Building My First Rocket Mass Heater (Tommy's Tea Dome)

    Hi there, my name is Sonny and this can act as my formal introduction to you all. I've been a long time lurker of this forum, and only now I feel I am ready to post my first thread. As I mentioned, my name is Sonny and I am 21 years old at the time of this post. I have studied and apprenticed at CalEarth Institute and taken every Superadobe course that they offer. Some of you should hopefully already know what CalEarth Institute teaches and for those of you who don't, they teach sustainable earth architecture based around strong shapes; such as domes or vaults. This is my passion, and I am so lucky to have already discovered what I want to do in life at such a young age. I know this could of course change, but I know for certain I am going down a good path; wherever it may unexpectedly lead.

My reason for posting this thread is that I, first of all, would like to document my progress on building my first rocket mass heater, as well as receive any advice from all of you knowledgeable folks here at Permies. I have read many books regarding the subject of rocket stoves/rocket mass heaters and I know the basic fundamentals and principles, and with that being said, I probably will not be asking many beginner questions. However, there are numerous questions that I do have, which are not easily answered by searching Google, or Permies for example. As I mentioned above, I build with the earth and I have a structure in particular that I built for my brother in Northern California. This structure is going to house my very first rocket mass heater. I have gathered 90% of the materials that I will need to build this rocket mass heater, and I am nearly ready to start assembling it. I figured I would post here first in order to straighten a few things out and organize my thoughts. Below, I will show pictures of the structure in which I will be building my rocket mass heater:

Note: if you are only interested in the rocket mass heater aspect of this post, scroll down towards the bottom and you will find where it says "Rocket Mass Heater Section"



As you can see, it is not by any means a conventional shaped building. This was constructed entirely out of earth, and once again for those of you who don't know, this is what CalEarth Institute teaches. This dome is a 12' diameter with a 2' spring line, and I know it is difficult to interpret how big or small this structure is so I will tell you it is 14' tall. The purpose of this dome is not to act as a usual home, but rather a change of physical environment for my brother Tommy who will use this structure as his tea house. I will quickly note that my brother Tommy suffers from muscular dystrophy and that I built this dome for him so that he can take his passion for tea to a whole new level. Here are some more pictures of Tommy's tea dome:





Below, I will show a brief reference to the building process. (In case it is unclear, this structure is built by filling long polypropylene bags with earthen material. Two compasses are used to ensure a symmetrical dome. Barbed wire is weaved between each layer of bag. Each layer is also tamped with a cast iron tamper to compact the material. The structure is then rendered and plastered leaving guests wondering what the structure is made out of.)

It all started with a scale model of what we wanted to build, a single dome with a short entry vault:



We then mapped out our building site and began digging the foundation:





Our first courses of bags:



Fast forward a few months:



And then fast forward to the completion of the bag-work:



That brings us to now, and it is very close to being a complete structure. All that it is missing is a source of heat, which is where this post is heading.




I'm going to draw up some plans for the rocket mass heater very soon, and I will edit that in. For now, I will post the original floor plan of the dome for reference:



Rocket Mass Heater Section

Here is a picture of the dome's interior where the rocket mass heater will be constructed:



I've narrowed the placement down to this spot between the center and left windows. The bench will run along the perimeter of the dome and it will stop at the very start of the window on the right:



Details About My Rocket Mass Heater And Questions For Whoever Is Reading

Hopefully, you can understand the shape I am trying to create for this rocket mass heater. It will be circular shaped, following the exact contour of the wall and it will be roughly 18" high and roughly 20" wide. As I mentioned above, I will draw the plans for the stove soon which should clarify my construction idea.

As of right now, there are a few things I am uncertain about. First of all, I plan not to insulate underneath my first layer of bricks or near the wall as this structure is all one big chunk of thermal mass which I want to heat up just like the bench. The floor is a 4" thick concrete slab with a vapor barrier underneath, and the walls are made out of adobe. I am curious on any of your thoughts regarding my ideas here, I believe I am correct in wanting to construct the stove in this manner. Let me know what you all think.

Second of all, I am uncertain of my chimney and how high it should be. In the picture above, I have a piece of 8" stove pipe that is 5' tall. I don't have a chimney yet but it will be placed roughly where the silver colored stove pipe is in the picture above. I'm going to carve an 8" diameter hole right above this pipe for my chimney to exit vertically. Is my chimney in the right place? How high should it be after it exits? These are things I have no way of researching as most people aren't building rocket mass heaters in dome-shaped structures.

These are my main two concerns at the moment, and I'm sure I will have more as I progress.

