Lu Bivona

Riiiiight, heat capacity! Thanks Eino. I always forget about it at my own peril when we talk about thermal mass :p

Okay, so to continue along this train of thought, another question: let's imagine I really over-insulate this thing and am only using it for the water heating (take the heat loss associated with the radiant out for the moment), to the point of say R-40 insulation all around, and it retains its heat incredibly well. So when I wake up in the morning, having heated it the previous day, the water temp is 105, totally reasonable for a hot water heater. If I fire it to run it again at that point, the ambient temperature around the stove will be higher than the ambient air temp around any  typical wood stove, because we're never trying to get the air in our homes that hot. Do we think that means it can actually run hotter (and hence burn more efficiently) than that identical wood stove surrounded by air instead, or does the fact that the water its surrounded by could still continue to go so much higher (all the way up to boiling) mean it'll just continue dumping energy into the water at a relative (but diminishing) cost to the temperature in the combustion area?

I guess I'd add that the snorkel stove is only dispersing heat to the water through its surface area and two closed holes in the body of the stove that water can flow through it. While that's a good amount of surface area, I imagine it doesn't offer the best possible transferance.

My understanding is that more consumer-available EPA certified wood stoves have quite good efficiency thanks to things like chambers for preheating the combustion air, secondary burning, catalytic converters, etc. If those tech were in my lil scuba stove I wonder if that'd bode better for the idea? Or perhaps the fact that the combustion area is not separated enough from the mass area is just too big a design flaw, since that's sort of a lynch pin of RMH's efficiency
EDIT: Maybe put air intake pipes in a circuit running between the tank and the insulation so that that air could get pre-heated by the water as its coming in and improve combustion efficiency? We're starting to complicate the design  now haha

Hi William,

Just to be clear, I'm also willing to entertain the idea of keeping this inside, or at least in a semi-inside/semi-outside state that I could vary with season by opening a door or something. Your points on the efficiency of the stove are the ones I'm most interested in - and I have some questions since I'm not all that knowledgeable on this front:

1) Air moves away from your wood stove more rapidly than water would, correct? So the stove losing heat to the water its heating is similar to a woodstove losing heat to the air its heating? I guess the difference here is that water is a much better conductor of heat?
2) If the unit were inside (or at least semi-inside, as in a partially insulated glazed attached area), the lowest the water temp would reach would be room temperature. If the home is as well-sealed, well-insulated, and passively solar warmed as I intend, I'm hoping the lowest that that would get in the winter would be 65-70. The water wouldn't be lower than ambient air temps, so less of a big deal, right? As with insulation, the heat loss is greater the greater the temperature gradient, so if I were trying to consistently heat cold or near-freezing water I'd think it'd be a bigger issue, but less so with room temp or higher water?
3) If my tank were larger, say more in that 200-300 gallon range, it'd take longer to warm up but would also take longer to lose warmth - and that larger capacity would give a bigger buffer when new water has to be added to the tank to replace hot water I've used. I'd also like to be creative with how all of my potable water is stored in general, maybe somehow within the insulated envelope of my building so it has time to warm up passively and the water going into my tank never has to be heated up that much in the first place.

I've been frustrated because books on wood stove and fire efficiency seem either too general audience oriented with no technical info, or oriented toward mechanical engineers with very little hope of me understanding. Structural engineers have made things pretty easy with span tables and simplified calculations, but understanding all this heat mechanical stuff has been a rougher go! If you have suggestions for education sources I'd love them!

Apologies if something like this has already been shared – the search function on here isn’t always ideal. Thoughts welcomed! The only post I saw that seemed to be getting at anything like this was this one: https://permies.com/t/60386/Radically-simply-hydronic-heat-boom

Background (feel free to skip): I’ve always liked Paul’s tinkering around with more easily deconstructed mass heaters (e.g. pebble stoves). They make the concept of mass heating much more reasonable when there’s a possibility you’ll have to up and move your house, and if the code folks ever come knocking when you’re experimenting with something not in the IRC and it ultimately comes down to them ordering removal/dismantling, it’s less of a sunk cost. Water is the holy grail for easy to obtain and easy to remove, low cost, high mass material, but the main problem (danger) is getting water to play nicely with high temperatures of an RMH.

