Andre Wiederkehr

What all do people use the term "bell" for? It seems like it's any non-piped, stratifying structure the exhaust passes through. So then many designs actually have two bells: the barrel over the riser and then a stratifying bench. This gets ambiguous, especially when, as in some of Matt Walker's designs, the barrel is replaced with brick, so that both bells are some kind of masonry. Am I correct in thinking "bell" applies to both? How do people clearly differentiate these parts? Riser bell and mass bell?

Anyhow, here's my next revision of the design, informed by reading the threads referenced here and by watching a bunch of videos from Matt Walker.

Instead of a barrel, a brick bell over the riser, just deep enough to fit the riser, but the full width of the mass (~3 1/2').
This:
-reduces how far this component sticks into the room, saving floor space
-fits better as a cooktop near my kitchen corner (under the two corner windows opposite the door)
-increases thermal mass and reduces rapid radiative heating, which I think will be desirable in such a small space
-looks nicer, in my opinion

Do you think this would work fine for exhaust flow? The riser would have basically no room around it on three sides, so all the gases would come out the top and go over one side, flow sideways under the cooktop, then stratify and fall to the exit into the main mass. Much more assymetrical than a classic barrel.

I would lean away from doing an additional heat shield between this brick bell and the plastered LSC wall, unless people warn me otherwise.

I imagine covering the top with two rectangular cast iron griddles to cook on. But would a steel plate be fine too? Lacking lived experience with a barrel as a cooktop, I'm not clear on how well thin steel works. Does it warp? Does it burn through due to being kept hot under a pot? (Paul suggests that this would be the case here, which surprised me, as I thought many people do cook on them.)
Has anyone tried both and have conclusions?

For the main mass bell in the floor, I'd like to top it with some flat stone I've found, but I have nothing that could do the full span. I am imagining using it as the finish layer on what is essentially a half-barrel cob bell. Matt discourages the use of steel to support a roof since it undergoes so much thermal expansion. How does that fit with the use of a half-barrel under cob? Does the sheet metal just warp instead of cracking things, or where does the expansion go?
Also, I am looking at a mass with awkward dimensions - an area about 3' square. Could I cut a barrel open and stretch it out to make an arch with a 3' base, or would the corrugations just crumple somewhere along the curve and leave it useless for supporting the weight? Alternately, since the space is so deep, perhaps I should just use an intact barrel, and make the cob sidewalls really thick, which would tend to send heat up rather than out (desirable, I think). Thoughts on that?
My main concern with that is getting enough ISA to extract maximal heat from the exhaust before it leaves, but I think I can.
I want to aim for a minimum ISA of 40 sq ft (recommended for a 5" batch, which I believe is considered roughly equivalent to a 7" J). As I plan to do a bypass (see below), I'm not concerned about making my ISA to big to start up cold.

I considered the plunger tube as a tunable exhaust, but I am leaning away from it for a few reasons. Telescoping stove pipe is more expensive. I don't have any easy way to access inside the mass for adjustment, and it sounds like it mostly needs to be adjusted when the fire's out, which prevents it being used as a startup bypass. So although it's not tunable, I plan to use a direct bypass from the bell over the riser to the chimney - basically a channel in the cob with a homemade sliding sheet metal gate. If I need to tune the chimney endpoint, I can, but I hope that would just be a one-time process, unlike the challenge of lighting up with a cold mass or warm weather.
Looking up the plunger tube design, I saw recommendations to insulate at least the higher part of it inside the mass to prevent the exhaust being overheated as it leaves, wasting energy. I imagine this applies to the juice box straw concept in general, not just an adjustable version. Would a shield made out of cob (maybe with higher straw content) be sufficient?

Two other design elements I'm still curious about including:
A baking option:
Maybe a way to take out the back of the firebox and slide a loaf pan into it after the fire's out. I plan to line the firebox with heavy firebrick, so there would be enough residual heat to bake something that way. Maybe better would be a door into the brick bell beside the riser.

A water heating option:
Maybe just a pot sunk into the mass so that it's flush with the floor. Or I can just keep a water pot on the top whenever I don't need all the space for cooking.

I welcome your thoughts. Hoping to start building next week, finally.

This isn't right on topic with your original question, as it pertains to healthy tubers with mechanical damage.
When digging potatoes, I often stab or slash a few with the digging fork. I've found that most relatively "closed" wounds will cause rot in subsequent storage. If I promptly trim out the damage (within a day or so) to leave a clean surface that can get air, then leave it to cure at room temperature for a week or so, the potato will form a dried protective layer that prevents spoilage, and keep until spring nearly as well as an intact potato.
If you have hundreds of pounds to dig in a short time, this helps spread out how fast you have to get through eating the damaged ones.

Glenn Herbert wrote:About the spacing of the floor joists, consider that the omitted central joist would only bear half the load of the side joists, so would not be such a problem for the tie beam to carry. I think the best bet here is to have a header which ties into the two other joists and also sits on the tie beam, to distribute the load over several elements. This would give no point load at the center of the tie beam, which is typically the most stressful position.

