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Walker -ish Tiny Stove Design Questions

 
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Hey all,
Just discovered the Walker Tiny Stove and I think I’m in love. I’m in Vermont, quite a long cold heating season, and I never liked the original RMH, because it took up so much floor space, especially with the barrel clearances to combustible walls, and because I couldn’t see whacking all my cordwood up into ee-weenie kindling and then sitting there to feed it all day every day. (Had not heard of batch boxes; had not heard of vertical bells. If I had, those might have changed my calculation, but I like Walker’s design a whole lot better because of its compactness and especially because of its multi-use cooking areas!)

My cabin is 16x20 plus a couple gable rooms upstairs, with a staircase/utility closet/bathroom dividing the downstairs into kitchen and sitting areas. The existing hearth and chimney is in the sitting area, which is a bit inconvenient for cooking but I do it some already (mostly an endless pot of soup). Chimney about 15’ of metalbestos, up the center inside the house and out near the roof peak. So a plenty warm stack. Because of the dividing staircase/closet, the sq ft of sitting area is quite small and I tucked the hearth into a corner. So I would need to fit any built masonry stove in a space no bigger than 50”x50”, INCLUDING clearances from combustible wooden walls on two sides, so say 42”x42”. Presently using a Fisher Baby Bear cast stove, which works fine, but it’s a small cabin so until the most brutal mid-winter sets in I’m burning small smoky choked-down fires a few times a day and the house alternates between sweltering and frigid. (Plenty of insulation in the walls/roof/floor but the windows bleed heat and I think I’d go completely cabin-fever crazy without them.) Presently I go through 2.5-3 cord/yr, depending on the severity of winter. I’d be only delighted to drop that down some. Also would be happy to just shut off the propane to my range for the whole of heating season. (Right now I still often want to cook when the house is already too warm to add fuel to the cast stove, plus the Fisher has no way to bake but to drop foil-wrapped things directly in the coals.)

Yes, I do understand that even if I figure out most of my design on my own, with your help, and from the Riser-Less Core plans published under Creative Commons, I should still probably send Matt a donation.

Here are my questions:
1. Looking at Matt’s design and the space I’ve got, I can’t possibly fit a bench bell but I might could fit a narrow vertical bell at the back. So: over the cooktop like he has it, down the one side like he has it (6.5”x20”x30”), underneath through the roasting oven like he has it (24”x20”x10”), then up behind into a vertical bell about 6”x30”x50”. I can’t think of any reason air-flow wise why that wouldn’t work – it would come in around 44 ft2 of the 57 ft2 ISA allowed for a 6” system, and even if 6” of that final bell went to the exit stack it would maintain a minimum 5:1 CSA – but it seems like it might be good to have a bypass for getting started in the morning (or for quick cooking when the house is already warm) and I’m having trouble seeing where or how I’d make one. Seems like it would need to T into the exit stack of the bell, but that would be begging for smokeback?

2. I make bread, which Matt says works well in the downdraft section. AND I roast things, which Matt says works well in the underneath section. And gosh I look at all that vertical space in the final bell and think, wouldn’t it be nice to set a bunch of drying racks in there, or even hang some jerky? (Though it might be too warm for that last.) However, seems like that many doors would be expensive, probably difficult to seal, probably too much immediate heat radiation into the room. Thoughts?

3. I understand the core needs to be IFB or ceramic fiber board. I’m curious about using cob in the shell rather than brick. I’ve seen recommendations that the shell be two separate layers with a cardboard spacer between, to make sure cracks don’t propagate through and release gas into the house. Seems like a good idea, but it also seems like it’d take up a lot of room and add maybe too much mass to radiate much, and also cob would be easier as 1 thicker layer than 2 thin, so is it necessary?

