Pebble-Style 3-in-1 RMH questions

Hello. I am hoping to get a lot of direction and guidance. This is absolutely new to me, despite how "crunchy" I am to most of my friends - I've never built anything, just sewn and cooked and made candles... I've got 4 kids and we are home-learning, so home all day. Money is very tight, like most people.
Our heater went out at the beginning of October. Our gas oven is in and out. It's getting COLD! We live in an older mobile home in KS and despite doing whatever I can to patch holes, insulate electrical sockets, etc, it's still drafty. The oven does not stay on at night (I shut it off when I go to bed, if it even turned on to begin with) and it's very cold when I wake up in the morning, already. Only 2 of the 4 burners work, too. It's been time to replace the stove and oven for awhile.

I purchased the newer rocket stove DVD set because we are in a mobile home and I want to build the pebble-style rocket mass heater. There is SO much to learn and I am nervous about how cold it can get!

I am looking at ripping out our livingroom carpet anyway to do a paperbag floor and wool carpets or cotton rag rugs as needed. This is the space I have for a pebble RMH, and it will need to be straight, not J/L shaped at the mass. I know the core will need to go where the giant picture is, and the mass will need to extend under the window.

I am only about 1 week into researching (see above for amount of children and the time limitations that offers!) and am trying to figure out... well, many things.
~ Is it possible to build 1 pebble style RMH that can function as heater, stove AND oven that could theoretically fit a 9x13 casserole inside, using a stove and oven at the same time? If so, how could this be done?

~ If that is possible, would it be possible to also shut-off the mass during the summer and only use it as a stove/oven?

~ I have not yet watched the shippable core section (again, see above about children/time) - once I collect every supply we need, how long does the entire thing take to build from scratch? I am almost done with the pebble-style video, so I know that can take less than a day for experienced carpenters, etc, but how long does the core and all those base heating parts take to cure, set, etceterea, before it can be installed in the system?

~ in the pebble style video, he said he used the RMH for a winter in the Fisher Price house, without any rocks/pebbles. He also had a gorgeous granite top which I am sure helped hold some of the heat in. What kinds of toppers can be used in the interim, if rocks cannot be found or aren't out of a budget at the moment? I am assuming a simple wood topper won't work quite right?

Thank you for any time and/or attention you can give to this matter!

Okay so I surmise you want something that is:  

1) easy and quick to build
2) safe for kids
3) low cost
4) compatible with a mobile home
5) will allow cooking and provide heat in KS
6) use for cooking in summer with no heating

I'm thinking that it might make sense to just start with a rocket stove and skip the "mass" part of it at least for now.  The reason being that it best matches the above goals.  You might want to start with the hardest part (and could be the most expensive part) which could be figuring out how to safely install a flue/stack/exhaust to go through the roof (or wall).  There are some youtube videos on this.  

For an "oven", some options are to heat a heavy pot on the bottom and use it like a dutch oven.  Or similarly you can make a small oven out of cob.

If you put chicken wire, or a scrap of steel fence around the hot parts that might make it more kid-safe.  

In the summer, I imagine one could wrap mineral wool insulation (or fiberglass if it wasn't too hot) around any stove walls not used for cooking.

P Mike wrote:


I'm thinking that it might make sense to just start with a rocket stove and skip the "mass" part of it at least for now.  The reason being that it best matches the above goals.  You might want to start with the hardest part (and could be the most expensive part) which could be figuring out how to safely install a flue/stack/exhaust to go through the roof (or wall).  There are some youtube videos on this.  

So, this would work well as a heater for the winter, too, even without the mass that radiates? Would it need to be fed more frequently? I've been impressed with the small bundle of wood that lasted for several days' warmth in the RMH. If I can still use that little wood for heating in a rocket stove, I would be very happy, as I would rather not get rid of our table from where it currently rests.

Thank you for your time, P Mike!

Something like this might worth considering - tiny house cook stove and heater

I think the main challenge with that is all the weight.  A brick weighs 5 pounds.  It adds up.  If a mobile home with a floor made of 2x6's can hold 40 lbs/ft2 , then that's only enough for 1 course of bricks laid flat (2" high).  I think if you want thermal storage you'd have to either add more support from the ground, or distribute the weight extensively.

