Masonry Heaters: 18-24 hours of heat output from a single wood fire
mha-net.org
mha-net.org
Specifically the important parts: “a mass of at least 800 kg. (1760 lbs.)”, “an overall average wall thickness not exceeding 250 mm (10 in.)”, “external surface of the masonry heater … does not exceed 110 C. (230 F.)”, “at least one 180 degree change in flow direction” of the released gas.
Essentially, it’s a rocket stove and a large thermal mass of masonry, and the masonry has a few design restrictions to meet certain performance characteristics (i.e. it should absorb most of the heat released by the stove and it shouldn’t release that heat too quickly from any particular point on the mass). Mostly I’m surprised it’s that easy; I would have thought anything that can radiate heat slowly and consistently would by necessity also absorb heat slowly and consistently, making it difficult to capture most of the heat from a fast-burning stove.
Something like a ratio of flame-side heat absorption at flame temperatures, to room-side emissivity at room-side temperatures(wavelengths)?? This could fall out from material properties without appearing as an explicit design detail (e.g. a surface treatment).
My understanding is that this is the motivating principle for the "bell" part of a rocket stove masonry heater. The hot exhaust from the combustion chamber is piped into a massively larger enclosed airspace, which has an exhaust port at the very bottom. Since hot air rises and cool air falls, most of the heat in the exhaust will be absorbed by the bell enclosure before finally being vented through the exhaust port. By enclosing the bell in masonry, it meets the design restrictions you mention.
I think it's also possible to build a masonry heater without using rocket stove principles, just a fireplace with a huge mass of masonry surrounding it. In that case, you'd want cladding with somewhat high thermal conductivity to capture the exhaust heat, but it's thick enough that the solution to the Fourier heat equation [0] is low enough to meet the design restrictions.
[0] https://en.wikipedia.org/wiki/Heat_equation#Heat_flow_in_a_u...
In fact it's how people have heated for a very very long time. Lots of these in European houses from days past. And in fact I know a guy in Germany that heats his living room, dining room and kitchen with one that's built into the wall between these three rooms. Not as fancy as what you see there or on Wikipedia. Just tiled wall basically. Old house and instead of ripping it out when he bought it, he cleaned it up and uses it to heat.
A kachelofen uses a maze-like passage created out of firebrick to release gases and smoke from the wood fire slowly, allowing the firebrick to retain as much heat as possible from the gases and smoke. The ceramic tile surrounding the stove also acts as insulation to retain heat. Such stoves were carefully designed so that the minimum amount of heat would escape, only as much as needed to warm the flue to maintain a proper air draught. The firebrick used in the construction holds 80% more heat than ferrous metals such as cast iron, while its heat conductivity is 1/45 that of iron or steel: A kachelofen is efficient enough to warm a house for up to 6 to 12 hours after the fire has stopped burning
Yes. Related: Russian stove:
https://en.m.wikipedia.org/wiki/Russian_stove
And this article show that many of these variants are related, and that the basic principle is very old, even prehistoric:
It’s a pretty ingenious modification of a known concept. As far as I can tell, this is indeed new.
There are a couple factors at play that help it absorb heat faster: 1) Convection rather than radiation. As the hot gasses travel through they stove they are in contact with the mass and can transfer heat more directly to it, which then spreads through the mass via conduction. This is aided by... 2) Newton's Law of Heating and Cooling, which basically says "the greater the temperature difference, the faster the heat transfer". Since the gasses from the combustion are around 1000 degrees or higher (at least initially) they transfer heat much faster to the mass (several hundred degrees delta) than the mass transfers it to the room temperature air (one or two hundred degrees delta).
Similarly, much of the radiation of the hot inner mass is absorbed by other parts of the hot inner mass; only the parts near the outside actually radiate heat away from the mass.
Aside from the technological measures mentioned in other replies, I'm not sure this follows. Thermal energy transfer is proportional to the temperature difference. So a high thermal mass storage can absorb energy quickly from a high temperature source, experience a small change in temperature, then expel the heat slowly into a sink slightly cooler than itself. That is a gross oversimplification but it at least it shows that the effect is not too surprising.
This is exactly how fire stoves and fireplaces were built in Europe for centuries, there is nothing new or unknown in this technique.
Usually they have water pipes running through the masonry, and they were not only used for cooking and heating the house - but also for heating the bath water. Very efficient.
sounds like a reverse offset smoker. the key is the doubling back of the flow of the hot air so it is directed under the cooking surface, then allowed to rise up to fill the cooking area with the smoke and hot air. the chimney is on the same side as the heat source, so it is pulled back across the chamber. the advantage is to keep even temp for the entire length of the cooking chamber where the traditional offset smoker tends to have a cooler spot directly under the chimney and a hot spot closest to the fire box.
