smallest rocket mass heater

I was thinking about trying to build a small rocket mass heater that I could just put on my hearth and have the exhaust go up my chimney. 

Does it work if i just make the thing small or is there a lower limit on size. 

Im guessing if i make the burn chamber with firebricks it might be too big.  Can i make the burn chamber out of cob?

What's a small barrel I could use?  any ideas?

can i cover the entire barrel with cob to act as mass since I won't have much of an exhaust run before it goes up the chimney.  or will covering it with cob make the gas in the chamber cool too quickly and have the rocket stove not work?


thanks!

Certainly small is possible. I have seen a picture of a rocket lamp that runs on twigs. Everything can be scaled down. One thing to look at is what do you do to the fireplace. Here is a link to a drawing that shows a retrofit to a fireplace with a masonry heater:

http://mha-net.org/graphics/chernov04.gif


The thing to note is how it is insulated from the inside of the fireplace so as not to loose heat up the flue. I would not worry about covering the "barrel" with cob, Normally it would be open to air and radiate heat away. Initially the cob may keep the part cooler than normal, but as it heats up it would actually end up hotter. The purpose of the exposed barrel, is to give "quick" heat. Covering it will mean you have to wait to feel the heat, but maybe it will last longer. The only worry I would have of putting cob right there is cracking. The temperature differential from bottom to top of that barrel is quite high. Also, any straw that is too close to the surface might smoke.

If you make it small and inexpensive, maybe you can try different things. Maybe use 36x36 paving stones to protect the original in case you want to take it out (like to sell the house). Make some models outside first.

Also check out Paul's portable rocket mass heater for some more ideas (he has it in a wood box).

Here is another try at what you are doing:
http://www.iwilltry.org/b/build-a-rocket-stove-for-home-heating/

Cob used around hot surfaces should not contain straw.  Depending on your design, you may want to use some non burnable reinforcement material such as chicken wire. 

Cob around rocket stoves it will most likely crack, unless lots of time and care are taken to dry/harden the cob very slowly well before the stove is fired up.  Most people want to test out the stove right away and so cracking does happen.  But look at outdoor cob ovens as an example, in which more care is taken in the overall thickness and slow drying process.  When done right almost no cracking is evident, so this 'cob' issue can be addressed as part of your design and process.

ok, i'm having a hard time finding the metal tube I need to build this and I understand that the vertical rocket part of this needs to be well insulated.  So I was reading up and thinking of maybe using concrete mixed with vermiculite and hand shaping the burn chamber and the rocket burn chamber out of this vermiculite concrete.  Maybe I could use some chicken wire or hardware cloth as a form to mold the concrete over.  I can get a bag of concrete for 2 dollars and vermiculite for less than 20.  Then I get a dryer duct for the exhaust for 10 dollars and find a barrel and Im in business!

comments.

oh and thanks for this link

http://www.iwilltry.org/b/build-a-rocket-stove-for-home-heating/

that was cool.  if i knew how to weld and had access to a welder, that's exactly what I would do!

neoplasticity wrote:
thinking of maybe using concrete mixed with vermiculite and hand shaping the burn chamber and the rocket burn chamber out of this vermiculite concrete. 

I'd use castable refractory cement instead you might have to call around a bit but where there are commercial boilers to be serviced you can find the stuff

but this seems like a decent substitute http://www.visi.com/~darus/foundry2/refractory.html

neoplasticity wrote:
ok, i'm having a hard time finding the metal tube I need to build this and I understand that the vertical rocket part of this needs to be well insulated.  So I was reading up and thinking of maybe using concrete mixed with vermiculite and hand shaping the burn chamber and the rocket burn chamber out of this vermiculite concrete.  Maybe I could use some chicken wire or hardware cloth as a form to mold the concrete over.   I can get a bag of concrete for 2 dollars and vermiculite for less than 20.  Then I get a dryer duct for the exhaust for 10 dollars and find a barrel and Im in business!

The idea of the vertical tube is to get things really hot. Concrete that uses portland mortar as its base starts to loose its strength at about 700C, clay might be better... still mixed with vermiculite.
What I have read suggests that once it looses its strength due to heat, it does not regain the strength on cooling.

On the other hand, I have seen a site where they use cinder block  to make a rocket stove for cooking. It was however experimental and the video I saw was of it's first burn. It would be interesting to see how it would stand up to a long hot burn.

You could make a really small one with a paint can... 1gal for tiny and 5 gal for small.

hmm, i didn't know that concrete loses str at high temp.  see.. this is why i ask!

so clay would be better.  I wouldn't have to fire the clay since using the stove would fire it up right?

good idea for the paint cans.  im sure those are plentiful.  i think a 5 gallon paint can would be a good size for a trial run of a small rocket heater....

if my yard has alot of clay soil, think i could just dig a foot down and harvest the clay from there?  or should i go buy clay?

