Rocket mass heater build for a small dome?

Here is a question for all you experts on rocket mass heaters.

I am planning to build a "small", at most 20 ft diameter, dome house out of native materials. I plan to use the basic design of indigenous peoples of the Northeastern (now) US. This would be a bent pole framework, coverings tbd. I do not want to use any platform or concrete pad, just the ground.  

Could the entire floor be constructed as a rocket mass heater?  There is plenty of native stone here (the foothills of the Allegheny chain in upstate NY) as well as a nearly pure clay frangipan accessible.  Since we have LOTS of rain, now coming in more concentrated events due to climate warming,  I was thinking of first putting down a layer of rock as a base. The finished floor will be a clay plaster of some kind (I'm still researching that).  In between can be the rocket mass heater build.

So the BIG ASK: what kind of design could be made to have a rocket mass heated floor?  It would also be a big perk if there could be a stove as part of it.  It will be a relatively small house after all!

Thanks in advance to everyone who can offer suggestions and/or links to resources.

For Mother Earth,
Freyda

I, unfortunately, am not the expert you are looking for.

I see two ways this could be done.

Building the rmh so the pipes go under the floor or using water to heat the floor aka a water heater.

Here is a thread that might be of interest to you or others:

https://permies.com/t/192718/Rocket-Mass-Heater-tent-Gerrrr#1595829

Another idea that is of interest to you or others:

https://permies.com/t/190806/built-rocket-water-heater

Now that I bump this maybe that expert will show up.

Freyda, I think you need to look into:
Mike Oehler
John Hait
and
the Wofati design by Paul Wheaton

All of them deal with how to have the ground be a heat source rather than a heat sink.

Hi Freyda,
Have you taken a look at the Walker Tiny Cookstove?
Not a large footprint for something that offers a pretty good balance of instant heat with thermal mass already built in and if course the ability to cook your meals.
For such a small space, it could be that you'd have to run any stove for too long before the floor was heated, and cook yourself out instead.

Hi Freida;
So If I understand correctly, you are thinking of placing large rocks down as the base of your dome.
Then you are thinking of a clay/sand cob over your rock as your walking floor.
You are hoping to bury pipes in the clay layer to heat your floor.
You did not say what style/size RMH you were thinking of, but an 8" J tube is my guess.
If that is so, your going to need a pretty thick cob layer to protect the pipes from the pressure of walking on them as well as a mass to hold that heat.
You want a fairly thick cob layer below your pipes and quite a bit more over the top. Failure to do that will result in poor performance and too hot or too chilly feet.
I'm thinking 16" cob floor minimum with 3-4" insulated cob (straw) then your 8" pipes and 5-6" over top minimum... that is a lot of cob to put in a 20' circle.

I assume this will be your home and not a meeting area, so you will be living in your dome.
I highly recommend you consider a heated bed/bench rather than the floor, for the most warmth while the fire is out.
I realize space is at a premium in a 20' dome but you will need a place to sleep and a place to sit down, both are extra nice if they are heated.
Cooking can be done on a J tube but your cooking space is limited a batch box is much more suited to cook on the flat top.
Another option would be a full brick bell. They can be any shape or size (within reason) and the large mass of brick will radiate for hours.
They use much less space and provide lasting warmth and can be built with a cooking surface.

Please keep us posted on your progress with posts including many photos!

I'm also interested in a heated floor from a rocket mass heater. I'm wondering about a sort of bell under the floor, with paving slabs, but have not seen anything similar as yet, so will watch to see what else gets suggested here.
In terms of dome structures have you seen the RMH in a Teepee thread here? That shows how the mass of the heater can be used as a peripheral sleeping bench.

thomas rubino wrote:Hi Freida;
So If I understand correctly, you are thinking of placing large rocks down as the base of your dome.
Then you are thinking of a clay/sand cob over your rock as your walking floor.
You are hoping to bury pipes in the clay layer to heat your floor.
You did not say what style/size RMH you were thinking of, but an 8" J tube is my guess.
If that is so, your going to need a pretty thick cob layer to protect the pipes from the pressure of walking on them as well as a mass to hold that heat.
You want a fairly thick cob layer below your pipes and quite a bit more over the top. Failure to do that will result in poor performance and too hot or too chilly feet.
I'm thinking 16" cob floor minimum with 3-4" insulated cob (straw) then your 8" pipes and 5-6" over top minimum... that is a lot of cob to put in a 20' circle.