The original idea – Wood-fired hot water heater: Well, about five years ago I took the plunge on one of those scuba/snorkel stoves that you submerge right in a tank to make a wood fired hot tub (here's the manufacturer I bought from: https://snorkel.com/product/scuba-stove-and-fence/). It had some good times and fun parties while it lasted, but for the past few years its been in storage since we’ve lived in rowhomes, friends basements, etc… Now that we’re finally planning for our tiny little place on our own land, it occurred to me that if I put that scuba stove, which is only collecting dust for now, in a much smaller tank (say 80-100 gal) and insulated the tank well, we’ve got a pretty direct, easy to use wood fired hot water heater. The scuba stove is capable of heating up a 470 gal tank 15 degrees F/hour, so this puppy would require very little firing time, even if I occasionally let the water get down to room temps). In the summer it might be annoying to have the excess waste heat from the stove in the house, but I figure I could find some design way to isolate the tank (maybe in its own little isolated attached mini greenhouse ventilated to the outside) during that season. Or I could run a warm-season only simple solar hot water loop through it and not fire it at all between late March to Mid-November (I’m in zone 8a NJ).  In the winter, the “waste” heat from the stove and chimney would actually be desirable, which leads to the next point…

Adding extra mass: Thinking about that “waste” heat and how it’d be convenient in the winter – especially because a wood stove immersed in water is going to generate a lot more humidity to soothe those nasal passages in the dry winter (one of my biggest challenges with wood heat since I get stuffy really easily) – it occurred to me that maybe I could make the room-facing sides of the insulation (the top and any exposed sides) removable and just let the water’s mass radiate into the room. Sure, it might mean firing the stove again sooner or that my “hot” water gets slightly colder than I’d like, but maybe it’d make for more comfort? Once I started thinking about just letting the mass radiate I started to ponder if hydronics would also make more sense? I could have a shutoff valve that I only open during the winters, with a small circulator pump in the circuit, and have water circulate just like a more mainstream hydronic floor. If I used larger piping, it would up the amount of mass and make the temp swings somewhat more gradual. Though I did the math and if tubing spacing is just one foot, I’d only be adding about 25-30 gallons of mass for our whole 450 sq ft place if I used 1.5” hydronic PEX – so not getting that much volume. Closer spacing would add more water (mass) and improve the heat distribution, but adds $ :/

Other thoughts:
- Why I like this: It’s easy and fast to build (anyone who can operate a drill and a ratchet and put a couple of waterproof gaskets on the tank side can install the scuba stove), can be scaled up or down and customized depending on space and need, adds humidity to normally over dry wood heat, and seemingly very simple to maintain and operate. It also seems very safe because the tank (and therefore the hydronic line running off of it) are ultimately not pressurized and the stove is not relying on heating coils that could get to hot in error or something like that can cause major safety issues. A few issues I can think of with it might be…
- Cost: This is not necessarily a cheap project – the scuba stove is $900 and 500 ft of PEX tubing is $1,000, add in the stock tank, pump, and various plumbing fixtures and I’m probably looking at around $2,500 (if I didn’t already own the stove) for just the heating supplies, not to mention the sand or earthen floor or whatever you’d immerse and support the hydronic tubing in (although if this would be part of your finished floor anyway, that price should only be moderately factored in). But I’m willing to pay a little bit more on the front end for cheap, easy, and reliable. My understanding is that even if you’re pretty miserly about scrounging up materials a rocket mass heater is still likely going to run you $1,500-2,500. An amazing savings over a professional mass heater but still not nothing.
- Durability? The snorkel/scuba stove is designed for being used recreationally in a wood-fired hot tub. I don’t know how it’d hold up to this kind of consistent firing. If it could last as long as regular tank water heater that’d be darn impressive, but maybe asking too much? IIRC from the manufacturers directions, the water helps dissipate the high heats facing the stove, prolonging its life and keeping it from burning out. Might ask the manufacturer about their durability! If I only use it for the hot water heating and drop the space heating idea, I imagine it would only need a very brief heating each day, so maybe that’d prolong it too?
- Stove efficiency? I don’t know enough about stove designs to comment on the efficiency of this design? It’s not a rocket design – actually the feed is a bit higher than the manifold where the stove meets the chimney. I really appreciate rockets for their clean burn so maybe the lack of that alone makes this a less desirable design? Another thing to inquire with the manufacturer? Like a rocket, the feed does go down, and if I understand rockets at all, this is a feature that helps generate that strong air flow, and the chimney is above on the other side of the feed where the chimney can act as a tall riser that stays hot. So maybe it actually is relatively rocket-y? My hunch is that not getting particularly hot is a part of the stoves safety design and therefore combustion might be incomplete? Input from more knowledgeable folks welcomed here 😊
- Amount of storage necessary? I may need a much bigger tank for the radiant idea to make sense (if it does make sense at all) – maybe 300-400 gallons? In which case that seems a bit overkill, although that makes the idea of housing it in an attached small greenhouse even more palatable, where any wasted heat, again, wouldn’t be wasted. I have some calculations from a solar hot water book that could help me here, just too lazy to run them yet

