Rocket stoves in Greenhouses , our own forum topic

Mike Haasl wrote:Thanks Peter!  Great video, wish I could speak French.  So when you say "to a degree of success", does that mean it's a compromise that doesn't usually turn out as good as other methods?  Or it's the best thing ever but it's a bit more work?  Or neutral?

Most of the time it's a compromise in the sense of door sizing and air inlet area/placement. But that can be modified one way or the other.

Mike Haasl wrote:I was mainly concerned with the interface between the steel of the modified wood stove and the masonry of the heater core.  How do you make that connection air tight when the materials expand and contract at different rates...

It isn't clear in the video but I would use strips of Morgan Thermal Ceramics superwool to provide a flexible joint and cob over it.

Mike Haasl wrote:If I could cut the face off of a wood stove and bolt it to the front of a batch box, I can maybe see how I could use some wood stove rope gasket material to seal between them.

Stove rope gasket is also a flexible and heat resistant material to provide that flexible joint.

Mike Haasl wrote:Oh, one more question that kept me up last night.  Does an 8" system need an 8" chimney?  It seems like if the air inlet is 10 square inches (cold air) and the riser is ~50 square inches (very hot air), might the chimney just need to be 4-6" in diameter since it's transporting warm air?

Always use the same diameter as the system, trying to get away with one that's significantly smaller is likely to end in tears. The system size is what it says it is, apart from the port (which is a venturi) it's the size of the smallest passage inside. Without delving in the details of aerodynamics: gas stream in a vertical duct tend to have a fast and higher temperature core. The stream is divided in layers, the closer to the wall the slower the velocity becomes. So the usable high velocity core is already about half of the vertical stove pipe.

Awesome, thanks Peter!  Let me know if you'll happen to be traveling through northern Wisconsin this year

Im planning to put a rocket mass heater in a 3000 soft greenhouse.  I have several ideas of how I would like to do it.  If anyone has experience, I would like to ask you some questions.  
1. Can I put the feed on the outside, run the burn chamber under the wall, and the riser and the bell inside.  2. Then run the exhaust pipe under the ground over 100ft to exit under the wall on the other side?  Can I run the exhaust that far?  I’m sure I will lose some btu to the ground, but I would think the majority of the heat would radiate up into the greenhouse.  Do you think this length of run would reduce the air pull and if so, could I add an in-line fan at the end to help pull the air to the end?
How safe do you think it will be running the burn chamber under the wall and if so, how long can I make the burn tunnel?

Good Morning Andy;  Big Welcome to Permies!
Lets see, To start, I want to recommend getting a copy of the rocket mass heater builders guide. It is readily available on Amazon. It will answer questions you haven't even thought of yet.
What part of the country are you building this greenhouse in ?  3000 sq' is huge, it's going to be a real challenge to try to heat if you are up north.
OK)  The earth will steal any heat...  Insulating under your pipes is a must.  An 8" RMH can only push 50' horizontal without bends. Adding a fan might help but you would be relying on the power staying on all the time, if it went out your rmh would stall and start to flow backwards!!! Not a good thing.
Burn tunnels and every dimension in the core burn unit must not be changed, or your rocket will not roar. So you can not extend the burn tunnel.
I suggest looking at batchbox's and keeping the whole unit indoors.

Were you thinking a simple plastic type greenhouse?  Or were you thinking of a more permanent structure such as) Mike Haasl has built in northern Wisconsin. Mike wants to add a batchbox heater to his greenhouse. (See Mikes posts all about his greenhouse here at Permies) Here is a link to Mikes post ) https://permies.com/t/76165/Mike-passive-solar-greenhouse-design
I have a hard plastic greenhouse/artist studio  in northern Montana , 8" J tube rmh keeps it warm all winter with no fire all night. But its 12x20 with 16' ceilings, so nowhere near 3000'

Well Andy, I hope I haven't bummed you out too much. It can be done! Its just a little bit more involved than a guy thinks.  

Thomas,
Thank you for such a quick reply. This greenhouse is a Conleys. It has a 4 foot louver on the roof which opens for passive cooling. It also has two large fans on one side and a wet wall on the other side. It also has roll up sidewalls.  I have been thinking since posting my question. And I believe you are correct and that putting the entire rocket mass heater inside is the best decision. I can put it in the front as you walk through the door on one side. The wall limits sunlight from getting to that side of the greenhouse so putting the rocket mass heater there Will not take up space or at least usable grow space.  I’ll check out those Resources that you listed and thank you. I am in Southern Oregon. The summers are hot and the winter’s although cold are very mild Comparison to Montana winters Or Wisconsin.

I have been searching through all of the wonderful information here and I believe my head is now spinning. I am trying to gather everything to build a RMH in my greenhouse....right away. I've seen so many different options and designs.
Question 1. My greenhouse is wood/glass but dirt floors. We are VERY much clay in this area. I am wondering what the base needs to be.. I mean, do I need to put in a slab? or could I build it on the ground? I understand the design portion (I believe) but I obviously need to figure out the foundation before I begin.

Question 2. Is a steel pipe ok for the burn tunnel? or should it be fire bricks?

Thank you in advance for any assistance...

Hi Amy, welcome to permies!  

I'm not even an amateur yet, but I'm thinking #1 depends on if the soil in you greenhouse is ever going to freeze (frost heaving).

I think #2 is "no", metal in the hot fire zone will spall and burn away to nothing over a relatively short period of time.

Now for the experts to come in and give you some good answers........

Hi Amy;  Another big Welcome to Permies!
I would like to start by asking if you have a copy of the Rocket mass heater builders guide? Readily available on Amazon. It will answer questions you haven't even thought of yet.