A few details I should mention are that I am building a J style rocket mass heater based off of Erica and Ernie's work. I have copied their 8" system dimensions leaving me with a 50" heat riser, a 24" burn tunnel that is 7.5" x 7", and a 16" feed tube. My barrel is stripped of paint, and ready to go. My bricks are refractory good up to 3000 Fahrenheit as well as my heat riser split bricks. I'm going to use clay mortar for all of my bricks, and this is something else I am a bit uncertain of. Should I use just clay slip, or clay and sand? The bricks are pretty symmetrical, only a few gaps here and there. I have a ceramic refractory blanket which will insulate some of my burn tunnel and all of my heat riser. This will be wrapped with a wire mesh. My manifold will most likely be made out of brick in an octagonal shape, although I am tempted to buy another barrel to use instead. Any recommendations on this?

My stove pipe will run about 15' before making it to the chimney. It will follow the wall to the far right window and then loop back towards the chimney. I'm going to utilize many 90 degree elbows in order to make this work.

Once the heart of this rocket mass heater is complete, I will be using cob to render the bench. I think that is the gist of my build and I am very curious to know what you all think. Once again, this is my first time building a rocket mass heater. Though I have done my research as much as I could before posting, as you can see I am working with some variables which are not all that common. I appreciate any feedback or advice and thank you for reading my long post if you made it this far.

- Sonny

Hi sonny; Welcome to Permies!
Beautiful tea house you constructed for your brother !  Superb work !
As far as your thoughts on your RMH.
Under your core will heat up too much without insulation of some kind, and eventually destroy the concrete underneath !
Under your mass will not be that hot.
Your dimensions all sound good.
Making a thicker mix of fireclay and clean sand , then spreading it by hand on each brick works better for me than dipping in clay slip.
Making a brick transition area worked better for me. Others like using a barrel, its personal preference. As long as it is large enough!

Now about your bench. Have you considered using half barrels as a stratification chamber bench rather than a piped bench?  Faster heat, easier to build and no flow restrictions.  

Keep us posted with pictures as you build.

Welcome to permies! Beautiful structure. Do you have a feel for how many work-hours are in it?

Thomas, do you have a reference handy for the suggested half-barrel option? I do not think I am picturing it correctly...

Hi Dillon;  Page 82 of the builders guide shows it well. If you don't have a copy yet then I'll try to explain it for you.
The bottom half of a 55 gal barrel is custom cut to clear the core unit and still allow the upper barrel to clamp to it.
An exit is made in the bottom barrel going to your mass. This exit must be large enough.

Gotcha, thanks. I was trying to picture it replacing the bench, in the orientation of the bench..

Wow, great build. Love Calearth and what they do to teach.

I'm going to reiterate Thomas and mention the importance of insulating under the burn chamber. My first build was similar and it eventually burnt away the cement beneath the burn chamber.

Dillon, i think you were referring to Thomas' mention of the stratification chamber rather than the double barrel initial bell technique show in my photo. Thomas prefers using bricks for the transition area.

The half barrel stratification method is using barrels cut in half lengthwise and used for the second bell of the system as demonstrated here http://donkey32.proboards.com/thread/716


Sonny, if you do stick with the piped bench, remember to calculate the total distance with +5 feet for every 90 turn. If you use a lot of elbows you'll need to figure how many 90s you end up doing before the chimney which looks like it will be in a good spot near the barrel. You should clear the top of the structure by several feet.

If you are unsure about cutting your bricks for the octagon riser, you could do a square riser with corner pieces for the first half of the riser which is successful at giving my stove the "rams horns effect". I've included another photo of this.

Good luck! and post some photos.

Hello Sonny and congrats on a cool design for your house. I have concerns with not insulating under the heater. I built one with the same thoughts as you that the floor would also act as mass. Boy was I right. The problem is nothing was done in advance to insulate under the cement. I see you did this also. The Earth is a heat sink and some of your heat will go through and keep going, never to return. Insulation under the mass or even better, elevation off the floor will go a long way at keeping the heat of your mass inside the room. Then the concerns of using building structure for heat. Will it take the expansion and contraction of the high heat and then cool? Was it built for that? In your case it might be fine. You will have to see. I would also like to have the chimney pipe exit higher if not right through the point. The ceiling will be the warmest there and the pipe will draw wonderfully even on cold start. Insulated pipe through the masonry and above.
Brad

thomas rubino wrote:Hi sonny; Welcome to Permies!
Beautiful tea house you constructed for your brother !  Superb work !
As far as your thoughts on your RMH.
Under your core will heat up too much without insulation of some kind, and eventually destroy the concrete underneath !
Under your mass will not be that hot.
Your dimensions all sound good.
Making a thicker mix of fireclay and clean sand , then spreading it by hand on each brick works better for me than dipping in clay slip.
Making a brick transition area worked better for me. Others like using a barrel, its personal preference. As long as it is large enough!

