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..
 

I got 9+ hours of burn from using anthracite nut coal.  I can buy it from Tractor Supply. They are willing to take bulk orders.  It comes in a 40 lb bag.  

I did a test burn in a vintage cast iron laundry stove and was amazed at the heat output and length of burn time, without any maintenance.  I used about 2-3 scoops of coal (big red scoop that I bought from Rural King for bird seed.  Probably holds 1 quart, or less).  It put out 300°- 400° heat.  If someone could incorporate coal use into a rocket mass heater, it would be amazing.  No overnight tending of the burn at all.

Hi everybody.I haven't seen answers to any of the questions I have so here goes....
   Zone 6 Greenhouse and RMH build. This is a 120 sq. ft. permanent greenhouse. We will be building a larger more "pro" multi-function stand-alone version somewhere else on the property in the future
   The Purpose: Start plants in early spring for transplant into the garden. (SMALL farmers market and 3 adults). Start cuttings for nursery propogation. Extend season for planted vegetables, (maybe through winter?). Dehydrating fragile foods, (strawberries, nettles, mint, raspberry leaves, etc., in mid summer. Future heat source for home?
  Nothing will be planted in the ground. There will 2 foot wide racks 15 feet long on 3 levels with a rocket mass heater somewhere under the bottom rack. The bench will run for about 25 feet.  I want the maximum battery possible within constraints, rather than ease of use, or quick heat, or multi-function, (cooking, char, water heater, etc.)
 The layout is: A house roof over a deck over a greenhouse with it's own lightly insulated roof. Greenhouse has glass on the S, (equatorial) and E sides, W wall is exterior of house, (Hardi-backer siding). N wall is unfinished wall with metal door. Dirt/ sand floor with 3/4 inch rocks 4 inches deep on top. Perimeter walls have 2 inch R-10 rigid foam 16 inches deep. Greenhouse is roughly 10' X 12' and slopes from 10 feet down to 8 feet tall.
 So here's my questions......
1. I assume I should insulate under the RMH and bench. I was planning on using 1-1/2" R7.5 rigid foam and covering it with 6 inches of 3/4" rocks, then laying the base bricks for the RMH on top of that. Is that enough rocks to keep the insulation from combustion? (it "is" combustible). Should I use something along with the insulation, (reflective?)?? How hot does the bottom of a RMH get? How wide do I need to make the insulation in relation to the bench.
2. Since I'm striving for maximum battery, and more cob means more battery, should I make the biggest bench possible and make my exhaust pipe either spiral or serpentine or something to maximize contact within the bench?? I could make the bench serpentine as well I guess, (slightly longer?).
3. Can I cob over the entire barrel except the very top? (more battery, slower heat).
4. Should I cob over the exhaust on the outside of the greenhouse as well? I could make the exhaust look like a tree and cover it all the way up, but would that help or hurt the flow?
5. Should the exhaust exit the greenhouse down by the ground or should it go as high up as it can inside the greenhouse before exiting.
6. I wanted to make cob steps where needed as part of the bench to sit on or stand on to reach the higher shelf, will the cob handle daily traffic as a step and chair ?. The whole bench will be topped with some type of pavers or soapstone or something durable and waterproof-"ish".

There is an excerpt from Paul Wheaton's new movie on free heat that might be useful:

Hello from Fort St. James Canada everyone!. I am new to the forum, and this is my first comment post. Me and the wife are building an A-frame greenhouse on our off-grid new-ish homestead (been here 2.5 years now), and we are on disability so have been sourcing out everything we can possibly get for free. I've (Adam here) got a big trench dug to put in a RMH, but I had some questions for any RMH guru's that could possibly help us out?.

Question 1: What size of RMH would you guys recommend building?, a 6 inch, or an 8 inch? for an 18 foot long A-frame greenhouse that is about 12 or 13 feet high at the peak? (haven't measured to the peak yet lol, but figured that should be a close enough ball park, also the pics I've attached should give a good sense of the size). I already have a couple salvaged 55 gallon drums as well so would those work for a 6 inch system?.