I have an 8" J tube in our greenhouse / studio. The floor under the mass is dirt.
To compensate for losing heat. I simply put down 4" of heavily strawed cob.  Makes a fine insulator and keeps that heat rising instead of heading down into the earth.
2" EPS foam board with 4" of concrete on top would be better but cost and time both rise.
In my auto shop I have an 8" J tube feeding a large brick bell. I did put down 2"EPS with 4" concrete on top for that build.

#2) Sorry any metal used in the core burn unit will fail.  Your core can be all firebrick or hand cast with fireclay and perlite.
The newest designs are using ceramic fiber boards for the core and ceramic fiber blanket for the riser ( five minute riser). A mixture of insulated full fire bricks and split firebricks is also commonly used.
Dimension's in the core burn unit are critical. They must be correct or you will not get the results you are hoping for.

What size is your greenhouse ?
Are you hoping for year round use or just extending the seasons?

No matter what design, style rmh you want to build. Our helpful crew of rocket scientists are eagerly awaiting your questions!

Thank you so much for the welcome!

My greenhouse is 20x25. I do use it year round, however, it is mostly for seed starting. We don't have horrible winters here, but we do tend to have crazy freezes in the early part of the year. Last year I lost a bunch of my seedlings and I don't want that to happen again. I just need to keep it above freezing at those times. My plan was to build a 6" J tube and use the warming bench for my seeds that like to sit on a heating pad. LOL. I've honestly even thought about just doing a small wood burning stove surrounded by bricks, etc for thermal mass, no more than I'm actually gonna need to use it. However, I do have a large hoop house as well and will be building another and would love to plan for larger RMH systems for those.
I ordered the plans for the 6" system so that I could have a close look but I honestly think I'm even more confused now!?!? Maybe I need a nap!

another quick question.... if I cast the riser from 'fireclay' and perlite, what are the ratios for making that? The only clay I have worked with on the farm, has been the dirt! Great for cob!

Amy;
For the riser it is highly recommended that you get a bag of fireclay.  You can use your farm clay everywhere else but the riser gets very hot and  your local clay will work... but ultimately will fail sooner.
Fireclay should cost between $7-20 a 50# bag. The mix is heavy on the perlite, just enough clay to make it stick together.
Rather than that. I recommend springing for a suitable size piece of ceramic fiber blanket.
Fit it inside of a section (or 2) of standard stove pipe and your done (5 minute riser) will take temps in the 2500 F so it will never fail, they have been in constant use for several years now and so far, not one report of failure!

Thomas, thank you so much. Hey the stove pipe and ceramic fiber blanket  sounds like a great idea for me! But let me make sure I'm not misunderstanding. You said fit the ceramic blanket inside of the stovepipe for the riser? would that mean that I need to go up a size in the stove pipe? I apologize for all the questions but I sure appreciate all the help!!!

Hi Amy;    Yes , 2" larger.   You use 1" thick cfb ,so an 8" riser you use a 10" pipe.
Here is a link to a post all about it. https://permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket

Hi Amy;
20 x 25 is fairly large. Even though you don't have a northern winter, I would suggest building an 8" over a 6".  Burn times are longer on the 8 and the temps get hotter, so your mass gets hotter.
Our greenhouse is 12 x 20 and the 8" can keep it warm with no fire all night long.
Using the bench as a warming table for starts is perfect, their little roots love it.
If plain clay bricks are available cheaply, consider boxing your mass in with them.  That allows your cob mix to be not so critical , it mainly just fills the air space between your large rocks.
It also is more mass that heats up and then shares its heat overnight.
I've included a few photo's from my greenhouse build. You can see the cob lasagna of cob & rock.
In the last photo you see the 180 turn of my pipes.  That is exactly how your plans will show it built... other than not having a clean out door.
Don't build it that way!  Where your 180 turn is, you create a  small box from brick or cob or even a metal barrel. Run your pipes into that box , give it a clean out access and its good to go.  
The reason behind that variation is, by building the box you have eliminated the resistance of that tight corner. Your RMH will burn better for it.

Thank you so much! That was incredibly helpful! I will absolutely take your advice on this. I will switch to the 8" system and will definitely build the clean out box. I put in the 'cob' base a few days ago and I am hoping to begin the building process today.

That's awesome Amy!  Starting your build today!!!
You will be glad you choose to go with the 8" and with the box design rather than the 180.
Are you using a barrel as your transition area ? Or are you building a brick box like I did?
Both methods work well, just no metal cutting needed if you build a box transition area over a metal one.

Here is a link to a post here at Permies with many different ideas. https://permies.com/t/61657/Flue-exhaust-transition-plenum-pictures

You guys are awesome with all the info. On the box transition area, my husband can’t pass up a chance to work with metal so his is currently building me one...while I pleasantly hurry him along. 😂
But I have another question because I’m finding sooo many different answers. What mix of cob is best for the bench area? I’m using rocks in it as well but I need to know the best cob mix to surround everything.
I seriously appreciate all of y’all’s experience and willingness to lend a hand!!!

Hey Amy;
As much rock or other solid items as you can, the cob is just the filler, at least until you get to the surface.
Are you building an exposed cob bench then?
If so, then the top few inches will be the important ones . You will want cut up straw in the mix, to help bind it all together.
After that, there are "finishes " you can  apply to help seal it up.
Cob mixture varies with the purity of your clay.  
Mine was pure, so I added three parts of sand to one part of clay. That's a lot of sand to load and haul!
Other folks have a perfect clay sand mix right out of the ground! No real rule but if its to sandy , it will rub off. If its to clayee then it will crack.
Be prepared, that almost all cob cracks and will need patching... not a big deal.
When you light off your rmh in the beginning , it will drip "black stuff " from the pipe joints , even though you taped them! Again no big deal, they all do it to a certain extent..