Actually you can scale an RMH down below 6" and have any dependability. Laminar flow eats the duct diameter down till it stops the flow of exhaust gasses. You might be able to do a very short system but that would negate the heating effect of the stove and you would have to babysit the stove anytime it was burning.

the smallest system we have been able to get working reliably is 6". However; you are welcome to try a 4" system. I would build it outside first and make sure it works before i put it in my house.  Smaller cooking rockets work very well but they should not be mistaken for the RMH  due to the different tasks and configurations involved.

A rocket cook stove is a single tube with intake at the bottom and exhaust out the top. this allows for very short systems and you laminar flow has no time to establish itself.

when you say 6 inches, do you mean diameter of the rocket tube or the height?  I was thinking that there is some minimum size for it to achieve the flow dynamics to work.

I'm definitely going to build outside on my porch and test it out before I build it inside.  my wife thinks that the pictures i've shown her look ugly and doesn't want it in the house but I think if i make it cute and i show her that it works, she'll let me build it inside.  I dont mind babysitting it.  I like playing with fire javascript:replaceText(' ', document.postmodify.message);

also, I live in socal so the coldest it gets here is mid fifties during the day so i dont need to run it that much.  I just have a ton of scrap small wood in the back from trimming all the trees while i setup my food forest that i think would be nice to put to use.

typo the word is cant  The size we are talking about is a 6" system (six inch steel stove pipe) this gives you a cross sectional area in the feed tube and burn tunnel 5&1/2" by 5&3/4" or some configuration that is roughly square.  4" ducting like that used on gas furnaces is to small for a RMH due to the narrowing effect of laminar flow in the duct.

I assume you have shown her this picture.

From dana annex rocket stove plaster

here is another slightly better for your application i think.

I will be posting these to an album with descriptions of the process. this one does not burn as clean as a RMH but its a real good adaptation of some of the principals.

From ernies pics

Ernie, I'm intrigued do you have the how-to-build info for this white stove in the pic?

Thanks

Hi Jami yes we do and we will get it up on our picture site soon. we have  a small back log to clear so it may be a few days. Its a pretty thing isn't it?

It *is* pretty -- I like it!  I've thought all along that eventually, as the concept matures, people would figure out ways to make the RMH look nice for inside their houses.  I personally don't mind the barrel sticking out, but I was raised on a homestead in Alaska where we heated with a barrel stove (now THAT is an ugly thing, not so much in appearance, but for the huge amount of wood it burns, and for the [lack of] safety factors -- one of my brothers and I both got pretty badly burned when we were little).  I think that what you like has a lot to do with what you grow up with.

Kathleen

Ernie I love it for how similar it looks to a Rumford stove -

http://mha-net.org/docs/v8n2/v8n2page2.htm

Would it be possible/efficient to paint the barrel black, then build an open latticework of brick around the sides of the barrel?  The top of the barrel would be left as is for tea kettles and such, and the open brickwork would be decorative and still allow the "quick heat" to radiate off the barrel.  This might also act as a protective barrier to keep small hands from playing with the hot barrel.

you can paint the barrel black... no problem with that. or with painting it with patterns or what have you. the brick work is going to limit the amount of quick heat but thats not a problem either if you want to do it. the book is just a guideline for building the stove itself. if you want to plaster the whole thing over you can do so you will just need to make the design changes that will allow it to work with your modification.

its made of cob you can sculpt it to what you want.

Muzhik wrote:
Would it be possible/efficient to paint the barrel black, then build an open latticework of brick around the sides of the barrel?

Maybe build the latticework out of roof tiles (tejas), so as to have a lower cross-section blocking the radiant heat.

I could see such a latticework also working to shape convective airflow around the barrel.

Hi forum newbie here
Question about the minimum chimney size of 6" due to laminar flow in smaller pipe
Has anyone tried a flow disruptor in the chimney or any type of baffling?
Just curious

Thanks
David 

yep. our observations where that it allowed slightly greater flow for increased distance. however not enough to consistently overcome both internal drag and environmental variations. 

Go fiddle with it and let everyone know; it would be nice to see someone else's data.