Question 2: I was wondering if anyone has thought of building the ducting that runs out of the base into the mass part out of brick?. I was considering using brick to make the exhaust running under the ground along the trench so I would not have to worry about moisture in the ground deteriorating the metal ducting?. I do have a bunch of 6 inch ducting as well though so was wondering if that would work OK?. I plan to put in maybe 2 or 3 inches of drainage gravel/rock in the channel first to allow for any water to run underneath the exhaust ducting/underground chimney and under the rocket mass heater itself. Am I overthinking this?. Or could I just use the 6 inch ducting, and make a simple 6 inch system, and fill the ground in around it with pea gravel, and then maybe some grates, or sidewalk concrete pieces between the grow beds where we would walk over the venting in the ground, so as to not have the weight crush the ducting/venting underground?.

Question 3: If I CAN make the venting running out in the ground out of brick, I found a way to lay the brick in there so that the opening stacks best to make the opening 4.5" wide x 6.5" high (you can see it in the pics, I laid out a couple bricks showing what I mean for the exhaust tunnel), which gives me a square inch total of 29.25 square inches. Would having an opening that is not perfectly square, that is taller than it is wide, make any difference or matter?.

I basically have everything else figured out and my head wrapped around it, but was wondering if there could be an advantage to making the buried part all out of brick, and have them be my thermal mass, along with pea gravel filling in the rest, so yeah just wondering if anyone else here has done something similar to that?. Oh and I do plan to use a perlite/clay slip layer after the drainage rock layer before putting in my ducting (whether it be metal ducting, or brick). The spot we built the greenhouse is on a naturally sloping hill, so it should work great for venting the exhaust as it has a natural incline. We basically plan to put the RMH on the low end of the greenhouse, then run the venting in the ground going under 3 raised beds, which will be filled with old logs, and hay layers, then soil and peat moss on top. We already have our grow beds built on the other side as you can maybe see in the pics. I figure that I can also maybe put some sort of grating down over the trench before putting in the logs and soil etc. to keep it from crushing the ducting if we do go that route. Anyways, I hope what I said makes sense. I've added a picture I took showing what I have done so far to give a better idea. Hopefully I can get this done and the poly on before snow flies here. Anyways, thanks for your time guys and I really appreciate this forum, it has been a huge help already. Oh and I did construction for many years, so as far as the work goes, I'll have no problems with that. Anyways, I look forward to hearing from anyone with advice on this for us. Thanks in advance!. - Adam

Welcome to the forum!

I like the look of your greenhouse as it looks neat!

I hope one of our RMH Gurus will be able to answer your questions.

Hi Adam,

Welcome to Permies.

Anne Miller wrote:Welcome to the forum!

I like the look of your greenhouse as it looks neat!

I hope one of our RMH Gurus will be able to answer your questions.

Thank you soo much for the warm welcome, and kind words guys!. Much appreciated!. <3 Sending love from BC Canada!.

Hi Adam;
Welcome to Permies and Welcome to the wonderful world of rocket science!

I'll try to answer your questions.

#1)  I suggest an eight-inch core over a six inch.
The feed tube size on a 6" is very restrictive, an 8" has much more room for crooked wood.
An 8" will also require less tending than the 6" and produce more heat.
Yes, 55-gallon barrels work with 6 or 8-inch rockets.

#2) Yes, you can use brick or even flat rock for the piped mass it has been done before.
As you want your heat to rise and not go down and heat the earth, you will want to insulate under your brick tunnel.
Perlite and clay with straw added is a good option,  sand is another. Pebbles are prone to rolling and might not be a good choice underneath.
Here is a new line of thinking for you.
You can create a bell with 1/2 barrels and not use a tunnel at all...
You would still need to insulate below.
A cob mix would cover the barrels.

#3) The volume of your tunnel must match or exceed the volume of the feed tube, the shape does not matter.
Sloping your tunnel is not necessary.

My questions are, How are you planning on using this greenhouse?
Is it for early starts and late-season finishing?
How cold do your nights get and how warm are you hoping to keep it inside?
Were you hoping to run it all winter?

thomas rubino wrote:Hi Adam;
Welcome to Permies and Welcome to the wonderful world of rocket science!

I'll try to answer your questions.