I think you're describing an idea for how to add a third joist on the non-stove side of the floor (so joists at 1/4, 1/2, and 3/4 of the width). Correct?
I think the solution you describe would be quite workable. For my expected loads, however, I think just having the two joists at 1/4 and 3/4 is actually sufficient. The spreadsheet calculator I built says at that span (4' 2") 2" thick white spruce boards should be able to carry 900lbs uniform load per ft of floorboard width, which is far more than I expect to pack into such a small area.
I could use your solution if I wanted to make the mass wider and move the outside joists farther apart, however, so I'll keep it in mind.

Great feedback; thanks Glenn!

I'm new to all of this, so I have some questions to unpack what you shared.

ISA = internal surface area?

Adjustable plunger - I'm not familiar with this concept, but I am imagining you are talking about having the exit pipe be able to slide up and down so that it can extract exhaust from the bell at various heights, effectively bypassing part of the volume of the bell if needed. Correct? That is a cool idea.
How would this be plumbed in practice to make it sufficiently gas tight?

I like the idea of enclosing part of the barrel with cob if needed to adjust radiation vs absorption rates. Again, I can't quite picture how to make the barrel removable without damage to the cob. Most barrels I've seen have protruding horizontal corrugations every foot. And how does a removable lid affect barrel removability?

For both of those, if you can point me to other threads that illustrate how to do it, that would be great.

The walls will be LSC with earth plaster. I believe this is considered non-combustible, so I'm not too worried about getting more clearance unless someone tells me I should worry. And there are no wood members by the barrel.
I would definitely prefer to have the flue behind just for feeding and cooking access and aesthetics, however.
Moving it behind the barrel would mean piping it horizontally right under the riser or burn chamber, correct? If so, wouldn't I lose the option of adjustability you mentioned by having a fixed extraction height? Is behind the barrel better than sort of beside the barrel and feed, as in my new picture? This is more compact (but doesn't move the barrel out, if that's a concern).
I would probably need to jog the pipe forward again up in the roof triangle to clear the rafter (rafter's shadow is added to the new floorplan as well).

Well, winter's coming and I'd really like to get something built this fall. Still lots to figure out for that to happen!

Here's a specific proposal that I'd welcome feedback on:

7" diameter system to match the insulated stove pipe I happen to have.

The building is on stone plinths, with stilts to deal with the slope. I'd propose putting the stove in the uphill end, building right on the ground (on a rubble base?).
The mass would be built on the ground and form a section of the floor (flush with the rest of the floor). This would save space for other uses in a very small room, allow me to build on the ground rather than on footings or extra joists, and have the bonus of making the barrel a more comfy cooking height, I imagine. The under-floor space is not currently insulated, so the mass would be insulated with light straw clay walls 12" thick, with a 3-4" air gap between the LSC and mass to allow convection from the sides of the mass up into the living space. This gap would be covered with some sort of grate.

Space constraints under the floor indicate a maximum size for the entire stove footprint of ~3.5'x6'. I'd like to do a stratification chamber for the mass as it's a way to not need so much metal pipe. The end where I'd put the stove core is 20" from floor level to ground; the ground slopes down toward the other end, making a depth of 33" there. I imagine I could dig it down a bit on the short end to give more depth there. I know this is quite small for a RMH. Would I somehow need to add drag to the system to extract an efficient amount of the heat from the exhaust?

The flue pipe would go straight up and exit near the ridge of the roof. It's a little in the way of feeding to put it in front of the fuel feed, but unless it would be practical to run it right under the fuel feed and then up, I don't see any other option.

Details on what constrains the footprint:
The building is framed with six large posts. The middle two are tied by a pole under the floor. This would make it hard to extend the mass past the midline of the structure. The floor boards are to run the short direction (8' 8"'), carried on the long sills and two joists that run the long direction of the building. Since the aforementioned tie pole is not designed to be weight bearing, the simplest solution is to run them full length on either side of the mass. For distribution of the floor load between these four members, on the non-stove side of the building, I'd prefer to not put these joists farther apart than 1/2 the width of the span (that is, placed at 1/4 and 3/4 of the span). This limits the mass to the middle half of the width; including the air gaps, that leaves only about 3.5'.

Does that make sense? Does it seem like a viable plan?

If that mass seems way too small, I could consider expanding it to the entire half of the building floor, and find a different way to support the joists on the other half.

It can be helpful to figure out what specific disease(s) you're dealing with. There are various things called "blight," as Mary pointed out, and they behave differently. Fortunately, there's lots of info out there because everyone has problems with these diseases, including large growers. Not all the management information will be appealing (potatoes, for example, are often sprayed repeatedly through the season), but there are lots of pictures that help with diagnosis and some cultural practices that are easy to implement.

Without double checking, here's what I took away from my reading on late blight (phytophthera infestans) in potatoes after we got hit badly a few years ago.

