub) with water, then dust on some powdered fireclay, mix it up, and repeat until each particle of perlite has a coating of clay on it. It should be able to form into a firm snowball and pop apart with firm finger pressure. Too little clay will make it crumbly, but too much clay will make it less insulative. After firing a few times, the clay on the inside will turn to pottery and become quite rigid and strong.

Shane Call : Color me happy , When a new future Rocketeer shows up here we want to make sure that he has the best information possible, its easier on everyone

to get it right the 1st time, and if we haven't pointed out problems in your design you might not mention the change you made, leaving you and us in the dark as how

to fix it later !

As this is your 1st and possibly only build we want to get it right the 1st time. The next sentence below is my Personal Opinion based on Direct observations !

For a 1st time Builder the 6'' J-Bend RMH is the smallest you should go. It is forgiving of minor errors, and will certainly heat your space ( depending on your layout )

With that style of Rocket a 1.5 inch gap Between the top of the heat riser and the underside of your Barrel should be 1.5 '' to 2.0'' ; 1.5 '' is a minimum dimension !!


You still have many more choices in the final metal type of Radiating metal 'drum' you place over top of your Heat Riser -to include using a 30 (U.S.) gallon water heater

tanks , this is something found as scrap at most Heating Appliance Dealers !


I am dwelling on this because in spite of the U-tube video you have seen -the use of metal in the Feed Tube and the Burn Tunnel will give you problems with smoke back

into Your house, and will eventually fail from metal fatigue due to " High Temperature Hydrogen Attack'' and ''Steam Embrittlement'' !

Remember that while the Heat Riser is inside Your metal heat radiating drum, the drum will set on top of the burn tunnel*, the burn tunnel can then use more Cob and more

insulation in its base !

In order to match the performance of the 5 year old RMH from the earlier video you must use Firebrick for the Combustion Core, that is the Feed Tube, and Burn Tunnel

and Heat Riser.


You are getting into personal opinions with the difference in Perlite and Vermiculite. If you are using a powdered fire clay ( Lincoln 60 ) You will use just enough clay- to

-perlite to to ridgidize that structure say 1 part Clay to 3 parts perlite.( and water to wet, not saturate ?) The vermiculite will soak up more clay needing more to stabilize

and thus will be less insulating ! Close to 1 to 1 .

I do not like the RMH so close to plywood walls and recommend an air gap then heat barrier then 2nd air gap to protect all flammable materials that close.

For the Crafts ! Big AL

* refer back to the ''Kirk'' video where the barrel was removed only the top of the bricks that made up the burn tunnel were in view the barrel sets on top of the Heat

Riser rather than surrounding it ! A.L.

Im I'm sorry but I'm still confused on how to make the heat riser. I thought I wasn't supposed to use metal, but one of you said I need to use steel as a liner and someone else said fire brick,someone el se said you don't want any metal inside by the heat riser.

Is there not a cut and dry way to make the inside heat riser using perlite clay?

Firebrick would be difficult because it seems like it would fall apart since it's just bricks on top of each other.

Yes, you don't want to depend on steel inside your heat riser. But there is nothing wrong with using light sheetmetal as a sacrificial form, knowing that it will burn away in time leaving the refractory material exposed. Firebricks are a tried and true method if you have enough space for them, as long as you enclose them in insulation, usually inside a metal or wire cage. With some kind of enclosure, or refractory cement mortar, they will stay together perfectly.