I sometimes wonder though, is better to have a negative pressure living space pulling dry, cool air into the house rather than positive pressure, forcing warm, moist air through any and all crevices throughout the house. I guess time will tell..
Its just a matter of cultural change in society.
resistance is futile.
What you want is tradition not reasoning.
From a pure Logic point of view metric is better.
And it's used in the USA.
Even inches and feet are based on the metric meter.
GP was taking about the US so imperial would be normal. It would be extremely not normal for a Pennsylvanian to describe their home in meters squared
Internet suggests that "normal" for 65% of HN is metric.
If for nothing else, units conversions is either trivial or stupidly complex, and everybody is doing it all the time so it makes actual difference. We used to have tens if not hundreds of similar systems to imperial one here in Europe during medieval times (my own country at least 5 of those), but we moved to much greener pastures since then and now it just looks... ridiculous to keep it polite.
It also seems weird on principle. If 65% of HN readers spoke Mandarin, would you make all your comments in Simplified Chinese?
That's what happened here. Somebody posted in their native unit, somebody converted it to the audience-standard unit and you took issue with "normal" and a lot of words were thrown on the bonfire of internet squabbles and here we are.
I don't expect people to change their units themselves. It's nice when they cover both bases. But it's also nice when people offer a conversion so others get the picture without having to do any mental gymnastics. I think what we could all do without is this bit.
Using a projected area of 70 cm², the floor space should fit ~33142 bananas when tiled end-to-end.
Negative pressure will pull air through crevices (which will be lower down, like under doors and windows, and from the foundation) drawing in dust and allergens. Ideally pressure should be neutral but that is easier to achieve with a "positive pressure" fireplace like that.
But it can be fancier too, more arts-y tiles or building the heater in a shape where you can sit down etc.
Anyways it’s incredibly good! You really only need a single small fire and the heat can last a long time. Perfect for cold winter nights.
We have same in Czech republic and Slovakia, physics of it are simple and discovered probably millennia ago and people also install it in new homes (I know few newly built homes it massive stove like this). You can actually ie bake bread in it if construction allows it.
My parents have another kind of stove - they can switch mode to a regime which doesn't burn the wood violently in 1h but rather lets it burn slowly for around 8-10h. The stove looks like generic small ones, but if you load it in the evening it still radiates a bit of heat in the morning from just few pieces of wood. They can easily make it through harsh winter with just this and moderate pile of wood, even if cut off form everything.
Unfortunately building/making wood stoves is a dying art, you can find less and less people who are good at it. And the trouble is that if you build a defective wood stove you risk all sorts of nasty stuff, like dying from carbon monoxide intoxication.
It was dual purpose cooking range and a huge mass of bricks for heat capacity. It was built at the spot where multiple walls met in the centre of the house to increase mass of bricks in contact and to heat multiple adjacent rooms. The walls in those rooms would typically be too hot to touch.
One weakness of our setup was that it would suck the air out of the house through the night. Another was that complex internal tunnels made the maintenance difficult.
If I built one today I would make sure to fix it and create a sort of heat exchanger for coming air.
Having been out all day, I needed to use the restroom. Turns out the bathroom shared a brick wall with the back of their wood ovens. So this bathroom was like a fucking oven, and super compact. I touched the wall like an idiot and burned my hand.
The pizza was excellent.
Also did you have a la piedra or al molde?
Resistive heat has the property that efficiency doesn't change with temperature. So you can heat a small amount of bricks to oven temp with no loss. But heat pump efficiency is very dependent.
So yeah in theory you could use a heat pump to heat a heat reservoir. Where I get stuck is detailed calcs are needed to suss out the economics. That people don't tend to do this says the economics aren't strongly in favor.
A rule of thumb from https://www.greenmatch.co.uk/green-energy/central-heating-ca... suggests 1 kW of heating per 10 square meters (that's likely the output that's required to maintain an adequate temperature during the n-th percentile coldest days). So on a cold day, a 50 square meter apartment would need 24*5 = 120 kWh.
I'd say it's pretty impractical unfortunately. Water is already the best (practical) choice in terms of heat stored per weight, and likely also per volume, and cost.
Roughing it I think if you used batteries you'd need to supply maybe guessing here 80kwh during the evening and night time. With a heat pump you'd need 20kwh. That's doable. Course likely over all more efficient to let the utility handle the battery end of things.
Then again maybe a masonry heater makes sense if you often have excess solar and your utility is screwing you.
That may sound low, but the thing is houses here are very well insulated, so when you heat the masonry walls and floors to 21c it takes a very long time for the temperature to drop - the heat pump is just maintaining the heat
We live in a slightly older house (which doesn't meet current insulation standards) and last year the heat consumption was 70kWh per m2. That's for heating and hot water.