Paul talks with Ernie and Erica about rocket mass heaters in this podcast: rocket mass heater podcast

They also talk about how small you can reasonably go with rmh's

i just finished building a very small rocket heater. i used 3" pipe and so far it works very well. i made some videos of the build here,

cheers. Jaime.

nice stove i would like to get pictures and see how it works over time. I shouldn't think you would have any issue with drag in a stove that has no mass to push exhaust through. hows it heat and how much wood does it use.
Can the extinguisher be removed to clean off the riser? do you have i diagram?

very nice work by the by.

it throws off quite a lot of heat. downside is that the burn chamber is small, so frequent feeding is required. i built it to heat a 100sq ft cabin. i havent yet had a chance to evaluate its performance installed. hopefully this weekend. the extinguisher isnt intended to be removable, but it will probably loosen in the casting over time and become removable. the updraft pipe can be cleaned through the ash clean-out in the bottom. i didnt use any diagrams, it was all done on the fly. did you watch all 4 videos? the last shows the stove in operation. cheers. Jaime

sattlelight connection and heavy cloud cover wont let me. How much heat s coming out of the chimney? I assume that you are using wood. have you tried a harder wood like oak?

the chiminy gets pretty warm, just a little too hot to touch. the extinguisher gets hot enough to dance water. havent had the time to run a whole lot of wood through it.

jaime merritt wrote:i just finished building a very small rocket heater. i used 3" pipe and so far it works very well. i made some videos of the build here,
http://www.youtube.com/watch?feature=player_detailpage&list=PL01FFC7A3CB99848B&v=4QbTzU-MG5s
cheers. Jaime.

I like your style, Mr. Porkbrick!
I would probably use carpet tube where you had the white plastic form, burns out a little cleaner, but the fittings are pretty slick.
I think you may have accidentally made aluminum silicate, a high-temp refractory material, from your concrete and the perlite (it's a silica-rich glass foam). Or maybe it's not accidental - your version of 'lazy' is a lot like other people's 'resourceful'.

Our shop heater that Ernie's dad made was a lot less work than yours, but didn't look as awesome either. I love the 'firefighter's nightmare' styling, quite a statement.
Pocket rocket diagram: feed, bucket/30-g barrel, chimney. Can cob, insulate, or put bricks in the bottom for weight stability. Not quite as clean as the updraft-downdraft, but this down-up model makes a good radiant heater.


On the main topic:
When we say 6" for rocket mass heaters, we are talking about the ones that heat a thermal mass after the combustion unit. You might get away with a tiny thermal mass, or a vertical chimney.
The fireplace-insert mass heater would need a small liner all the way up the chimney, or you might get downdrafts of exhaust in the chimney as cold brick cools the exhaust.

For a small rocket-style stove or pocket rocket, where you have no more convolutions on the exhaust than a woodstove, you can get away with very small dimensions. We've done mini-cookstoves with steel drink cans, or a tea stove with a 3" chimney in refractory ceramic: Image is too big, so visit on our blog: Ernie and Erica's Joint Adventure: Oysturkey and the Tea Stump
-Erica

Erica Wisner wrote:
I would probably use carpet tube where you had the white plastic form, burns out a little cleaner, but the fittings are pretty slick.
I think you may have accidentally made aluminum silicate, a high-temp refractory material, from your concrete and the perlite (it's a silica-rich glass foam). Or maybe it's not accidental - your version of 'lazy' is a lot like other people's 'resourceful'.

Our shop heater that Ernie's dad made was a lot less work than yours, but didn't look as awesome either. I love the 'firefighter's nightmare' styling, quite a statement.
Pocket rocket diagram: feed, bucket/30-g barrel, chimney. Can cob, insulate, or put bricks in the bottom for weight stability. Not quite as clean as the updraft-downdraft, but this down-up model makes a good radiant heater.

On the main topic:
When we say 6" for rocket mass heaters, we are talking about the ones that heat a thermal mass after the combustion unit. You might get away with a tiny thermal mass, or a vertical chimney.
The fireplace-insert mass heater would need a small liner all the way up the chimney, or you might get downdrafts of exhaust in the chimney as cold brick cools the exhaust.

For a small rocket-style stove or pocket rocket, where you have no more convolutions on the exhaust than a woodstove, you can get away with very small dimensions. We've done mini-cookstoves with steel drink cans, or a tea stove with a 3" chimney in refractory ceramic: Image is too big, so visit on our blog: Ernie and Erica's Joint Adventure: Oysturkey and the Tea Stump
-Erica

thanks. yes, carpet tube would have been cleaner and if i make another stove like that i will use it or weld the whole system. the use of aluminum based cement was intentional, tho i didn't know if it would work like i wanted.

i see now what you are referring to when you talk about laminar flow. mine and similar designs get additional draft from the vertical, thin walled, chimney. running the gasses through a long run in a thermal mass causes too rapid cooling and loss of draft. is this correct? i like the tea stump! very cool. i haven't seen to many designs for a rmh with a more vertical mass (think masonry heaters), it seems to me that if that configuration works well it would be easier to fit into a retrofit/remodel situation. my next project is a rocket water heater for a hot tub. ill post that in another thread. cheers. Jaime.

Another idea?