Basically, the question is in the subject line. I've read that wood's durability typically boils down to a combination of 1) the extractives/compounds that help to deter fungi and other rot-inducers and 2) the density of the wood itself. Black locust seems to be an example of a wood that has both in spades and contributes to its superior rot resistance. My question is, how important is #2? I think there are some interesting applications for sawdust that is rot resistant (e.g. loose fill insulation in subgrade applications like a basement, when mixed with wood glue or some other binder could be an outdoors-compatible wood filler, I'm sure there are other applications) so it'd be cool to know.

Curious, maybe someone's accidentally left out a pile of pine sawdust, a pile of maple, and a pile of white oak and seen a difference in decomposition? Maybe some unintentional natural experiments going on out there? Just probing the hive mind for inklings of an answer :)

Couldn't figure out how to just edit my last reply, so here's just some more info:

1) Wow, no joke about it being here soon: I found a scientific paper from 2011 saying it's most northern range at the time was Wilmington, NC. Reports on iNaturalist now show verified observations https://www.inaturalist.org/taxa/79388-Triadica-sebifera near Richmond, VA. At this rate I'll be harvesting tallow wax here in NJ by 2030.

2) Found a patent granted in 1989 to a team from Japan for making 'hard butter' from the seeds: https://patents.google.com/patent/US4844940A/en. To get to behave like butter, you can apparently fractionate it through 'winterization' - i.e. cool it down slowly and then remove the crystals that form at different rates. All that being said, virgin coconut oil is unfractionated and still has a shelf life equal to or better than refined coconut oil. I guess the only reason you'd want to do this is for aesthetic preference (more neutral taste - maybe tallow tree tastes bad?, color, etc.)

That's all for now :D

ust thought I'd update this for any folks in posterity who were interested - never went back and did so with my goldfish brain! Neal's idea to consider species that generate wax was a good one, so I decided to look into waxy species. I went on Plants for a Future, searched for 'Other Uses: Wax" and put in my zone info (7/8), and I went down the (very short) list of results to see if any had the fat listed as edible. The only possible candidate that came out of that search was Triadica sebifera, the 'Tallow Tree', and it seems like it checks all the boxes I was asking about:  it is highly productive and does produce an edible fat. However, Dean at Eat the Weeds Tallow Tree had trouble finding more information and much luck preparing it himself, and the only scientific journal articles I can find of making an edible fat from it seem to be industrial food chemistry folks, but the reports of its edibility date further back... another puzzle to solve! Wish I knew Mandarin!