This mean that the design of the whole thing will change to modulate the heat flows appropriately.
https://www.elnur.co.uk/product/ecombi-hhr-high-heat-retenti...
My parents use these as their main heat source, and once you figure out how they work, the work fine. If I was building new today I'd go with a heat pump, but they have no ducting or central heating, so retrofitting that would not be easy. These could still be useful as a supplementary heat source if you have solar, as they are much cheaper than batteries for storing energy.
A big downside of conventional wood stoves I find is that, if I'm not going to be hanging out at home--mostly around where the fire is--for the day it's not really worth it a lot of the time. I fire up my wood stove mostly when it's especially cold and I'm not primarily working in my office.
When I run it during the work day I'll generally work in the same room just to keep an eye on it. Some days that's not worth it. Some days it's -20F out and totally worth it.
It stores about 60kWh and releases it gradually over 24+h. The problem is that it's power is about 3kW of released heat. Which is not enough for my home. I still need to use heat pumps as my main heat source (not complaining as they are more efficient anyway).
So it's a nice fireplace that retains heat and the cats really love it. But there is a tradeoff between power and efficiency so if you are looking to put one in, do the math and figure out whether it is the right solution for you.
They also claim that over 90% of the burning energy goes to heating the home and only 10% is lost. Tho as mine uses inside air I suspect that as cold air is drawn in, it might be less.
[0] https://www.tulikivi.com/en/Products/Fireplace_collection
We have a passively heated house, and use a standard modern wood stove. It has some stones inside it to hold a bit of heat, but only stays warm for about an hour after the fire is out. It is >80% efficient and has similar emissions to these. They run $1-2K in the US. Here are some examples:
https://www.blazeking.com/product-category/wood/
(These tend to be heavily regulated with different manufacturers in each country producing similar stoves.)
It has a built in thermostat/oxygen regulator, and can run for up to 24 hours without reloading (8-12 hours is more realistic; they advertise 40 hours on bigger models).
Instead of using exterior air through an inlet, it pulls air from the house. The outside of the stove looks traditional, but is actually a cleverly designed convection heatsink, so it doesn’t need a blower. (For tiny homes or trailers, you still need an inlet from the outside.)
Anyway, for modern construction at mid to low latitudes, I think this approach is much better than a masonry stove. In particular, we can turn the stove down in the morning, and up in the evening by turning a knob.
We had a masonry-style stove where I grew up, and it was fine. That house was older (and shaded by trees), so it didn’t really passively heat during the day. That’s an appropriate use case for big masonry thermal masses.
I was under the impression wood-burning stoves produce very high amounts of particulate pollution, is “clean” really an accurate claim?
This doesn't sound right to me, because there's heavy soot production whether hot or cold stove. Are you sure it's not just because a large masonry stove (and its long chimney) has a lot more surface for soot and particulate to stick on compared to e.g. a tiny portable one?
But when we talk about burning wood cleanly we're really talking about smoke -- and that's almost entirely organics (including large amounts of carbon monoxide, methane, and VOCs) which will combust given sufficient heat and oxygen.
If you're interested in the chemical and physical processes that comprise combustion, I cannot recommend the Faraday Lectures highly enough. Michael Faraday was an experimental physicist, and the data resulting from his experiments were what James Maxwell used to derive Maxwell's Equations for electromagnetic phenomena. Separately, he gave a lecture on candles and combustion, which has been recreated in the original language here: https://www.youtube.com/playlist?list=PL0INsTTU1k2UCpOfRuMDR....
The important intuition is that combustion is a gaseous process. When burning (solid) wood, the heated wood will give off gasses and particulate matter (soot). Those gasses and particulates will combine with gaseous oxygen in an exothermic reaction called combustion. In a conventional fireplace, the oxygen and temperature constraints result in incomplete combustion, leading to particulate pollution as you mention.
The search term to learn more about overcoming those hurdles is "rocket stove". In these sorts of stoves, there's a few modifications that result in more-complete combustion. There's separate chambers for the wood and the combustion. In the first chamber, the same processes as in a conventional fireplace occur: the wood burns, creating soot and hydrocarbon gasses. In the second chamber, hot air is added to provide a secondary source of oxygen, and the airflow velocity is reduced (by carefully calculating chamber size using the Bernoulli fluid equation), resulting in complete combustion. By having a right angle between the input from the wood chamber and the chimney, you can achieve a chaotic mixing (in terms of chaos theory) between the wood byproducts and the added oxygen, which also helps ensure complete combustion.
So the entire system has the following parts: a chamber to burn the wood; a chamber where hot external air is added and full combustion achieved; a tall chimney to ensure a good draft and airflow; a "bell" to slow air velocity of the hot exhaust and ensure good thermal transfer; a final exhaust port.