Need more info.  Is the home built on a concrete slab?  Is it on the ground or up on blocks?  If it's off the ground can you reinforce the floor and add a column to the ground?  Can you build up from the ground?  Can you tunnel into the ground for mass?  Can you cut a hole in the floor?  Can you install a RMH outside, or underneath the home?  

The pipe in a RMH bench is often over 20 feet long.  If you ran that straight (instead of doubling back as it's often done), then maybe you could add a little mass to it without it being too heavy.  Was that what you were originally suggesting?  


> So, this would work well as a heater for the winter, too, even without the mass that radiates?

yes, but if the surface area is less, then it's less efficient.

>Would it need to be fed more frequently?

yes.  But maybe your kids could help.  Maybe if you prep it and make it easy to light in the morning you can get some quick heat?  Maybe make two and leave the other one ready to light in the morning?  

My advice is to not worry about the thermal-efficiency or thermal-mass-storage right now.  It can be improved later.  Also I think you shouldn't put too much weight on the floor.  Maybe aim for 50-100 pounds since it may be rated for 200 lbs maximum in one spot.  This is actually good because it will save you time, money and space, and you'll still be able to get heat and cook on it!  I would look at portable type stoves, with most of the bulk in insulating-refractory materials since these are in the right weight range.  They can still produce quite a lot of heat.  You can either make or buy a simple stove that will run on any fuel you pick.  Keep it simple.  You can always add to it or improve it later.  I would focus mainly on building something and making it safe, both in terms of being fire-safe and free from carbon monoxide in the air.  Whatever it is, you can test it outside first to burn off any residue.  Then when you're confident it's safe you can bring it inside.  

I would definitely give thought early-on as to where to put the exhaust stack.  I guess the quickest and cheapest would be to make a temporary installation through the window.  Just make sure it is well insulated and well sealed to prevent fire.  Maybe ask someone to help you with it.

Later on, like next spring, you could build the thermal-mass outside on the ground or underground.  You could even tunnel into the ground and use it like a "ground-source geothermal" system.  You'd need a way to get the heat back into the building, but you could even use natural convection for this (since "heat rises").  But don't worry about that now.  Don't waste time on it now.  I only mention it to show that in theory it can work and there is flexibility with that part because there are many possibilities.  

Right now you should probably focus on just building or buying a small (portable) stove.  I'm sure there are many plans all over the internet.  I can help you with the design and theory if you want, because it's interesting, and I was thinking of doing the same.  I also need to build a stove ASAP so I don't catch cold this winter!  haha.  Time to quickly come up with some design plans and find the materials and just build it quickly.  Nevermind if it isn't perfect, it can be fixed or replaced later.  Just take action and make something basic and safe and do it now.  

A rocket mass heater without mass will suffer the same issue as a conventional wood stove: no heat when the fire is not burning, and potentially too much heat when it is. You need some mass to absorb and spread out the heat. You will need some support from the ground to the bottom of the floor, which can be as simple as a couple of layers of concrete patio blocks and a row of regular blocks up to the floor, stretched out over the length of the mass. Add shims as needed for a tight fit to the floor. This is not a real long-term solution, but will get you through the winter without breaking the floor. You can build a foundation matching the rest of the house supports next summer.

You mention that the core and barrel will go by the picture on the wall; the barrel needs the same kind of clearances as a wood stove, something like 18" minimum IF you have a metal heat shield on the wall spaced out 1" with standoffs and free ventilation behind it. You probably also need a heat shield on the ceiling above the barrel.

Matt Walker's cookstove does everything you want, but would be too big a job to do right now without major outside help. If you can get firebricks or old red bricks (not modern cored hard bricks), you can build a temporary 6" J-tube with fireclay mortar and a 33 gallon drum over it, contained in a wooden box like the Wheaton Labs one. A half-barrel bell in the bench instead of ducting would be less expensive and probably work better, but involves cutting that you may not have the tools for now. A couple of loops of 6" duct in the pebble bed would cost more if you buy new material, but would be easy to do. (Check craigslist for old ducting and bricks, you may get lucky.)

Having the exhaust from bench to chimney right next to the barrel would let you install a bypass so heat could go straight out for cooking in summer. That would also give easier starting in heating season as you wouldn't have to establish draft through the whole mass until the fire is going well.

Caitlyn's situation is that it's mid-December and it's getting cold.  She's heating her home with a conventional oven that is old an only partially working.  She's tight on money and time because she's taking care of her kids.  She has never built anything like this before, even though she has skills like sowing.  