Aware of it now, I can see most of the posts on PFAF include references to how invasive it is. It's always hard to judge through rhetoric how severe an issue is, and the USDA only lists it as moderately invasive, which is not the impression you'd get reading articles about it from state extension services etc. But I guess if it's invasive as the most fearful folks say it is, I won't have to plant any, because it will be growing here soon enough (currently it's restricted to zone 8 areas of N Carolina, but it exists in significantly colder ranges in E Asia).

At the very least, I'd say this tree puts to bed the theory that climate necessarily precludes plants producing solid, edible fats. Seems the tropical biodiversity argument was the better explanation.

Last year I developed a mini natural swimming (more like soaking) pool/wood fired hot tub design and installed it in my back yard (I know, this isn't a pond, technically, but I figured you all would have the best idea of what's going on with lotus plants). The pool is 6 foot stock tank and there is a two-part natural filter system made up of IBC containers.  a The first filter holds cattails and these seem to be doing fine, while the second filter is where I keep American Lotuses. It's a pretty cool design, and assuming I can get it working better, I'll have to do a write up some time.

The lotuses currently aren't looking great. I started some from seed and others from tubers I purchased. There are some large coin leaves, and the ones from tubers appear to be doing a bit better. However, some of the smaller leaves have started to brown and a couple are even slowly skeletizing, where the leaf looks like its burned away and just the net-like veins remain. The bigger leaves look like they're still ok, but are slowly turning lighter shades of green and beginning to look chlorotic. My hunch is that I'm not lacking in nitrogen or phosphorous, because the algae seem to be doing just fine :/ From what I've read, it seems like these might be micronutrient issues. However, I've read that under no circumstances should I fertilize my lotuses until they have their first aerial leaves. But it's been over a month now  and no aerial leaves in sight, and there are no shoots coming that make it seem like there will be aerial leaves any time soon.

I feel a bit stuck here, and these lotuses doing well is critical to my natural filter design. It's night time now, but I'll make sure to post pictures tomorrow. Thanks for your help!

I have to say, I come from the agroecology and gardening end of the permaculture community, so when it comes to engineering topics, or basically has anything to do with generating power or heat, I'm much less technically inclined. But I am trying to break through these walls a bit and understand them! Still, I'm confused about a number of things, even having poked around these and other forums quite a bit. I have a lot of questions, and they sort of jump around but please bare with me!

My thinking was that a simple project to introduce me to understanding this world would be to build a simple charcoal maker/kiln/retort (first question: is there any difference between these three things, or are they basically synonyms? I've read conflicting things on different pages!).

In a resource-constrained, too-frequently polluted world, before settling on a design, I wanted to understand what retort/kiln design would be most efficient (both in terms of minimal fuel input and maximum charcoal out) and generate the least harmful emissions (e.g. particulate matter or other nasty gasses). (Second question: What IS the most efficient design? From what I've read both TLUD and Rocket Stove designs can work well, but what are their pluses and minuses? Are there others? After reading about several different types of designs, I'm still unclear, probably because I don't understand the basic concepts well enough)

In trying to read about charcoal making, I ended up frequently on pages that discuss wood gasification. I did not understand before today that in charcoal production, the "smoke" exiting the chimney is actually wood gas (at least that's what I've read and heard in articles and videos trying to figure this out!). But it was my impression that pyrolysis (which is what we're looking to encourage to make coal) occurs at lower temperatures and lower oxygen levels than gasification (which is what you want to be at to produce usable wood gas). And my understanding was that in gasification, your goal is to burn the fuel so hot that you're NOT left with charcoal (or at least as much of it), and that it essentially all gasifies. So I guess third question: Could one design a charcoal maker/kiln/whatever from which one also harvested the wood gas exiting the system? Or are these two mutually incompatible processes, where efficiency in one (e.g. good at producing coal) would be bad at producing the other (e.g. low quality or low amounts of gas)?.