Here's some resources:
https://www.youtube.com/watch?v=IsO9vLn8JHw
https://en.wikipedia.org/wiki/Rocket_mass_heater
"A rocket mass heater (RMH), also known as rocket stove mass heater, is a form of slow-release radiant heating system, designed to primarily heat people and secondarily to warm areas in line of sight around it."
https://en.m.wikipedia.org/wiki/Russian_stove
Now when the end game is known, perhaps it is better to just adopt the existing designs instead of trying to perfect the current product.
I'd at the very minimum put large flat rocks in the fireplace, then put them under the bed.
PS (They lost - instead of concentrating on food, they concentrate on the shelter, which was too sophisticated and took too much energy)
PPS I'm also surprised that most of the contestants rarely move from bad locations. I.e. "this location is so bad, there is no good place for the shelter, and no food", but then instead of moving they just stay.
It's pretty clear from watching it that it's set up so you'll starve no matter how good you are at hunting/foraging. Then it's all about the energy expenditure. I noticed that the bigger the shelter, the thicker the logs in it, the sooner they tap out from starvation. Me, I'd start with two trees about 10 feet apart and use them as my main posts. Lash a log between them about 7 feet up, and that is the roof ridge. Lean 1" logs against it. Voila! a decent, strong shelter with minimal energy expenditure. Next I'd build a bed a few inches off the ground, to keep it dry and so hot rocks can be pushed under it.
Then I'd tap out because I don't know how to hunt or fish.
One woman built a decent fireplace out of clay and rocks, but it was a bit too small.
I really liked it. It was intense and unique - one person from the pair had to hike to location using compass. So they didn’t start together.
In Patagonia season one of the women built top-class camp as well and she wasn’t starving too!
I disagree about that it’s setup for starvation - quite often there is single or maybe two participants that get enough food. It is surprising though that quite often they obviously don’t pay enough attention to food and just starve. Ie they would try to fish in the same spot and not get fish over and over and so on.
I don’t have enough training to do that, but strategy-wise I would tap out if me trajectory was negative. Ie if I’m obviously not getting enough food or fail to have warm shelter as cold arrives, I would just tap out.
There is a "rörspis" - a free-standing masonry stove with integrated "rörslinga" in the house as well but I have yet to renovate it so we do not use it yet, also because we can keep it warm enough using the two heat sources we now use. Next to those there is a wood-burning oven built into foot of the chimney (or "murstock" as it is called in Swedish) which sees very occasional use. I also made a connection for a stove on the 2nd floor but have not connected it yet because of a lack of need, it gets warm enough the way it is now.
Total wood consumption for the house lies between 4m³ and 7m³ per year depending on winter temperatures and types of wood, this includes wood used for cooking. I have yet to cut a living tree for firewood as there always is enough storm-felled wood in our 15 hectares of forest to satisfy our needs.
Edit: did a bit of research, apparently it is a “particulate per hour” restriction, not a general wood fireplace restriction. This might meet the requirements.
2. It would be interesting to revive the old Roman hypocaust system, with the benefit of modern rocket-stove design knowledge.
3. Going the other direction, a hyper-modern system based of full gasification, followed by clean combustion of the syngas -- something that looked more like a chemical plant than an oven -- would be interesting, if maybe not actually simple/robust enough to be a good idea. Would like to see it tried though.
What could historically lead to prevalence of such a bizarre design?
https://www.samharris.org/blog/the-fireplace-delusion
It smells nice but it is damaging your heart and lungs and those of people within a quarter mile radius.
1. He wrote that in 2012, citing a study from 2007. Standards for wood stoves have changed. Old wood stoves and open fireplaces are unquestionably dirty - in the 30-50 grams of particulate released per hour, sometimes more. But is it fair to compare them to, say, modern EPA standards which have a 2.5 gram limit per hour which took effect in 2015, or the 4.5 gram limit from 2012? I don't believe so. Sure, if you still have that ancient 80s wood stove still in your house, it's unquestionably far more dangerous than a modern product. The masonry heaters in this original post have been hitting 1-3 grams of particulate for over 30 years now. Some heaters managed to score less than 1 gram of particulates released in MHA EPA-certified testing in 1998, beating standards decades ahead of time.
2. "The case against burning wood is every bit as clear as the case against smoking cigarettes." This is an argument that proves too much - because the exact same arguments about "creating pollution you can't dispose of" is equally valid of cars. It's also, arguably, equally valid for when you buy unnecessary products on Amazon, or do any CO2-emitting activity. You would have to show it is a particularly unconscionable level of emissions - which, with modern efficiency meaning you can get about 20~25 fires in a modern device for the damage of a single campfire... if you are going to burn wood, I'd prefer you had a modern wood stove habit than a campfire habit.
https://www.virustotal.com/gui/url/3d43fe6f2a5b96525b5b76817...