Given those circumstances, the amount of mass in the heater is far down the list of priorities.  It's much more important to try to make or acquire something quickly that is safe (or at least safer than a conventional oven) and will provide at least some heat through the winter.  It sounds like she isn't going to have the time, money, and know-how to add to the foundation in mid-December seeing as how we're in the holiday season.  That is why she doesn't necessarily "need" mass, as Glenn said here:

Glenn Herbert wrote:You need some mass to absorb and spread out the heat.

I mean I agree with Glenn's suggestion that in theory it could be not too complicated to temporarily reinforce the floor, as he says here:

Glenn Herbert wrote: ...some support from the ground to the bottom of the floor, which can be as simple as a couple of layers of concrete patio blocks and a row of regular blocks up to the floor, stretched out over the length of the mass. Add shims as needed for a tight fit to the floor. This is not a real long-term solution, but will get you through the winter without breaking the floor. You can build a foundation matching the rest of the house supports next summer.

However that is not necessarily required to get at least some heat.  Having a "conventional wood stove" might be a *good problem* for her to have.

She is not 'suffering' the issues of a conventional wood stove, she is 'suffering' the issue of not having any stove! (except for a kitchen oven)

Glenn Herbert wrote:A rocket mass heater without mass will suffer the same issue as a conventional wood stove: no heat when the fire is not burning, and potentially too much heat when it is.

So Glenn might be offering a good answer to maybe a slightly different question.

Now, what Glenn talks about below may be a good place to start because it's the most difficult and/or most expensive part, and it's safety-critical if you're locating it near a wall.

Glenn Herbert wrote:
...the barrel needs the same kind of clearances as a wood stove, something like 18" minimum IF you have a metal heat shield on the wall spaced out 1" with standoffs and free ventilation behind it. You probably also need a heat shield on the ceiling above the barrel.

Glenn Herbert wrote:
Matt Walker's cookstove does everything you want, but would be too big a job to do right now without major outside help.

That's the main challenge I think.  What's a good way to take the large project of a RMH and break it down into smaller projects?  It seems to me the most obvious solution is just to leave the mass part of it off for now.  

I mean really, it seems to me you could just start by building a small Rocket Stove, maybe with some kind of pipe made of steel, ceramic or reinforced cob to vent the exhaust out the window (or wall), with fire-proofing insulation around the part going through the window/wall.  I know this is a shoddy hack, but it's easy enough that you might actually be able to complete it this season.  I'm not sure how cold it gets there, but maybe then there would be less pressure on you and worry about not having any heat in the winter, and maybe then you could take more time to plan a longer-term solution.  

That's my opinion anyway, for what it's worth.  

I think a woodstove without mass, especially a homebuilt or old one, will invite the issue of being too hot much of the time and being run slow and dirty, making lots of creosote, potentially causing a chimney fire.

Building a wooden box for the ducts and core like at Wheaton Labs would allow filling with pebbles as resources allow. Much advice will depend on the resources available at present, so talking more now is probably not productive.

Thank you so much for all your dialogue and responses so far!! I have been reading them and chewing on the information and trying to sort through it.
A friend loaned us a radiant space heater, I've been keeping the fans on in the house to circulate warmth, and putting blankets over the curtains on windows at night and our home has been deliciously warm - warmer than when our forced air heater was working very well. It's warmer at much lower temps and stays warmer longer... It's definitely a convincing sell for a RMH, which will be less work and probably less risky than an oven being turned off and on all day.

I definitely want to start with securing my foundation. It is a single-wide mobile home in a trailer park, on a dirt floor and cement blocks as uhhh... "support?" This means we cannot put in a cement foundation here. We will have to basically do whatever they did at the Fisher-Price House... I want to understand that section of the video a LOT better. My husband can help do this when he is home on the weekends. If I give him information and directions, he gets a LOT done, but I need to make sure to get it to him before he starts working, as he is open to the information and works very well from it before, but... not in the midst of working. So I am hesitant to get started until I understand as much as possible to successfully reinforce under the mobile home.