#1)  I suggest an eight-inch core over a six inch.
The feed tube size on a 6" is very restrictive, an 8" has much more room for crooked wood.
An 8" will also require less tending than the 6" and produce more heat.
Yes, 55-gallon barrels work with 6 or 8-inch rockets.

#2) Yes, you can use brick or even flat rock for the piped mass it has been done before.
As you want your heat to rise and not go down and heat the earth, you will want to insulate under your brick tunnel.
Perlite and clay with straw added is a good option,  sand is another. Pebbles are prone to rolling and might not be a good choice underneath.
Here is a new line of thinking for you.
You can create a bell with 1/2 barrels and not use a tunnel at all...
You would still need to insulate below.
A cob mix would cover the barrels.

#3) The volume of your tunnel must match or exceed the volume of the feed tube, the shape does not matter.
Sloping your tunnel is not necessary.

My questions are, How are you planning on using this greenhouse?
Is it for early starts and late-season finishing?
How cold do your nights get and how warm are you hoping to keep it inside?
Were you hoping to run it all winter?

Wow thank you sooo much Thomas for the wisdom and help!. We REALLY appreciate it!.

Yeah, I was leaning towards an 8inch system, but I was wondering if I'd have enough brick, but I don't wanna cut corners and want to give the greenhouse it's best chance to thrive, so 8 inch it is!.

As far as the piped mass goes, we were hoping to have it basically sit flush with the ground so we could have a couple 4x4 foot, 3 foot high raised beds over top with walkways between so we can get to the other side of the green house. (you can see in the pics how we've spaced the beds on the other side of the wall with approx 2 foot wide walkways in between). So basically it would be the RMH at one end, a 2 foot gap, a 4 foot raised bed, another 2 foot gap, another 4 foot bed, another 2 foot gap, and then a small 2 foot bed at the end with the chimney coming up at the back of the bed and right out through the roof. So thus the reason I was thinking of making a brick tunnel.

Oh and yes we plan to use perlite and clay the whole way on the bottom underneath everything. I was just thinking to add rubble, drainage roc underneath just in case any water ran into there during spring thaw etc.

As far as the "1/2 barrel bell" system you spoke of, I have not heard of that before and am not totally sure what you mean?. Are you saying to run out of the heat riser at the bottom and instead of into a brick or ducting "tunnel" then chimney, to go into a barrel cut in half?. I have not seen such a thing so am having trouble seeing it in my minds eye?. Is there examples or drawings anyone has done of this?. I am open to any and all ideas here.

Basically, yes we want to grow year round in it, as we are off-grid and trying to become totally self sustaining here, being that we are both on disability, and want to become free of "the system" etc.

Our weather here gets quite cold, we are basically near the dead center of British Columbia by Stuart lake, and last winter we had a few days that went as low as -30C (-22F), and an average of -15C (ish) (5 degrees F). We do have a friend who lives 2 hours away and he grows year round using hoop tunnels and big steel wood stove drums to grow year round, so we know it can be done. We plan to try it out this winter and see how it does, and may even add another 2x2 wood strips gap and add yet another layer of 6 mil poly to add another insulation gap. The goal is to eventually replace all the poly with that plastic greenhouse sheeting, but could only afford poly right now.

Oh and I'm not sure what difference this makes, but our ground here is maybe 6 inches of topsoil, and then it goes straight to a tan colored clay (basically like the background of this web page lol). So we have PLENTY of clay here, and it goes DEEP. Our frostline is around 7 feet here!.

Anyways, thanks yet again for the advice. I'm super interested in hearing more about the half barrel bell idea, as I am willing to go that route if we can make it work in the space we have. If utterly necessary I guess we could sacrifice having one of the 4 foot beds. Oh and we also plan to put an IBC tote up above the RMH riser if we have enough room, so we can keep the water warm through winter as well. :)

But yeah, any and all advice from anyone on here will not go unappreciated that's for dang sure!. We are picking the Perlite up on Tuesday, so hopefully we can button the rest of this greenhouse up very soon here!. The nights are already getting chilly here!. The race is on! lol.

