question or problem with build

I built a rocket stove- I have used it a couple years- stove burns ok as long as you use small wood but top temp will only get to 650 degrees no matter how small wood is or how strong you increase draft- I made a little video maybe you can look at it and tell me what I have done wrong. My video wanted to cut out -we made several videos one thing not quite said in this video is this inside of the outer barrel there is an inner barrel surrounding the 5" burn chamber in the air space between the 5" square riser and the wall of inner barrel I have sand- my idea was that the sand would be a containment for heat and continue to radiate heat- this does not seem to of that much help. I am wondering what the lower temp problem is- can you help- please watch video to see my design- thank you
link to video below

Hi Howard, that stove looks pretty cool!

The thing about sand is that is is a pretty poor insulator. Some people use it as thermal mass for storing heat, but it is not great at insulating. If you were to change the sand out for loose perlite or vermiculite the performance of your stove will improve quite a bit. In the video you mentioned that you undertstand that steel in the burn tunnel or riser of a rocket stove can be expected to burn out. So if anyone has ideas for a more permanent fix we can wait for their answers.

I may not completely understand the dynamics of the operation. I am responding to the answer given. Let me try to Aks this correctly- the insulator around my burn riser is sand( although I thought this would be some mass storage) . What am I insulating from? I am not fully understanding- what does the insulation around the burn riser do exactly? or how does it help? I am not doubting I am trying to wrap my head around it. I will do whatever need be to make a better stove. I have some other locations (homes) to put some stove in so I have to do some more building. All help appreciated.

Hey Howard, you don't need to apologize for asking questions. Asking questions is what we do here :)

The insulation is to keep heat in the riser. The burning wood in the feed tube lets off smoke and heat. If we insulate around the smoke we can keep the smoke hot. That superheated smoke then burns as it mixes with air and travels up the riser. You are basically creating a chimney fire in the riser every time you use a rocket stove. But to make that happen we have to keep as much heat in the riser as we can. We do that by insulating the riser.

If we try to extract heat before the smoke is completely burned we lose the heat that would have been generated by burning that smoke. Sand is going to absorb more of the heat than a really good insulating material will. So your stove is probably not burning as efficiently as it could be.

When you burn your stove for awhile, after it gets up to temperature, can you see smoke coming from the chimney?

When the stove gets burning it does have some smoke but not anywhere near as much as in times past when I had a conventional wood stove. I may be getting some of the effect but not full as I have the good rocket sound but not have the afterburn?-maybe if that would be correct. Because I had wooded floors and not a ton of space extra in the house my thought was that the sand barrel around the riser would kind of be like an inside mass so that although I didn't have a cobb bench I may get benefit from the barrel inside filled with sand. I am not sure that idea produced that much lingering heat. I have been impressed by the people who have the long cobb benches and fire their stove up for a couple hours and then don't light it or burn it anymore until the next day. I can't quite do the same and no being an engineer or maybe scientist I simply said hey I've got some sand- the sand should hold some heat after it heats up and continue radiating like the cobb bench. It is possible it does yet the inside barrel is a good 1 1/2 inch away from the outside barrel so how much heat will actually go from that inner barrel?? no so much as we get good heat with the heater as I said around 600 degrees which is much lower than others but you do feel the heat in the house and you do have the rocket sound- the problem or issue kind of is you must feed it all day long with these very small pieces of wood or if it goes out the temperature drops rather quickly in house.

Hi Howard,

There are several things that I see in your build that I would recommend changing so that your rocket can really take off.

- Increase the riser top gap from 2" to at least 3". This is a bottleneck area that will add a lot extra friction to the flowing gasses as they shift direction 180 degrees while still falling within the ratio guidelines for the J tube.

- 1.5" gap between the barrel and the heat riser is a bit cramped also. If you still want to use the hot water tank core as your primary heat exchanger, then you could offset it from the heat riser. In other words, it doesn't have to be equally spaced all around which leaves more room for the gasses to exit out the bottom, often referred to as the manifold.

-It would be very helpful if the burn tunnel was insulated. Of course this means that the steel will have to be replaced with firebrick or other heat tolerant material as its life behind a layer of insulation will be very short.
Right now, there is so much heat being lost through the metal that by the time it gets to the riser, it has cooled to the point where complete combustion will not take place in the heat riser.

- Above all, eliminate the sand from the riser. Instead, make a heat riser from either perlite/clay or a single layer of Morgan superwool, often referred to as the 5 minute riser. This will likely be your biggest modification that will improve your situation.

Substituting one function for another, expecting the riser to act like a mass is not its purpose and therefore cause problems.

After all these modifications, a draft fan should not be needed. You may decide to keep it installed if its not impeding the natural flow of gasses with it off for shoulder season use or easy starts, but for regular use, keep it unplugged. Having to depend on its use to me is a sign that something is not right with the system.

Wish you well on your upgrades and do keep us posted of your findings.

One thing people have not said in so many words is that the core combustion chamber design you are using is taken from youtube videos, not from rocket mass heater researchers. This design is fine for an outdoor cook stove, but not at all suitable for a rocket mass heater (regardless of how little mass you use). Welding is not needed for most parts of making an efficient rocket heater.

The best available resource for building an efficient rocket heater, massy or not, is The Rocket Mass Heater Builder's Guide by Ernie and Erica Wisner. It will give you all the dimensions and materials that are best to use. You can have a small mass of cob or brick on the outside of your barrel; just because you can't support a ton of mass in one spot does not mean you can't have anything. If you could support a decent sized wood stove, you can support 4" of cob or brick on your barrel. This would hold some heat overnight at least.