If they're incompatible, from what I've read, being a pragmatic hobbyist on a suburban-size lot, there doesn't seem to be much reason for me to develop a wood gasification system. In my short time exploring this topic, I've learned I have NO interest in storing wood gas (it seems like a good way to accidentally make a bomb, if you're as non-tech savvy as I am...), and so wood gasification is only really valuable if you can use it right away, say hooked up to a gas-powered generator. And those wood gas cars are cool, but I feel like my bike is infinitely more efficient, and I at least know how that works :p So at best, I'd really only be putting all of this work into developing something that could power a back up generator? Would wood gas make more sense to use as a cooking fuel since it burns cleaner, or can I just use my charcoal grill and feel pretty good about that? I guess I'm just trying to understand: in what ways would/could wood gas fit in more urban/suburban permaculture homestead settings, if at all?

For that matter, from my limited understanding, the power density (as described by Vaclav Smil) seems bad for wood gas and even worse for coal gasification. From a global perspective, does gasification really make sense as a form of sustainable power compared to solar, wind and hydro? It seems like the land footprint that would be required to gather sufficient amounts of wood would eventually lead to deforestation, as it did in the 19th and early 20th centuries? Is the idea that it makes more sense in areas with comparatively little solar exposure?

Finally, getting back to the charcoal making, putting on my permaculture hat of "designing for multiple yields," it seems that the yields of a charcoal maker are the charcoal, potentially the wood gas (see questions above), and heat, and a large number of the designs and videos I see people routinely not taking advantage of the latter (burning in the middle of summer, outside, and no mass around the kiln). Are there any charcoal retort/kiln designs that have thermal mass around them to catch and slowly release that heat, a la the rocket mass heater? Is this another case where doing one thing well (capturing the heat) would make another function work less well (e.g. producing less charcoal, or a less efficieny burn?). At the very least, perhaps it makes sense to just save your charcoal making until winter, so that you're at least getting a use out of the heat you're generating?

Yeah, I'm leaning toward the latter - that it's essentially a miracle that even just a few plants have this exact mix of traits, and so much of our planet's biodiversity is concentrated in tropical hot spots that it's probably a matter of odds. Still an interesting question!

Great point on the bayberries - I was violating my own (only plant sources) rules and have been thinking about beeswax as a sensible replacement in cosmetics. It may be silly of me, but I just wasn't really thinking about waxes as a fat that shares similar qualities to these oils. But myrtle/bayberry wax pretty much checks off all of the boxes I'm looking for in that sense. I think I'll do a bit of experimenting with it! Thinking of waxes as having these similar properties definitely opens things up quite bit

For culinary uses comparable to how one would use butter, I also had a little flash of insight: in this whole thing I've been thinking about what temperatures these fats (coconut, shea, cocoa) melt at. I read an interesting article about how, because they are composed of several different chains of fatty acids, coconut oil and the like don't have one "melting point", and that different parts of the fats are actually melting at different temps. Beeswax candle making is the same way - beeswax melts between 145-150 F. Only thinking about the melting points, I was failing to think on the flip-side - what are the freezing/solidification points of other oils? There might be some that are liquid at "room" temp, but only because we generally only keep our rooms in the comfy 60 - 75 F range. Maybe there are some oils that are solid at 55 or even 60 and could just be kept in the fridge to be treated like butter in a spreadbility sense? A quick look at some common oils shows that avocado oil starts to solidify around 50 F, and peanut oil in the high 30s. Again, I'm counting avos because I've seen them survive into zone 8 (with plenty of protection). I'll post back here if I find anything interesting to add!

That's my thinking as well, but I don't know enough about where and when these fats are synthesized in the plant (e.g. the same combination of fatty acids that is in the coconut is not necessarily circulating around in the plant - they might synthesize in the fruit itself, so transporting them around the plant is not such a big deal. There are also places where coconut trees grow well. Also, the melting point of these oils is relatively high (coconut oil 76, shea butter in the 90s), yet there are climates where these plants grow just fine in quite low temperatures - there're coconut trees in Tampa, FL, where temperatures in the cold months are in the 50s on average, sometimes getting considerably lower. You'd think if it were a problem of the plant equivalent of arteries getting clogged, that it'd only live in the most tropical climates.