I want a pebble-style because my husband and I would like to eventually build a hobbit hole in the ground somewhere, and gift this mobile home to someone who needs it, which means they may not like a rocket heater/stove and a pebble style will make for easier removal. This also removes the idea of doing a RMH from outside and coming in somehow. I adore the one in the Fisher-Price House and the only thing that will need to be changed is the topper for the mass, because I cannot find affordable-to-us granite, even on Craigslist, right now. I do wonder if that could just be a huge plywood sheet until the rocks and pea gravel go in - but I don't suppose that would help hold heat very well!

Our plan is to go up through the roof for the smoke stack/exhaust, as in the videos we watched it seemed the wall didn't work too well on their models and they'd always go to a roof one anyway. It also looks so lovely!

I am a pretty determined person - I could list many things, but the one that rises to the top of my mind is clipping my own tongue tie with sterilized scissors and doing bodywork on myself before and after for weeks, so that I could function better. I took a video of it, so that people could see it's not an impossible thing to be done. Most of my friend's still think they could never do it themselves!
That said because I can will myself to do many difficult and sometimes painful things knowing there is a good end in it. I also know my limits in which something is dangerous and will not cross it - when I've been B12 deficient with body numbness and mind haze, I choose not to drive. I won't lift things that I know are too heavy (like a core by myself or even with just 1 or 2 other people). But I can do a lot on my own and thrive off of the feeling of doing challenging things that are for good ends.
My parents offered to get the part for our heater, but I resisted letting them know the part because I WANT this RMH and I know we won't rush to it if the forced air is working....
So reinforcing the floor is an absolute must, right away for me. It looks like aside from the terror of going under the mobile home, dark and cobwebby, I could reinforce myself, if needed? I think my husband can do it in a day or so, with my help from outside the trailer?

We go to a Mennonite church and there is a source of help there, along with belonging to a local Buy Nothing gift economy group on Facebook... I think we will have muscles and construction experience when the day comes to put in the core. I am hoping!!

I do like the idea of breaking this down into little projects.
We are sticking with as much wood as possible for this, for cost and weight reasons, so the mass will definitely have to be wooden like in the Fisher Price House.

I think right now I HAVE to take time to watch the shippable core so I even know what materials I will need for probably the most expensive aspect!

Thank you so much for your help!

I wanted to chime in with some suggestions for keeping warm in the interim.  Maybe you already know these things.

Wrap up with scarves and hats inside.  It may seem strange, but it really helps to conserve body heat.  Wear a hat to bed!

Everyone gets a hot water bottle for both day and night time use.  If it's cold during the day, I'll keep one on my lap as I knit/read/etc.  I take it to bed with me in our unheated bedroom.  My son too, but his dad doesn't usually need one, although he sometimes borrows one of ours.  I refresh it with hot (not boiling) water from our electric kettle as needed.

Go out during the day to a heated public/community space like the local library.  Or make a visit to a different neighbor every day.  Anywhere warmer than home

Go out for a brisk walk;  I walk my son to and from school every day and am warm for the next hour at least.  Send the kids out to play a rough/noisy/vigorous game.  Hide and seek?  Tag?

Have hot meals if possible.  I eat a hot breakfast and dinner, but really notice a cold lunch in a cold house.  Breakfast for me is scrambled eggs, and dinner is usually in the slow cooker, so not a huge time commitment either way.  I need to figure out a quick hot lunch, I think.

Warm up in a hot bath right before bed;  for us, it's more of a soak than anything, as we're no-poo (no shampoo) and mostly no-soap.  Usually my son gets in first, then me, then my husband.  We don't empty the bath until the last person's done--although if we get in and it's too cool, we'll let a little out then add more hot.  In the summer when we actually get sweaty/dirty we take our own showers/baths, but in winter the water stays pretty clear especially without shampoo and soap in it.  I don't let my long hair get wet (I've gotten to the point where I only wash my hair with water about once or twice a month) unless the house is warm too--but short will dry quickly enough.  I guess blow drying is an option.

I recently built my very first RMH (a batch box with masonry bell) myself;  my husband was recovering from surgery and my son is seven, so I really did it all by myself, complete novice that I am.  It's not pretty, and it took me three weeks and some frustration, but if I could do it, I'm sure you can too.  

Galadriel Freden wrote:I wanted to chime in with some suggestions for keeping warm in the interim.  Maybe you already know these things.

Wrap up with scarves and hats inside.  It may seem strange, but it really helps to conserve body heat.  Wear a hat to bed!