Volunteers:
In Ontario, it only lives on living plant tissue, not in the soil over winter. Some places two differing strains that can mate and produce oospores that do overwinter on their own, so maybe check that. In this climate, tomatoes fully die on their own over winter, so potatoes are the concern - the tubers can be infected and, when shoot in the spring, can infect your new planting. For that reason, it's recommended to dig any volunteers and get rid of them (compost or bury very deep). In the fall when digging, do the same with any tubers you're not eating (damaged or diseased), or leave them fully on the surface so they can freeze and die that way.
When planting, I prefer to use tuber pieces that are big enough that they need to be cut at least once. This lets me see if there are signs of disease within the tuber. Otherwise you could be planting blight that you yourself kindly overwintered in optimal conditions.

Moisture:
Late blight usually gets into your garden by traveling on wind, so you can't keep it away if it's in your area. You can limit your plants' susceptibility by keeping them as dry as possible. The main tool for that is keeping foliage density a little lower so that plants can dry promptly after rain. For tomatoes, that means pruning all suckers on indeterminate varieties for a while, and for both tomatoes and potatoes it means planting far enough apart (you need to determine what that is in your conditions) and removing weeds. I don't like plastic, so I don't do this, but I have friends who have had success with the mini-greenhouse idea, so that you can keep the leaves entirely dry and water at ground level.

Infected plants:
If it's still just a few, remove them promptly and hot compost fully covered or bury deep. If possible, do this on a dry day to limit spreading spores. This probably won't stop the infection, but may slow it down.
For potatoes, a key understanding is that the top gets infected first, then spores from the top wash down through the soil in the next rain to infect the tubers. So once you identify infected tops, remove them immediately (before the next rain), and you can likely still harvest clean tubers. I like a sickle. Wait a few weeks to dig, to allow any spores surviving on the surface to hopefully die.

Varieties:
Good genetics go a long way with disease resistance, although blight is also constantly evolving new strains, so we lose resistance again. For potatoes, I've had some success with Sarpo Mira surviving in the presence of infected neighbours. For tomatoes, LYC 859 El Salvador and Bosu seem to have good late blight resistance (these are OPs, so seeds are as cheap as your labour!). When looking for seeds, google lists of resistant varieties and see if your favourite seed company has anything on the list.
If you prioritize disease resistance, it might mean letting go of focusing on your preference for colour/texture/flavour. It's a tradeoff.

Now with diagrams.
As before, let me know if you have feedback from trying it out.

Tony, thanks for more info about that article. With the date I was able to dig it up in the Internet Archive. Looks interesting. Not sure whether it's what I want, but definitely will keep it in mind.
The Energy Saving Stove

Coydon, you're in a pretty cold location for yurt living! Although I have run into a few in Ontario at Ottawa latitude and in central Vermont, both of which also get pretty cold winters.
Your reflections on the materials are worth hearing. Thanks for sharing them.

Coydon Wallham wrote: One element of "local" is how to view the existing production and distribution networks. 'Scrapping' makes use of readily available materials that would otherwise become landfill waste. But if use of such materials becomes part of a codified, growing system, it helps reinforce production methods that produce waste and excess. It would seem a tough call at what point making use of 'waste' products becomes encouragement to produce more of that waste. Using scrapped materials though often makes an RMH more reliable and reproducible than creating a similar element from local, scratch material. I guess that is the trade off you are looking to find help deciding on?

My own journey is a mixed focus on here-and-now impacts and modelling visionary alternatives. I sometimes follow the salvage mentality in areas where I am choosing to not give up a technology (e.g. some internet access: I got a computer from community computer recycling NGO), or where doing so has a positive footprint impact (e.g. a more efficient biomass cookstove from scrap metal makes less air pollution than a three stone fire - maybe important given how many people we have right now). I think the modelling of true alternatives is important too. As you say, depending on salvage ultimately requires the continuation of the present system. So I don't want to eat primarily on dumpster diving, or mulch my garden with cardboard, or get all my clothes at thrift stores, even though those things are possibilities in the wasteful context I'm in. We need people dancing to different music, expanding our collective vision for what is possible and how.

For this particular need, I'm not looking for help deciding which path to lean toward so much as putting the question out there about what options people are aware of so I can make a more informed decision about what to try. What I take from your writing is that you would suggest that I build a proven design so I know what good operation looks like, and then experiment from that baseline later.

Coydon Wallham wrote:As such, I've discovered it would likely have been better to install a 6" system.

Can you explain why 6" would be better than 8" for that situation?

Your notes on cooking are helpful. Sounds like it'd be okay, but not any too much cooking power. I'm a bit spoiled having recently switched my summer cooking from a bare firebrick J-tube to a TLUD based on Paul Anderson's Champion design. It boils water fast, and fries quite well.

Coydon Wallham wrote: I have to wonder what objections you have to wool insulation? Are you referring to rock wool/super wool? Or actual sheep's wool?

Yup, I was vague. I meant ceramic wool - high embodied energy and high-tech/centralized processing needed (very hard to localize). I have no problem with sheep's wool in reasonable quantities. I guess the latter is pretty fire resistant for an organic material - are you suggesting it could have a place in RMH building?

Thanks, Tony. I'm having trouble finding the article you referred to on the MHA site. Can you link it for me?