I assume this will be your home and not a meeting area, so you will be living in your dome.
I highly recommend you consider a heated bed/bench rather than the floor, for the most warmth while the fire is out.
I realize space is at a premium in a 20' dome but you will need a place to sleep and a place to sit down, both are extra nice if they are heated.
Cooking can be done on a J tube but your cooking space is limited a batch box is much more suited to cook on the flat top.
Another option would be a full brick bell. They can be any shape or size (within reason) and the large mass of brick will radiate for hours.
They use much less space and provide lasting warmth and can be built with a cooking surface.

Please keep us posted on your progress with posts including many photos!

Thank you Thomas for your very complete and informative answer to my question about  whether a RMH could be used to heat the floor of my planned small dome.

I realize now that I should have perhaps been more forthcoming about my limited knowledge regarding Rocket Mass Heaters.   I have been reading posts at the Permies forums for a few years now and have viewed many videos on rmh's, but I must admit that my ability to translate the concepts to appropriate plans is limited.  Although I have been many things during my life, including a union Carpenter, a lab technician and a long-time homesteader, I am more a naturalist than an engineer. There is so much information on rmh out there that I just cannot learn it all, and so my initial question was posed with the intention of learning if building an rmh to heat the floor of a small dome house was even possible.  I don't like overheated houses, I'm more comfortable at 65 degrees than at 70, but floors tend to be cold and  drafty and the one house I visited with a heated floor was heavenly... cool head, warm feet!

I really appreciate the details you gave in relation to depth of materials, stone, cob, etc. which would be necessary for running pipe in the floor for such a system. You are right, that's a huge amount of material and would be way beyond my ability to bring in to build.  My idea of laying a base of "stone" was to serve two purposes, to bring the floor up off the ground away from moisture and provide a stable base. The rocks are what is separated from soil when planting trees in the orchard and other agricultural activities and ranges in size from a few inches around to too big to toss. It will be enough work to move enough to build that base so I'm letting go of the heated floor idea.

As for the ubiquitous heated bench/bed/seating, I had considered that but didn't think it would suit me. I sleep "hot" and think the extra heat below me would be uncomfortable.   Then there is the amount of space it would require, as you mentioned. I don't sit much and, if I do, I want the flexibility of sitting at a work table.  
The full brick bell is something I've not read about before.  That sounds like it might be a better option for my needs. Could you point me to some more information about it? Design principles, materials, examples, etc.?  

And as my plans and build progress, I will certainly share photos and updates.  But this winter is the learning and planning stage.

Thanks again for your help and advice!
Freyda

[I have heated my rural house (approx 900 sq ft) with airtight wood stoves for decades and I have cooked on wood cook stoves, not airtight, as well. Even though wood supply is not an issue with my large acreage of forest, I would rather leave the deadfall on the ground and burn the least amount of wood with the smallest release of combustion gases. As I get older, and with arthritis as well, the less wood I have to handle the better!]

Hi Freyda
Here is a thread from my shop stove build.  A brick bell with an 8" J tube feed https://permies.com/t/94980/Brick-Bell-Shop-Heater
And here is the thread where I converted to a Batch box design,   https://permies.com/t/138779/Breaking-News-Shop-Dragon-admitted
Here is a recent rebuild,     https://permies.com/t/181421/Batch-box-rebuild
And here is finishing things up   https://permies.com/t/190599/Finishing-touches-Shop-Dragon-rebuild

That should be enough info for you to think about or it might be enough to make you run screaming in the other direction...

Brick bells (stratification chamber) are fast becoming the most popular building style.

Nancy Reading wrote:I'm also interested in a heated floor from a rocket mass heater. I'm wondering about a sort of bell under the floor, with paving slabs, but have not seen anything similar as yet, so will watch to see what else gets suggested here.
In terms of dome structures have you seen the RMH in a Teepee thread here? That shows how the mass of the heater can be used as a peripheral sleeping bench.

Thanks for your suggestion and link. I didn't see the build but I did view a video a ways back taken in the tipi when I was first searching around learning about RMH.  I've been in tipis before and have great love and respect for the nomadic structures. In fact, I spent quite a long time investigating living in a traditional yurt or ger.  For me, the natural sense of space in a tipi was made claustrophobic by the massive bench.  It just feels all wrong for me personally, although it's fine if other people find it comfortable.  

I'm giving up on the heated floor idea for now since reading Thomas Rubino's information about how much cob structure it would take to make it.  I'm following the brick bell idea right now as I investigate further. It looks like something I could handle and is more familiar in building style to me.