Everyone gets a hot water bottle for both day and night time use.  If it's cold during the day, I'll keep one on my lap as I knit/read/etc.  I take it to bed with me in our unheated bedroom.  My son too, but his dad doesn't usually need one, although he sometimes borrows one of ours.  I refresh it with hot (not boiling) water from our electric kettle as needed.

Go out during the day to a heated public/community space like the local library.  Or make a visit to a different neighbor every day.  Anywhere warmer than home

Go out for a brisk walk;  I walk my son to and from school every day and am warm for the next hour at least.  Send the kids out to play a rough/noisy/vigorous game.  Hide and seek?  Tag?

Have hot meals if possible.  I eat a hot breakfast and dinner, but really notice a cold lunch in a cold house.  Breakfast for me is scrambled eggs, and dinner is usually in the slow cooker, so not a huge time commitment either way.  I need to figure out a quick hot lunch, I think.

Warm up in a hot bath right before bed;  for us, it's more of a soak than anything, as we're no-poo (no shampoo) and mostly no-soap.  Usually my son gets in first, then me, then my husband.  We don't empty the bath until the last person's done--although if we get in and it's too cool, we'll let a little out then add more hot.  In the summer when we actually get sweaty/dirty we take our own showers/baths, but in winter the water stays pretty clear especially without shampoo and soap in it.  I don't let my long hair get wet (I've gotten to the point where I only wash my hair with water about once or twice a month) unless the house is warm too--but short will dry quickly enough.  I guess blow drying is an option.

I recently built my very first RMH (a batch box with masonry bell) myself;  my husband was recovering from surgery and my son is seven, so I really did it all by myself, complete novice that I am.  It's not pretty, and it took me three weeks and some frustration, but if I could do it, I'm sure you can too.  

Is there some way to give people big, warm fuzzy hearts here?? I love this all the way through. Thank you for taking time to write and tell me all of that goodness! I am also partial to your username - I actually did name my son Aragorn.

Caitlyn Pierce wrote:
Is there some way to give people big, warm fuzzy hearts here?? I love this all the way through. Thank you for taking time to write and tell me all of that goodness! I am also partial to your username - I actually did name my son Aragorn.

Thank you

To be honest, the hardest part of building my RMH was ignoring the little voice in my head that said I couldn't do it and actually just starting it.  The build itself was fairly easy, barring a few minor setbacks.  

Glenn Herbert wrote:Matt Walker's cookstove does everything you want, but would be too big a job to do right now without major outside help.

Glenn, can you direct me to Matt Walker's cookstove? Thank you!

Here's the link - Matt Walker's Tiny House Cookstove and Heater

Those ceramic cooktops are very brittle.  If something falls out of a cabinet they'll shatter.  I've seen it happen.

How are you going to support all that weight inside?  Maybe you should put all that mass on the ground.  Or else don't use a bench mass.  Instead spread it out across the floor and make it like so called "radiant floor heating" (even though technically it's more convection than radiant).   Or build it as a large bed frame.  Do whatever you have to do so that the ceramic/brick/cob is only 1 to 1 1/2 inches thick, so that the floor can support it without custom reinforcement.

You could also use water-based thermal storage since it holds 4 times as much heat (at these low temperatures) for a given mass.  You can't let it leak though or it will destroy the fiber-board/MDF floor and your mobile home could be a total loss.  However if the water was completely absorbed in some spongy or absorbent material, then the risk of a spill should be much lower.  A waterbed won't give you a good night's rest, however I think the idea goes in the right direction.   You could use a pallet or similar frame to spread the weight out or to make it easier to move if it was built outside.  You could also build an insulated cement block or other foundation for the mobile home and use that as thermal mass.  You might have to do this anyway to support the floor.  

In this case, the cheap "foundation" you have is actually beneficial, because it will be easy for you or your husband to add support that will be just the same. As long as the support is equal across the house, you won't get differential settling/heaving. Identify the area where you want to put the RMH/bench, and put in blocks to match what is there.

I don't think the "40 psf" load limit is significant; if a few heavy people can stand in a group without collapsing the floor, it can stand the weight of a modest RMH with some foundation blocking to transfer the main load to the ground.

I wouldn't use a wooden top for the bench. 18" square concrete pavers are pretty cheap and will do the job fine. You can paint or stain them to make them more attractive.