[youtube]http://www.youtube.com/user/porkbrick#p/u[/youtube]

if you want the hot tub design let me know i have one worked up that should do really well. After all a hot tub is simply a vat for cooking people. add some veggies and a bit of rue and you have soup.

i would love to see your design. i started working on mine this afternoon (the stove that is, ive been working on the design for quite some time), hopefully i didn't go off in the wrong direction.

jaime merritt wrote:
i see now what you are referring to when you talk about laminar flow. mine and similar designs get additional draft from the vertical, thin walled, chimney. running the gasses through a long run in a thermal mass causes too rapid cooling and loss of draft. is this correct? i like the tea stump! very cool. i haven't seen to many designs for a rmh with a more vertical mass (think masonry heaters), it seems to me that if that configuration works well it would be easier to fit into a retrofit/remodel situation. my next project is a rocket water heater for a hot tub. ill post that in another thread. cheers. Jaime.

Exactly.
A woodstove or non-thermal-mass stove just has a vertical chimney.
Your gasoline-can-fire-extinguisher rocket has essentially two chimneys vs. one downdraft area, lots of draft to pull past the laminar drag.

Heat-exchangers' longer horizontal guts, and the corners especially, add a huge amount of drag, while cooling the exhaust so a second vertical chimney may actually create resistance rather than extra draw.
I wonder if the smaller cross-sections don't just lose heat more rapidly too, adding to the 'cold plug' effect.
The only systems I know about where 4" duct has been used to any advantage are either non-thermal-mass, like yours, or a large combustion-chamber system with a multi-exit manifold and skads of little 4" octopus-legs which all go straight out through a thermal mass. None of them are very long, and they have more total volume to offset drag.

Tall masonry heaters have a smaller footprint, but greater 'moment of inertia' (or more simply, they can fall on people if their toes aren't long enough). So the footings required to handle the load, and the broader hearth especially in earthquake zones, gets prohibitive. Some people just cut a hole in the floor and go for it, like . Others find it's easier to add a low mass, maybe take out the second-best sofa, and keep their options open by not destroying the hardwood floor.

(All you fellas, now follow Jaime over to a new thread and take the little water-heater gems along.)

-Erica

Would a copper hot water tank stand up to the heat of a RMH? It would look nice polished up.

Or I was thinking a perforated stainless steel shroud like they have around truck exhaust pipes would lookgood

aman

I dont think the copper could take the heat of the heat riser but it could handle everything else. however as Erica said Copper is tricky stuff it dont radiate worth beans it just gets damn hot. SS and brass would be good. titanium might work well, bronze, old real iron would actually be lovely, hmm i could steam punk one out. I have not played much with the looks cause i like our barrel and if i want it shiny i know where the sand paper and never-dull lives (ya its toxic, ya folks can get cancer from huffing the fumes, but its one of the few things that will polish and clean almost everything).

Ernie, I have a question. Is it practical to combine the 3 functions: heating space, heating water and cooking all into one centrally-located do-it-all rocket stove/tush-warmer? I would like to buy plans for such a combination unit. If you think it is better to not try to combine so many functions, which one(s) would be better separated. I am planning to build a house this summer and I feel that the rocket stove should be the heart of the house. The white heater/cooker is very attractive, indeed. In sum, I am comfortable working with cob and various masonry materials, etc. but I do not feel comfortable designing the thing myself. Do you have such a design for sale? or could you make one?

Thanks,
Maura

Maura
I would take this to Email and we can talk about it. the system you are asking for is going to be a balancing act and while i dont mind working some stuff out here on permie's this is not a design i would be comfortable having anyone else build at this time. my e mail is eawisner@gmail.com. Water heating and cooking are are year round and heating is not in most places so that would be the option for splitting functions. As i said please send an email and we can get this going.

jaime merritt wrote:i just finished building a very small rocket heater. i used 3" pipe and so far it works very well. i made some videos of the build here,

cheers. Jaime.

Thanks for the great vid Jaime. This has given me some great ideas for building one to go in my small home. Good simple design. Could you tell me what you used to make your hole cuts so cleanly?
Thanks again.

Fred

Jaime hasn't posted since 2012, so he probably isn't going to see your question.
I would probably drill a pilot hole, then use a scrollsaw/saber saw/whatever name you know it by, then file the edges to clean up.


And to answer the question you didn't ask, I wouldn't try to copy that build for a permanent installation. A 3" stove isn't going to put out a measurable amount of heat without feeding round the clock, and the metal innards will tend to conduct heat away from the combustion zone and cool the already tiny fire. If you want a small RMH, you need a highly insulated combustion zone of refractory (ceramic) materials to allow enough heat buildup for complete combustion and withstand the corrosive atmosphere inside a high-temp fire. Without complete combustion you will get creosote in your chimney and in any heat transfer ducts.

How small is your small house? There have been a few 4" systems successfully built and published here.

buddy its made of steel? if you are looking for a tiny rocket stove... check out this really neat one. tiny rocket stove