Thanks again from:
Adam, Melinda, and Kyla, the Alander family! :) <3

Here is a young Matt Walker describing the stratification system. https://www.youtube.com/watch?v=vXHmcKWfFw4
And a post comparing the two designs https://permies.com/t/154958/Split-barrel-bench-parallel-duct
And another https://donkey32.proboards.com/thread/1824/20cm-tube-half-barrel-indoor?page=1

Check it out and ask questions.

thomas rubino wrote:Here is a young Matt Walker describing the stratification system. https://www.youtube.com/watch?v=vXHmcKWfFw4
And a post comparing the two designs https://permies.com/t/154958/Split-barrel-bench-parallel-duct
And another https://donkey32.proboards.com/thread/1824/20cm-tube-half-barrel-indoor?page=1

Check it out and ask questions.

Oh OK, I see what you mean, I've actually seen the first video and some of Walkers stuff.

I think with the materials we have and the time we have that the extra work needed vs going with my original plan is not feasible before winter hits.

I have since found a website where some people did a very similar idea to mine but just bigger https://midwestpermaculture.com/epic-greenhouse-rocket-mass-heater/#Conclusions, and they said it worked great to keep a much larger greenhouse "well above zero".

So I'm thinking I'll just build a typical 8 inch J tube RMH with the 55 gallon barrel system. I'll fill the trench I dug with a few inches of rubble/rocks (sharp edged so as not to move around) which will allow any snow melt and water from watering plants to flow underneath freely, then I will do a clay slip/perlite insulating layer (approx 1 to 2 inches), with maybe some ceramic blanket or similar insulating along the side walls of the trench (the people on that site I mentioned used blue Styrofoam insulation so maybe that's an option). Then I'll build a brick tunnel from the RMH to the end but having the hot air go into it at the top portion, and the exhaust at the far end coming out the bottom to vent the cooler air up and out. (I was wondering though if I can make the tunnel a bit larger in square inches than what an 8 inch duct would be?, would that cause it not to vent out the chimney as well, being that it will be an 8 inch chimney pipe?, or should I keep it the same size the whole way of approx. 50 square inches?).  I am looking at grabbing some sidewalk pavers and then I'll cover the whole channel with that on the top. If we had more time I would do the bell thing, but sadly we do not have enough material to do that, nor the funds. So I figure hey, this will just be an experiment for this winter to see how well it performs etc. We can always just redo the venting part later and even add a ducted cob bench going the other way next summer. I will be sure to keep all you guys updated on the progress, and give a rundown after winter to tell you all how well it worked in our cold climate here. From the sounds of it, if I insulate the tunnel well enough from the bottom and sides, I should be able to radiate a decent amount of heat upwards, and being that we are on disability and always home, we can go stoke it as many times a day as is necessary to get us through the winter here. Hopefully our experience will help others in their RMH journeys!. I'll keep you guys posted with pics and such here over the next few weeks. We are headed to pick up a 4cu ft. bag of perlite, ceramic blanket, and sidewalk pavers this Tuesday!. Getting excited to get this all buttoned up!. I've attached some more pics of the greenhouse to show how well the plants are doing on the other side etc. I hope you guys like em!.

Thanks again for all the great advice!.
- Adam

Those are some great pictures of a great looking greenhouse!
I think the changes you described involving  a wider tunnel, venting the hot gasses from the rocket into the top of the tunnel and exhausting the gasses from lower in the tunnel all amount to using the tunnel as a long narrow bell.
Because of this, I think the width might not matter, but the overall interior square footage might.

I applauded your upcycling of materials and your willingness to just try things.
Looking at your existing raised beds,  it occurs to me that you could put a half barrel in the base of each of the new beds, and connect them with your already  dug tunnel.
The only drawback I see is the possibility that the first one in line would suck up too much heat.
I think that could be mediated by restricting the opening between the tunnel and the half barrel.
This would be like making each hole in a irrigation hose smaller than the next to even out distribution.

The only other drawback is if you need those barrels elsewhere, like for the rocket stove itself.
In that case, I bet you could build a nice bell out of bricks, stone, pavers, and clay.

Sidenote, I myself want to try building an adobe/cob bell over a pallet wood frame and just let it burn out during use.

I am looking to make a RMH to heat the floor of the greenhouse. The feeder  and chimney will both be outside the walls of the house. The length of the burn tunnel inside the mass will be 30 metres. What is the possibilty of making the burn tunnel so long? (30m)