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Pebble-Style 3-in-1 RMH questions  RSS feed

 
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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!
 
Posts: 18
Location: MA
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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.

 
Caitlyn Pierce
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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!
 
Mother Tree
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Something like this might worth considering - tiny house cook stove and heater


 
P Mike
Posts: 18
Location: MA
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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? 

 
P Mike
Posts: 18
Location: MA
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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.  
 
gardener
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Location: Upstate NY, zone 5
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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.
 
P Mike
Posts: 18
Location: MA
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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. 
 
Glenn Herbert
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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.
 
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