Thank you Gerry- I am slowly but surely getting this wrapped around my head better. A few years back ran across a video of a rocket mass heater and it was super cool. I knew I didn't have the capacity in home to build the Cobb style. I also saw a commercial model called a Liberator which I think I loosely pattern my build after. Has the idea changed about the height/ clearance of my J channel burn tube. When I was watching other video's of builds I do remember numerous people saying to make sure that tube came kind of close 1.5 to 2 in to top of your outer barrel. I don't have to use the fan all of the time but instead of running a preheat propane torch in the throat of the burn chamber to force the warm air I use the duct booster fan to get the air flow rally shooting up the chimney so I don't have a backdraft (which I have had) starting. I have experimented with running the fan when the stove gets going good to see if I could really get it a whole lot hotter- it does burn hotter but not crazy hotter. I may be expecting too much from my stove. There is no cobb or mass other than my inner barrel with sand surrounding my J-tube burn pipe. WE actually get fairly good heat from the stove and burning scraps has saved us money from our regular heat bill- the issue is you have to feed and fool with it all the time because when the fire goes out the place does not have that mass and so it gets cool kind of quickly. I thank you for your response.

Thank you Glenn- this seems as a good resource. I will continue to try to learn and am doing more research like this to improve for the future. I am understanding this a bit better now. My idea of insulating the J tube burn area was maybe ok (although wrong insulator) -the real flaw I see now is that even if that mass of sand heated up which I am sure it does, it is setting inside of the outer barrel and not much heat is actually going to transfer into house. Cobb on the outside of the barrel would give some mass to continue with heating room. I will see about the book. I do have a thought since you mentioned  the cobb. Let me know if I am even close to being right. Trying to get the right diameter barrels or tubes took me a while. in the changing world here you can't poke or look around in scrap yards- my outer barrel was a hot water heater and none of the scrap yards would sell me one even though you could clearly see them in piles. Enough of that story I do have a resource to buy flat steel for not to terribly high a price. How about if I took the J burn tube -built a  square box around it -filled that box with perlite or vermiculite, then build  another square box on the outside of that to contain the smoke to send out and up house chimney and then made a third box on the outside which could be filled with cobb. The idea being the cobb couldn't fall off as the walls of the outer box would hold them to the smoke box. Does this sound more reasonable then what I have done previously? Thank you.

Howard,   Best to start with a system that is known to work, then experiment later. The Wisners or Ianto's book will do just that.
Although there are certain specific guidelines to follow, a basic J tube is easy to build. Keeping metal mostly confined to the outside of the core is a good idea. The Liberator is metal on the outside but lined with insulation on the inside of the core.

Your thought of using perlite for insulation instead of sand will work reasonably well, as the heat riser is the most critical part to keep super-hot. For the outside, you really don't need a container for a cob covering; with chopped straw in the mix, cob has tensile strength enough to hold together as long as it is at least 3 or 4 inches thick and not spread over a long flat expanse. A finished cob mass 2' diameter on the outside and 4' tall will hold up fine. You can plaster the outside for a smooth strong surface.

Cob will have a density around 100 pounds per cubic foot, so 5 cubic feet of cob will weigh around 500 pounds, a static load any decent floor should be able to bear when spread over a 2' square, even better if you can spread it over 2 1/2' square. 5 cubic feet will cover 20 square feet at 3" thick, or 4' x 5'. 3' high x ~7' long will wrap around an 18" cylinder with some left over. So it is really not hard to add some mass.

Your metal core has held up well because it never gets really hot. When you have the riser insulated well, it will get much hotter and begin to spall. You can keep using it as long as you check it regularly for deterioration, and meanwhile start making the core for a permanent rocket mass heater.

Thanks to all who have answered my questions in response to my video. I have watched so many videos of these stoves which have been so amazing. I made another question post which I don't believe anyone saw so I am putting it here as so many seem to answer. I am getting ready to build another heater. I have some 5" square tube lying around so this again will make my J Tube- might as well use up stuff I kind of have. I am going to use the perlite as suggested rather than sand as insulator. I try to remember all I have watched and seen however in my curiosity I get some of it jumbled up. In my original build I did make the heater in many ways similar to the Liberator stove. The height of this is close to the video Paul Wheaton has using a 55 gallon drum as outside of the whole chamber. I believe some have said in videos maybe even Paul that the stoves don't have to be that tall. Question 1) Is there an optimum height of overall stove. Question 2) A previous reply said I need 3" clearance instead of 2" at top of J Tube- is 3" the optimum clearance for this Question #3 what is optimum clearance/space between  J-tube and insulation chamber and also the optimum clearance between insulation chamber and outer barrel. I have a bunch of junk laying around to use however if the chamber pieces I have are not a good diameter I can look around for some other things to use. Of course on the height/ clearance at top of J tube this is just a matter of cutting to size. I know the biggest difference or improvement will be the perlite compared to sand to which I thank all for pointing out. I would like to use the best height and clearance ratios to make the new stove a bit better. Thanks once again for your replies. I may not have this stuff perfect but I have been able to cut my dependance of fuel oil down using the stove I already built. As a bit of help or suggestion I get some woods chips for free delivered by tree cutting companies which I use for some mulch and if I have some of the chips dried out I drop them down the J tube along the burning process which is kind of cool as I don't have to split those up.

As I am sure you know, steel is not going to be a long lasting material to use as a hot face in any insulated rocket stove.
The top gap on a 5” should not be less that 2.5” but there is not really an upper limit so 3-4” will be fine.

As long as you maintain minimum gap requirements to keep the gasses flowing freely, it is then a question of how you want the heat to be directed.
For cooking or boiling water on top of the riser, you would want a smaler gap, while for general heating purposes, a larger gap would be better to minimize restrictions from ash buildup over time.
If the riser gap is closer on one side vs the other, the heat will tend to be given off more on the side with the larger gap.
Great when you may be close to a wall with your barrel and want to direct the heat away from it into the room.