I'm not an expert in this but I would caution you to check the weight loading rather carefully.  A 5700 lbs mass on a 2x6 joist could be sketchy.  


Normally, in a permanent installation, foundation piers bear upon reinforced poured concrete footings that are constructed below the frost line.  Even this probably isn't hard to do anywhere along the outside wall.  

My guess would be that at this time of the year, getting a simple box built, with J-tube core, ducting, and some gravel poured around the ducts, would be doing well. In any case, I believe this installation would end up around a ton or so, not nearly three tons, and if spread out over 20 square feet would be around 100 psf - not unreasonable for a dead load if the joists are supported from below.

around 100 psf - not unreasonable for a dead load if the joists are supported from below.  

Sounds reasonable to me.

Glenn Herbert wrote: I believe this installation would end up around a ton or so

All depends on the design.  For normal gravel, it must only be 1 foot tall at most to limit it to 100 psf.  Light gravel could be 1.5 feet tall.  

not nearly three tons

There are RMH that weigh that much.  One that I randomly found that is specified to 5700 lbs and up to 240 psf is Wisner's 6" Annex design.  

The density of gravel could be comparable to cob.

A 2 foot gravel bench could be 200 psf.  

Now that still might work with reinforcement, I don't know.  

What is the dead load rating for the floor?  It might only be 10 psf static, 40 psf dynamic.  

You're saying 100/40 = 2.5

But

200/10 = 20 , which is 8 times more, so be careful.

The 40 psf design live load is what the entire floor system must be able to withstand with minimal deflection, without added support. If there is added support in the form of blocks below the mass area, the original structure is not being asked to hold all of it. Floors are required to be able to support a concentrated 200 lb load at any one inch diameter spot on the floor, so as long as the joists are supported, the flooring should be able to transfer the load to the joists and then to the new blocks. (Cornell Legal Information Institute)

Glenn Herbert wrote:The 40 psf design live load is what the entire floor system must be able to withstand with minimal deflection, without added support. If there is added support in the form of blocks below the mass area, the original structure is not being asked to hold all of it. Floors are required to be able to support a concentrated 200 lb load at any one inch diameter spot on the floor, so as long as the joists are supported, the flooring should be able to transfer the load to the joists and then to the new blocks. (Cornell Legal Information Institute)

Okay... so, I am trying to understand all of this and am still wrapping my mind around it! My husband and I priced the 6" ducting, HVAC tape (and aluminum tape, which has a higher heat toleration), and the bendable duct joins. We didn't have time to price everything, so that's it.
I wanted to ask about the foundation support, because that's the first thing we'll be doing over Christmas break. Is this just cement blocks and some wood for "shims"? How far apart should these be from one another, if we will be doing a RMH with the mass?
I am going to do the mass without the stones and pea gravel until spring/summer, for cost reasons. Without the gravel, will this hold heat and radiate still, just to a lesser degree?

Burra Maluca wrote:Here's the link - Matt Walker's Tiny House Cookstove and Heater

I looked at this... I do not want to spend $85 to find this out, and after 45 minutes of watching some videos and looking over his website, I cannot see the answer. Is the mass filled with anything surrounding ducting? Or is it just the brick? I question if I could still build the mass with a wooden box and fill with pebbles, but make the rocket stove/oven the way he has?

I'm giving up on this thread.  Caitlyn, you either need to follow plans exactly and/or get someone more qualified to engineer it for you.  I seriously doubt a novice could do a custom design just from a few tips on an internet forum if they don't have the experience.  Unfortunately the correspondence here does not seem to be up to the level needed to complete this task safely and appropriately.  

Caitlyn Pierce wrote:

Burra Maluca wrote:Here's the link - Matt Walker's Tiny House Cookstove and Heater

I looked at this... I do not want to spend $85 to find this out, and after 45 minutes of watching some videos and looking over his website, I cannot see the answer. Is the mass filled with anything surrounding ducting? Or is it just the brick? I question if I could still build the mass with a wooden box and fill with pebbles, but make the rocket stove/oven the way he has?

Have a read through the thread I linked to - I think it will answer all your questions about that stove.  And if not, ask in that thread and it's very likely that Matt himself will answer.

It's a hollow bench, otherwise known as a bell.  Here's a video that explains it all.

