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Woohoo! Another RMH Process Thread!

 
Posts: 87
Location: Kansas City, MO
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My wee family of 4 is spending much of this year at my folks' farm.  We're currently living in a 280 square foot home, converted from a single-car garage.  Yikes!  

But to the point, I intend to construct my first rocket mass heater to provide our heating needs for this modest abode through the winter.  Timber is abundant, so duh.  

I'm thinking of casting my core to keep costs down, and planning to put the whole RMH on our front porch to save interior space.  I'm torn between using site-harvested made insulative with sawdust, vs just buying typical materials for reliability and predictability. Any advice on testing local clay mixes for high temp requirements?

I'm planning on building a small enclosure, maybe 6x6, around the heater, and using upper and lower vents to circulate the warm air into the space.  If this wasn't a stop-gap before we build our own place, I could consider more significant alterations to the existing structure, like an interior bench or heated floot, but as it is I'm needing to accomplish the most impact for the least time and cost.  

Any input on the veracity of this general concept is welcome.  I am married to no part of this plan.

I'm thinking of using Ernie and Erica's greenhouse heater plants (with the exception of the cast core), and enclosing the space with a corrugated plastic roof, as I have heaps of those on site.  I suppose I'll need to put heat shielding potentially on all sides of the barrel in these tight quarters.  

Here's some initial pics for reference.
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Front Porch / Side Sun Room Addition concept. I will build the front porch prior to the RMH. The rest will wait until next year at least.
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Dimensions of existing structure, prior to porch addition.
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First hack at cast core mold, and possible location of RMH under window.
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Another vantage of cast core. Will this work?
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Some crappy cob test bricks of beautiful black clay harvested on site. Seems promising.
 
gardener
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If you want to use native clay to cast the core, I would use solid clay/sand for the first inch or so around the feed tube and burn tunnel, surrounded with a light insulating mix. I think the sawdust should work for that. I have used dried grass clippings in a similar situation - they burn out where the clay gets hot, leaving lots of tiny insulating voids. The inner face of the clay actually fires to become pottery. I would make the whole riser of the most insulating mix possible, as that is not subject to abrasion.

I have built an L-tube rocket oven with the top arch of the firebox of native clay (which includes a lot of sand). It has held up for about four years of occasional baking use just fine, and is exposed to outdoor atmosphere but not rain. I would line the feed tube with firebrick for abrasion resistance.

The notion of putting the RMH in a separate 6' x 6' enclosure and venting it to the room sounds iffy. You will have to have a doorway for frequent access - going outside is not a reasonable option. You really need to have the RMH feed in sight of your main living area where you can monitor it to know when it wants more fuel. Also, confining the mass so radiation cannot reach the room and nobody can sit or lean on it in normal room use will drastically cut its effectiveness. You need to have the mass exposed to the room, preferably in it. If you can just remove a 6' section of wall, that could make the RMH function acceptably. A recessed bench to lounge on may become a favored winter spot.

It looks like the front porch is a concrete slab 14'-6" x 5'-10", with the door near the middle of that. Are you thinking of a permanent bump-out just for the RMH, or moving the door out to the edge of the porch? The potential bump-out location puts the heater at the far corner of the heated space, making good heating even more difficult. What is the internal structure and space use? If on a slab, I would put the RMH central to the space (in the most unobtrusive spot of course), and make a tall bell rather than a bench to minimize floor space. Then add a bump-out where convenient to increase usable space. Maybe a bunk alcove at the original RMH corner for a couple of kids?

 
pollinator
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Location: Penticton, Canada
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Beau,  A good video on the subject of cast core builds:
Matt Walkers Rocket Mass Heater Cast Core Build
There have been quite a few threads about cast cores so you may want to sift through a few of them too to see what others have learned and not make the same mistakes.....or should I say to help smooth out your learning curve a bit.
One of these curves is that you may want to look at dividing the core up into sections to allow for expansion and contraction. As the heat is not consistent through the whole core the materials will want to move at different rates and cause cracks to occur that may not leave your core as structurally sound as it should be.
 
Glenn Herbert
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Most thermal cracks you get in a cast core will be unimportant in the long run, but you don't want the ceiling of the burn tunnel cracking as that could lead to parts falling out. It has been found that a three-part core mold, for cores made in sections, isolates stresses to safe locations; this includes the burn tunnel roof as a separate part, together with left and right side/bottom sections. Just packing most of the core box up to the level of the roof edge, adding a parting layer like sheets of paper at the roof edges, then filling in the roof in one piece, should get you a casting that will crack in safe places.
 
Beau Davidson
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Glenn and Gerry, thank you heaps for your input!  The method of separating sections of the cast with sheets of paper is VERY helpful.  I also considered adding an arch to the burn tunnel of the mold using a paper slurry to increase stability, the arch being a structure with more intrinsic strength.  Do you think that would help?

I had initially dismissed the possibility of placing an RMH in a separate space, but encountered the concept on several other tiny home+rmh threads, which I can't find right now. Your feedback is causing me to lean back towards placing it in the space itself, to capitalize on radiant and conductive advantages, as well as provide the convenience of monitoring the unit.  

The interior of the space is a single room.  The South-facing wall has two large windows. I had considered purchasing a rocket masonry kit from Dragon Heaters and putting the bell between the two windows there, or running a bench along the underside of the window.  I am quite familiar with the mechanics of a traditional barrel + bench concept, but I am yet unclear in how a bell achieves the same air flow.  It is my understanding that the cooling effect of the barrel is a major contributor to ideal draft.  How does a bell work in that regard?  

