Woohoo! Another RMH Process Thread!

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.

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?

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.

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.

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.  

Beau,  The go-to place for all your bell questions: Bell Theory and Flues vs Bells

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.

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.

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.

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.

On bells.

https://www.mha-net.org/docs/v8n2/kuznetsov/freegasprinciple.doc

http://kuznetsovstove.com/en/about-the-new-system

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?

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.

"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?

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.

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.

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.

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.

We’re leaving town for a couple weeks so I tried to make the rest of the core so it could cure for a few weeks. I feel like I’m winging it a little more than I’d like, but I’m certainly learning a lot so at least there’s that.

Part of my constraints are my folks’ aesthetics, who have mandated “no barrel, no cob.” Which is making this a bit more time consuming, and teaching me a bit about laying brick in the process.

In fairness, it’s more or less a cob unit, with a bunch of brick in it, as the “mortar” is 100% site-harvested clay and sand.

The burn tunnel included some leftover masonry cement and a hearty inclusion of vermiculite.

The heat riser is cob with sawdust, vermiculite, and perlite, mixed to a very lite and crumbly and packed down one run at a time.

I think I’ll feed this into a simple stratification bench packed in sand, then out the chimney in the rear. Considering a small oven on top as well.

As inspired by many of your comments, I’m building an addition in the front to house this. If it’s clean a reliable, I may knock out the wall between to bring it in to the rest of the cottage.

As always, comments and criticism is welcomed.

Beau Davidson wrote:The burn tunnel included some leftover masonry cement and a hearty inclusion of vermiculite.

Do you know what temperature your masonry cement rated for? High heat rating is definitely needed in this area.

The heat riser is cob with sawdust, vermiculite, and perlite, mixed to a very lite and crumbly and packed down one run at a time.

By your picture, it looks like there is an inner of cob and sawdust layer and then surrounded by vermiculite and perlite. Cob is pretty dense as a material for a heat riser, unless you mean clay slip as the binding agent?  

I think I’ll feed this into a simple stratification bench packed in sand, then out the chimney in the rear. Considering a small oven on top as well.

Dry sand is full of little air pockets that make for a better insulator than a solid mass would which is the opposite of what you want. Now this is where the cob would be most helpful (loaded with rock, urbanite or anything real dense) and then surrounded by a skin of something - brick, stone whatever that would hide the cob from your folks view. That oven sounds like it would be a nice addition.

As inspired by many of your comments, I’m building an addition in the front to house this. If it’s clean a reliable, I may knock out the wall between to bring it in to the rest of the cottage.

Lookin' good!

Gerry Parent wrote:
Do you know what temperature your masonry cement rated for? High heat rating is definitely needed in this area.

1500*. I know some can get hotter than that...

Gerry Parent wrote:By your picture, it looks like there is an inner of cob and sawdust layer and then surrounded by vermiculite and perlite. Cob is pretty dense as a material for a heat riser, unless you mean clay slip as the binding agent?  

I did indeed pack cob, 1’ thick, around the riser, then packed crumbly insulative stuff around that. My worry was that the crumbly stuff would not hold together as the form burns away. I was hoping that the inner sleeve would maintain structural integrity and the outer layer would keep it nice and hot. I’m willing to redo it to get better performance and longevity. What would you recommend?

Moving along on the enclosing structure, pictured.