Now about your bench. Have you considered using half barrels as a stratification chamber bench rather than a piped bench?  Faster heat, easier to build and no flow restrictions.  

Keep us posted with pictures as you build.

I will definitely be insulating underneath the core now that you have all chimed in and used some solid logic to convince me. This makes complete sense to me now, and yes I was thinking the slab would act as another "battery" so to speak to capture and retain expelled heat energy from the core. Now I clearly understand that it will be very important for me to insulate underneath the core in the long run. I'm thinking of using 2-3" thick of perlite mixed with clay slip underneath the core just as Erica and Ernie do and possibly line the floor with aluminum foil first. Then I'll lay my heater pad down and build the core. What do you all think of this idea? Also, in terms of heating the dome, would it be better to insulate underneath my bench thermal mass too? I understand that the concrete would most likely be fine in the long run if I didn't, but would it be smarter to insulate in terms of heating the space efficiently?

Thomas, I had not heard much about stratification styled rocket mass heaters before. I ended up doing a bit of research after reading your comment, and I would love to try this method in the future. As for the RMH in this thread, I have already bought 15' of stove pipe, crimpers, and elbows etc. So with that being said, I will be building with stove pipe as I originally planned. I did, however, find an excellent video explaining this style of RMH if anyone is interested. https://www.youtube.com/watch?v=vXHmcKWfFw4 I'm sure some of you have seen it before, but it definitely helped me understand the fundamentals behind it if anyone is curious.

Dillon Nichols wrote:Welcome to permies! Beautiful structure. Do you have a feel for how many work-hours are in it?

Thomas, do you have a reference handy for the suggested half-barrel option? I do not think I am picturing it correctly...

Dillon, I was honestly so concerned about getting the dome in question built that I did not even think about keeping track of the work hours. I couldn't even take a guess, I started it one year ago in January 2018 but there were times where months had gone by without any work done to this structure. I also built a few other structures around that time, while Tommy's tea dome sat untouched for quite some time. One of these structures I did, in fact, keep a record of the number of work hours that went into it but only in terms of building the structure. It took five of us to build this dome (below this comment) and it took 8 days. We each worked 8-9 hours a day, and this was even in 110+ degree weather in the Mojave desert! The size of this dome is 10' diameter with a 2' spring line which makes it about 12' tall, significantly smaller than the one I built in the previous posts although still quite similar comparatively speaking.

Brad Weber wrote:Hello Sonny and congrats on a cool design for your house. I have concerns with not insulating under the heater. I built one with the same thoughts as you that the floor would also act as mass. Boy was I right. The problem is nothing was done in advance to insulate under the cement. I see you did this also. The Earth is a heat sink and some of your heat will go through and keep going, never to return. Insulation under the mass or even better, elevation off the floor will go a long way at keeping the heat of your mass inside the room. Then the concerns of using building structure for heat. Will it take the expansion and contraction of the high heat and then cool? Was it built for that? In your case it might be fine. You will have to see. I would also like to have the chimney pipe exit higher if not right through the point. The ceiling will be the warmest there and the pipe will draw wonderfully even on cold start. Insulated pipe through the masonry and above.
Brad

Hi Brad, thank you for your comment and kind words. I feel that structurally, this dome will easily tolerate the constant swings in temperatures from the RMH. Though cosmetically speaking, we will see how well the interior plaster holds up to that. It is a type s lime plaster, which has some give to it and I feel that the RMH won't disturb it too much. In terms of the chimney, I am opting to exit vertically right next to the barrel. There would be no way for me to exit straight through the ceiling at its highest point, unfortunately. I'm hoping that the barrel further enhances this RMH's thermosiphon by having the chimney close in proximity to the radiant barrel.

Brad is right. Your floor and exterior wall will make fine heat sinks, but most of the heat will go into the earth or the air, not to your benefit. Insulate and/or isolate the mass from the floor and wall, and you will get better results from your mass. Also, running the chimney out at the peak of the roof will give the best draw, and incidentally the easiest, most reliable sealing at the joint to resist rain entry.

Cross posting... Good for insulating under the RMH. Making an actual air separation so air can carry heat from all sides of the mass to the interior would be best, but insulation is good. Aluminum foil is effective as a radiant barrier, but that only works when exposed to an air layer. Touching other material, it does absolutely nothing.

I'm curious what prevents you from exiting the roof at the peak with your chimney. You will need to go to the same height wherever you locate it, and anywhere else makes more pipe exposed to the cold exterior, and needing to be supported against wind.