Aside from footprint requirements, I am considering safety for young children and ease of cleaning and fuel transport.  I'll likely need to incorporate a guard rail or low wall to keep the little ones clear of the barrel and feed tube.  
 
Gerry Parent
pollinator
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Beau,  The go-to place for all your bell questions: Bell Theory and Flues vs Bells
 
gardener
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Beau, don't do the arch thing until you understand  rockets well. Just an example. The burn tunnel should be the narrower part, if there is discrepancies in sections of your build.

Tall and narrow burn tunnel is better than low and wide too.

I quite like the look of this one.

But it's far from perfect.

 
Glenn Herbert
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As far as safety for small children is concerned, a bell will avoid the large hot metal surface of a barrel, so there is that.
My youngest grandkids, 2 or 3 at the time they first saw my RMH, quickly learned that the J-tube feed was "hot" and not a thing to play with. At 5, they love to help feed it when I let them.
 
Beau Davidson
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Gerry, thanks for those resources. So in a bell system, does the exhaust ultimately exit horizontally through the wall? How far must it typically extend at the house exterior?

Glenn, thanks for the young kid experience. Very helpful.
 
Glenn Herbert
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Bell or bench both need a proper chimney, unless you have very specific and uncommon environmental conditions (i.e., you can guarantee that the horizontal exhaust will always be on the downwind side of the building.) The horizontal exhaust was an early feature in RMH development, popularized because the developer Ianto Evans did have the right environment.
 
Satamax Antone
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Beau Davidson
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Thanks all.
Supremely informative.
I have done quite a bit of reading and thinking. Here’s a few lingering questions:
-If I build a single brick bell directly over the heat riser, and adhere to 57 square feet for my 6 inch tube as per the above material, is there a standard rule on how far the unit needs to be from the nearest wall?
-Can I vent cool exhaust using regular ducting, or do I need some heavier duty stuff?
-Do I need to seal the interior of the bell with cob or other to prevent gas leaks on startup?
 
gardener
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Beau Davidson wrote:-If I build a single brick bell directly over the heat riser, and adhere to 57 square feet for my 6 inch tube as per the above material, is there a standard rule on how far the unit needs to be from the nearest wall?
-Can I vent cool exhaust using regular ducting, or do I need some heavier duty stuff?
-Do I need to seal the interior of the bell with cob or other to prevent gas leaks on startup?


Ehrmm... That bell of 57 square feet applies to a 6" batchrocket. A standard 6" J-tube is yielding half of the heat at any given time as compared to a batchrocket. The latter one will consume also twice as much fuel in any given time frame, raw btu's can't be made out of plain air.

A single-walled brick bell is subject to the same rules as a bog standard cast iron wood heater. So it need to be (if I remember correctly) 2" from the nearest combustible wall. In case the bell is double-walled with 8" as minimum thickness that distance is down to 4". Even then, I would prefer to have a heat shield behind it.

You can use regular steel ducting for the exhaust, yes. But you'll also need a proper vertical chimney, preferably straight up and insulated.

It's wise to seal the inside of the bell but it isn't really necessary. It's better to build it in such a way you are able to inspect the outside regularly and mend cracks when the need arise. Having said that, it's sensible to install a CO detector in the vicinity of the heater anyway.
 
Glenn Herbert
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"So it need to be (if I remember correctly) 2" from the nearest combustible wall."
Peter, I think you meant 2 feet, not 2 inches, correct?
 
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This seems to be a smallish rmh that just might suit

https://permies.com/wiki/40/63312/permaculture-projects/Rocket-Mass-Heater-Workshop-Jamboree#597864

Just need to make sure you get borosilicate PYREX as that apparently only cracks rather than explodes.
 
Peter van den Berg
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Glenn Herbert wrote:"So it need to be (if I remember correctly) 2" from the nearest combustible wall."
Peter, I think you meant 2 feet, not 2 inches, correct?


Yes, that's true, you spotted the typo before I did.
 
Beau Davidson
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Hay all, thank you for all the feedback, guidance, and research material. I’ve been reading heaps the last few days. I really like the arched brick bell batchbox build from the last jamboree that you linked, Graham, thanks for that. What would be the effective differences in converting that to a j-tube? Or should I consider just going batch box?

This is a bit of a “tell me what else I don’t know” moment, as I’ve never laid brick, done much cob, or built fire devices. I was mocking up some first steps of the build, pictured below. I know the fit, size, and shape may change a bit with mortar added.

edit - I’m having trouble posting out here in rural internet land. I’ll try to post them when I head to town later today.
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First run layout. Is this necessar to lift it from the slab?
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Foundation. These are 140-year-old stockyard pavers that performed pretty well in a rough rocketstove test setup last week.
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Cob bricks. May be replaced with cast core.
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Mold in place, possible footprint of j-tube, bell, wee bench, and exhaust exit.
 
Beau Davidson
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Went ahead and spent a free morning beginning construction.

I’m using clay/sand from on site as mortar. I’ll give this first push several days at least to cure to see how it’s behaving. Any insights or constructive suggestions welcome.
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Do not set lab on fire. Or this tiny ad:
dry stack retaining wall
https://permies.com/t/85178/dry-stack-retaining-wall
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