P Mike wrote:I'm giving up on this thread.  Caitlyn, you either need to follow plans exactly and/or get someone more qualified to engineer it for you.  I seriously doubt a novice could do a custom design just from a few tips on an internet forum if they don't have the experience.  Unfortunately the correspondence here does not seem to be up to the level needed to complete this task safely and appropriately.  

Thank you for ducking out when you know your limit is reached to respond to a newbie with grace. :) I don't mind following instructions. I don't mind playing with things. I am just in question asking stage right now and see there are a lot of variations even within the category of "rocket stove," so I am wading through what can and cannot be done and I don't have more than an hour or two a day for researching, with homeschooling, cooking from scratch, and keeping up with 3 children under 6, which might make me sound hasty or sloppy with questions or inadequate with the time I've invested in watching videos or reading websites. Thank you for the time you have given already - it has been valuable - and I hope you have a good holiday.

Caitlyn, you would probably be best putting in a continuous row of concrete blocks centered under the area where the heater mass will go, for its full length. Be sure to put solid blocks (maybe 4" x 8" x 16" laid flat and crossways to the row) on the bottom so the regular blocks don't sink into the dirt. Remove any grass, sod, or soft loose earth before starting to lay the blocks. A couple of inches of gravel (not round pebbles, crushed is best) will make it easier to set the blocks firmly. I presume the floor joists are perpendicular to the outside wall, so the row of blocks (with shims) will support every one of them.

You can install the ducting bare and get the instant heat from it, and have clean burning from the J-tube, but without some mass around the ducts it will be cold the instant the fire goes out. If you can pick up any kind of gravel, stones, old chunks of concrete, whatever is heavy and noncombustible, it will help hold heat.

The bell as Matt's video shows is an excellent way to heat the mass, but needs to be built in a permanent fashion (strong mortar between the bricks) to avoid leaks especially with a lightweight supporting frame that might flex a bit under load and cause cracks. Cracks in cob around ducts don't matter as much, and of course gravel won't crack If you can get a steel barrel and cut it in half lengthwise, and make a "half-barrel bell" (look it up for details), and seal the bottom edges and joints of the barrel to something like a cement board base, you could have a good bell chamber to pile gravel around with possibly less expense than new 6" ducting.

Matt Walker wrote:It's a hollow bench, otherwise known as a bell.  Here's a video that explains it all.

Thank you! I have only had a few hours to look through your website and it's pretty wonderful to realize you have been so generous, that I cannot have seen all the videos or gone over all the information, yet!! I did get to more videos last night, but not this one. I will watch it today. Thank you so much!

Ha! I have been watching this video and where you say you strongly discourage the use of pebbles... this is why I am asking a lot of questions and taking much more time than I hoped I could get by with. Normally, I am a researcher and take LONG stretches of time to make anything because I want to make sure it's perfected. I tried to go a quick and dirty route with the RMH simply because it is winter and these systems do look somewhat forgiving.
It is clearer that this is like anything else and I am going to be drawn into researching amounts no one else I know in my every day life would research most anything.
So... perhaps not a pebble RMH. More research. Thank you! This video is very informative. Again, thank you for putting out so much for free to inform and help, Matt!

For permanent installations I would avoid the pebble style; however for the lighter possibly temporary situation you face, it might be a reasonable choice.

Glenn Herbert wrote:For permanent installations I would avoid the pebble style; however for the lighter possibly temporary situation you face, it might be a reasonable choice.

I did see on one of Matt's videos that these can be put into trailers IF they would be there a year or more and deconstructed (more laboriously) than the pebble style.

I am also contemplating that with a pebble style, it looks VERY difficult to get a full size oven and a stovetop (in use at the same time), while Matt's is already set up for exactly all three needs at once, even though it is not as lightweight. I do have a friend that can gift us a stovetop for Matt's design, if that's what we end up going with.
Thank you so much for your time, patience, and wise contributions, Glenn! I am learning a lot and willing to learn more. Thank you!

About the support - that will need to be balanced on the other side of the mobile home from the RMH, right? So that it's not unbalanced within the flooring supports?

Thank you everyone for helping a newbie with a willingness to learn and a lot of questions, even if she is limited on researching time!

The style of the mass is entirely separate from the style of the combustion core. You could easily put a pebble-surrounded half-barrel bell onto the side of Matt's cookstove core.

You would not technically need to add support under the other side of the trailer if the first footing is centered under the new load, though it couldn't hurt to balance it out.