4. Our local clay is a very different substance from the kaolin-rich Carolina clay I learned to cob with, and the local potters tell me it is low-fire only. I made a little test block and set it in the coal-bed of the Fisher for a couple hours. When I picked it out, it appeared intact at first, but I could crumble it with my thumbs. Obviously cob made of this particular clay doesn’t stand up to even mild firebox temperatures. Of course it wouldn’t be in the firebox, it would be in the bell. So: How hot do the walls of the bell get? Because I might need to source clay from over the other side of the Greens if it’s too much.

4. Also, any advice on mounting airtight doors in cob?

5. Sigh … And about that suspended floor. The house is on piers with a sloped crawlspace beneath. About 3’ high under the area being discussed. Floor joists are 2x6 rough cut hemlock with 10’ spans between beams, plus a little extra blocking beneath the hearth for the (just a couple hundred pounds) weight of a cast stove. Grade soil a soft silty loam. Probably still fairly damp beneath the house (the water table is high here, drain ditches or not; vapor barrier and gravel floor at present; still higher humidity than it ought to be) but at least it doesn’t freeze hard down there so no frost heave. Not especially easy access, just a crawl-hatch about 24” high by 30” wide. I use it for a root cellar but always kinda dread hauling buckets in and out of there all hunched over. Miserable place to excavate, for sure. Obviously I’d have to do something. I do have a good supply of salvaged cinderblock, but it may be too old not to crack. Ideas? Would I be asking for trouble just to pile up cinderblocks on top the gravel and shove shimmed-up cross pieces spanning the joists above?

6. Hey, would this thing let me burn sappy resinous softwoods without gucking up the works? Because that’s mostly what I’ve got and I’m always scrounging for clean-burning hardwood in a spruce swamp. That’d be a perk!

Thanks in advance for all your help.
 
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I think your configuration with vertical bell in back would work fine, though I think it would be better with a minimum dimension of 8" in the bell since the air will need to move up and down in the same space. I would make a 4" bypass from the back of the stovetop to the exhaust flue; you will still get plenty of draft through that short straight connection. I am planning a very similar layout for my wife's sunroom addition.

That many doors that seal well would be expensive. If minimal functional sizes they might not be too much direct radiation. I would question the functionality of the bell for smoking meat, but not knowing the ideal temperatures for that I can't positively advise whether it would be too hot.

Cob would work fine as an outer layer of a bell, or as both layers, though it would possibly be too thick for your space. I have an inner layer of red bricks and firebricks on edge (so 2 1/2" thick) and an outer layer of cob. Mine is 6" thick and takes longer than I would like to transmit heat, especially at the corners. I would use a 3" minimum layer of cob with straw reinforcing and possibly large wire mesh. 4" would be significantly sturdier while not giving too much overall thickness. Cob as the inner layer would char the straw before long, so I would leave the straw out in that case as you don't want insulating cavities.

The coal bed of a wood stove will not get anywhere near hot enough to durably fire clay, so it is not surprising that your result was crumbly. The top interior of a bell over the core could get close to 1000F in spots, which wild clay in my experience will tolerate fine. It will get partially fired and not melt in water, even if it does not get much stronger than raw clay. If your clay crumbles easily, it may be too silty to be strong, and you would need to augment it with some fireclay or other commercial clay. Experiment before doing anything permanent!

An airtight door frame in cob needs a decent flange that sticks into the cob an inch or more, so there is a long tight path around it and it will not wobble. It might still need a gasket depending on how much positive pressure it sees.

How big are the pier footings for the house frame? I would probably make the footing for bell support two to four times as big in area, and I would dig down as close as you reasonably can to pier depth just so they have the same character of bearing underneath. That said, if the floor framing is reasonably beefy, you can probably depend mostly on that and use the new pier as backup, given the relatively small size of your proposed heater. As long as the salvaged blocks are not cracking as you handle them or give them a firm tap, they should be fine stacked up on a gravel bed.
 
April Wickes
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Thank you, Glenn!
I was considering a bricks-on-edge interior skin, yes. Did you use that cardboard-spacer interior gap between bricks and cob, or not concern yourself with it?

As for the door mounting, I was unsure because any flange or pin made of metal seems likely to do a lot of expanding and contracting under heat, and I thought that might be likely to crack the cob. How is yours holding up?