Freyda Black wrote:I'm giving up on the heated floor idea for now since reading Thomas Rubino's information about how much cob structure it would take to make it.  I'm following the brick bell idea right now as I investigate further. It looks like something I could handle and is more familiar in building style to me.

There's an explanation how bells work, how to calculate the right size and some examples on https://batchrocket.eu/en/building#belltheory
This information is geared towards batch box rockets, though. In order to get a good ballpark, keep in mind a batchrocket will yield twice as much heat in any given time as compared to the J-tube. In other words, a brick bell tailored to a J-tube ought to be half the Internal Surface Area of a batchrocket of the same system size.

Peter van den Berg wrote:
There's an explanation how bells work, how to calculate the right size and some examples on https://batchrocket.eu/en/building#belltheory
This information is geared towards batch box rockets, though. In order to get a good ballpark, keep in mind a batchrocket will yield twice as much heat in any given time as compared to the J-tube. In other words, a brick bell tailored to a J-tube ought to be half the Internal Surface Area of a batchrocket of the same system size.

Thank you, Peter, for the reference.  I read the article and have some questions.  It is clear how the bell works and how it extracts the heat to the masonry.  However, there is no mention of secondary burning of gases, which is not surprising since the system is designed to extract heat efficiently, cooling the gases.  This raises two questions.

1. If there is no secondary burn, does this not lead to greater exhausting of unburned gases and more air pollution?

2. Likewise (if #1 is true) is there not also a higher deposition of creosote in the bell, requiring cleaning as in a standard wood stove and chimney?

If these are not true, what is it I am missing in the principle of the bell?  Perhaps this reflects my lack of more basic knowlege of the general rmh principles? If so, please feel free to refer me to necessary reading to improve my understanding.

Also, I don't understand what you mean by "a brick bell tailored to a j-tube".  Perhaps the article you referenced is a bit of putting the cart before the horse for me.  Maybe I need to learn more about what batch rockets and J-tubes are?  You probably missed my earlier post explaining my limited knowledge.

Thanks again for your help,
Freyda

Freyda Black wrote:However, there is no mention of secondary burning of gases, which is not surprising since the system is designed to extract heat efficiently, cooling the gases.  This raises two questions.

1. If there is no secondary burn, does this not lead to greater exhausting of unburned gases and more air pollution?

Over time, I measured a lot of different heaters. I mainly used a gas analizer for that, not just my senses. The results of all that made one thing christal clear: complete combustion, if any, will happen inside the core, not outside of that. As soon as flames are rising out of the riser, that part of the burn isn't complete at all. The burn tunnel and the riser together is the hot and turbulent environment, the barrel isn't.
The notion that secondary burn happens in the barrel is nothing more than an urban legend, according to my findings. For the unbiased spectator it looks like flames in the barrel are beneficial but they aren't, rather the opposite.
So, exhausting a well-running rocket core, no unburned gases, no pollution.

Freyda Black wrote:2. Likewise (if #1 is true) is there not also a higher deposition of creosote in the bell, requiring cleaning as in a standard wood stove and chimney?

#1 isn't true, so no deposition of creosote in a brick bell. A standard wood stove and chimney requires regular cleaning, a rocket heater won't, not so much. My batchrocket heater with masonry bell has been in regular use the past 7 winters, just a month ago the chimney is swept. What came out filled a quarter of a 2 gallon bucket. The chimney sweep regarded it as "not much for a whole burning season".

Freyda Black wrote:If these are not true, what is it I am missing in the principle of the bell?

You are overthinking it, the bell's function, either in the shape of a brick enclosure or a barrel, bench or whatever, is to extract heat. It doesn't contribute to complete combustion.

Freyda Black wrote:Also, I don't understand what you mean by "a brick bell tailored to a j-tube".  Perhaps the article you referenced is a bit of putting the cart before the horse for me.  Maybe I need to learn more about what batch rockets and J-tubes are?  You probably missed my earlier post explaining my limited knowledge.

OK then, the table for sizing is tailored for batchrockets. Which, as it happens, are roughly twice as powerful as J-tubes within any given time frame. For a 6" batchrocket the recommended maximum internal surface area of a brick bell is 57 sq. ft., for a 6" J-tube half of that. The internal surface area of a bell is found in the walls and the ceiling. Not the floor of it, the exhaust is situated above the floor so this won't be heated directly.

Hi Freyda,
I had asked Peter a very similar question a while back in regards to flames coming out of the riser. Although he already answered it very well, here is another way of him wording it and also some experiments further in the post I did to see if it could be mitigated.
Flames out of riser question