As for the cob itself, pretty sure it’s not silt because it passes the jar test with remarkable purity and it wasn’t crumbly at all before I put it in the coals. But there are four or five major families of clay with different chemical makeups, plus innumerable variations within those. This is maybe a chlorite?? Anyway, the local potters say it melts to green goo under high firing (cone 6+). But if you’re telling me that test block crumbled because of *low* or middling temperatures … isn’t that precisely what the whole entire shell would be subjected to, always?

DOES anyone out there know the varying temperatures inside downdraft chamber and lower bell, both moving air and interior surface? Seems like it would help to have a range. Also help in knowing more about a sensible layout for oven usage. Probably the vertical bell would get too warm for drying things or meat. Would it rob all the heat out of the lower roasting oven?

I agree, it would certainly be better for both the downdraft section and the vertical bell to be 8” wide instead of 6”. The problem is, 22” of core and blanket + 4” interior wall + 8” bell space + 6” exterior shell on either side = 46” total. If I did that I’d have barely any clearance from the wooden walls. The interior wall between core and downdraft could maybe be bricks on edge, but the one behind the core would have to support another 20” of vertical masonry above it and would really need to be laid flat. So … would it *work* with 6” bells? Or is that just too much restriction?

Finally, routing from lower bell into vertical bell and exit stack. So far as fitting the stove within the corner I have for it and attaching with minimal headaches to the existing chimney, seems like it would work best to have the exit stack directly behind the downdraft section. (Bottom layer layout pic below.) As I understand it, the design is pretty much a wide open area underneath, with pillars supporting the core and interior walls above. What sort of baffle layout would be appropriate to direct the cooled gases into the exit stack? Does it need any block between downdraft section and exit stack? Any block between low bell and high bell? Or should I just leave the entire 10” lower layer as open as I structurally can and trust the gases to spread out on their own instead of all sucking straight up the exit stack?

Thanks to you all.
stove-bottom-layer.png
[Thumbnail for stove-bottom-layer.png]
 
Glenn Herbert
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My bell is 26" square inside by about 6' high, all bricks on edge. It is mortared with refractory cement as required by US building code, not clay slip. One face that will back up to the future masonry chimney has a vertical hairline crack but that has no signs of leaking. I didn't use a cardboard spacer on the other walls, but it is probably a good idea. I would have zero qualms about making the back wall from bricks on edge as it would be very difficult to put a big load on it to push in. If your tall bell is 24" x 8" inside, I would make it all from bricks on edge for the inner layer. The strength of bricks far exceeds the gravity loads you will have. The back bell will be the last part of the gas flow, so I wouldn't expect it to be more than 300-400 F inside at most, probably more like 200-300. This is low enough heat that 2 1/2" of brick and 3" of cob will have a surface temperature that can safely be close to wood framing. Just for insurance, a metal shield 1" from the wall with free airflow will guarantee safety.

The downdraft area has smooth flow in one direction, and I don't think it needs to be any more than 6" if it is 22" wide. In your sketch, I would block the opening to the back bell nearest the exit, so all flow goes across the base space and into the far side of the bell. It will rend to rise to the top and then drift down ending up at the exit opening. A 10" x 10" opening from base to bell would give easy flow. You could add a divider in the base of the bell to force all entering flow to rise if you wish.

I have a 20" x 30" steel access panel and a 24" x 15" "oven" insert both set into the cob, and have seen no cracking around them. They are both exposed to internal air temperatures from 500 to 800 F or possibly more at the top. Your cooktop will radiate a lot of heat, so everything following will be cooler by a few hundred degrees more or less. I would guess that the air in the downdraft would be 500 to 600 F at most, maybe up to 700 near the top if you are burning long and hard in the dead of winter. The base would probably be 300 to 500 at most (Matt said his base oven only got to low roasting temperatures.)

 
April Wickes
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Glenn, this is lovely. I really appreciate it.
 
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