Oven Stoves and Heat Walls (2008)
lowtechmagazine.com
lowtechmagazine.com
I think these are neat and the idea of running a stove for only a short time each day would be great, but the architectural drawbacks should be obvious. The heart of your house is now lost to a massive stone structure.
The drawback of taking a long time to heat up shouldn’t be overlooked either. Anyone who has lived in a stone or brick house could tell you that it will stay warm once it’s warm but bringing it up to temperature can take a long while. The same is true of radiant floor heating in a slab. This can make your home difficult to adapt to temperature swings.
But have you ever lived in a home with a tile stove like described in the article? The time it takes to heat up isn't really an issue in my experience. Yes, it takes a bit longer than with radiators but unless you're out on holiday your house wouldn't be completely cold, would it? You get up, it's still warm from the previous night, then you throw some fresh wood in the oven and start a new fire. Especially with well insulated homes as they're built these days it works well. But here's the actual problem, apart from taking up space like you already mentioned: The fire has to be maintained. I'm pretty sure that's the main reason they've fallen out of use. It just doesn't fit into our fast paced lifestyle with small families and busy parents anymore. Back in the day there generally used to be someone home, either the housewife or the greatparents or one of the 15 children or so. Wood fired stoves just got automated out of existence like most other stuff. Central heating can be set to a certain temperature and be forgotten about. Sure, you can do that now by getting a modern gas burning tile stove, or retrofitting an old one with modern tech. But by now most have already fallen in disuse. I find it unlikely they will be revived outside of niche cases for the simple reason that fossil fuel is on it's way out. Floor heating is great and can be electric, powered by green energy.
TBF masonry heaters with modern enclosed fireplaces can slow-burn at high efficiency, and even if the fire goes out because the fireplace remains very warm and the flow is active restarting it is generally easy.
Per the few friends who have masonry heaters, if you throw a log on before going to bed, with ventilation set low, odds are high you'll still have embers in the morning and will just need to throw a new log on and stoke a bit to get it going again.
So while the convenience is a bit lower than with central heating it's hardly a back breaker. The cost of installation and structural requirements (masonry heaters weigh literal tonnes), as well as awareness of the option, are more likely.
The latter factor should not be discounted, most people don't really go out their way to look for options (if they're even doing / ordering the building in the first place, which for masonry heaters is a requirement: retrofitting a masonry heater in an existing building is very risky), so they'll get one of the options they're provided with.
Yes, water has a high heat capacity, but a metal + circulating water system also has high heat conductivity[1] which means it releases the heat into the air quickly and doesn't hold heat for long. Stone releases heat more slowly since it doesn't conduct heat so well. Stone has heat capacity about 1/2 of water per unit of mass, but on the other hand the mass of stone in a oven is usually pretty high. I'd side with the author about stone being a better material for holding heat.
[1] For heat holding can probably conflate radiating with general heat conductivity here since the low conductivity results in low radiating heating due to the surface temperature of the heating element being farther from the hot core and closer to the surrounding air to be heated.
So I agree with you that stone is a better material for holding heat, by how much--I don't know.
Having a stone fireplace might make sence heating one room, but I still have my doubts on energy saved, unless you have some kind of adobe house built around a huge stone fireplace.
It just seems more practical to have hydronic heating in your structure than a stove in each room. I have tried to heat a home with a Wolf Stove in a fireplace. A Wolf stove use just a metal stove with a fan in it. It's a fireplace insert. It only heats up the room it's in. It does heat up the bricks on the fireplace, but not enough to keep heat in when the fire is out.
If anyone decides to put in a hydronic heating system, you will like it. It's good for allergies too. I put one in a bay area home, and the install is pretty simple. The books make it much more complicated than needed.
Basically put down pex piping, or registers (radiators, baseboard) in each room. Soneyinrd you need both. Buy a boiler that has been around awhile. Condensing boilers are new, but efficient. Buy your primary pump, and your sector pumps. Know the difference between primary, and secondary piping. You will need an expansion tank. I used a mixture of copper, and pex. And a thermostat.
(In the Bay Area, we use natural gas. An electric heating here is very costly. Stay away from electric. Oh yea, for some reason hydronic heating was stopped in the 50's here. Contractors went to forced air, even though most homes don't have air conditioning. This means many hvac techs do not know how to work on hydronic, but act like they do. So double check their estimates.)
Hydronic heating is usually the best. The problem with floor systems is poor adjustability, usually noticed on cool down times, but the initial heating can also be very slow. With registers you stop flowing the water and turn off the fan. Or do the reverse and make the air hot immediately.
Something like this: Imagine a standard central heating system: Radiators connected to a boiler through a water circuit.
Except now, the boiler's exhaust is additionally led through an oven-stove-like stone structure that acts as a "heat storage". The water piping goes through both the boiler and the stone structure.
You retain the flexibility and speed of central heating: If you want to heat up a room quickly, the boiler can heat up the water directly; if you want to cool a room down, disconnect the radiator. However, at the same time, the stone structure will store heat long-term and will hopefully allow the boiler to run for shorter periods of time.
I gotta admit, it's less exciting than an actual oven stove though, as it reduces the principle from "radically different ways of heating" to "minor technical improvement of boiler systems".
There might also be the issue that heat transfer from the stone structure happens indirectly again, which might make the system less efficient than an actual oven-stove.
The conclusion I came to is that heating systems like this with huge buffering capacity make sense for western Europe where weather systems are relatively stable. But continental climates swing too quickly.
What are the reasons? They are not obvious to me.
1. Metal wood stoves slow the combustion rate down by blocking incoming air. So they can burn logs over several hours rather than all at once. Obviously the stove must be airtight.
Russian wood stoves just use a mass of masonry and have lots of internal baffles to extract the most heat. They burn a large fire at a high temperature to have increased efficiency and extract the most heat into the stone.
2. Both types of stove use high temperatures and preheated secondary air to achieve complete combustion. Open fireplaces let 50% of the energy in the wood escape as unburned gases.
Yeah these are called Masonry Heaters [1] in the West. They work by forcing the heat generated through combustion to dump as much of its energy as possible into parts of the heater that live inside the house rather than venting hot air out of a chimney. The goal of an efficient masonry heater is to make sure the exhaust gas is cold so that all the heat energy has been extracted.
> Both types of stove use high temperatures and preheated secondary air to achieve complete combustion. Open fireplaces let 50% of the energy in the wood escape as unburned gases.
The alternative here is to use a catalytic converter to start a secondary reaction and burn exhaust gasses themselves releasing even more heat and resulting in a cleaner burn and fewer harmful exhaust gasses. Modern American wood stoves can come with EPA certifications [2] indicating the use of the catalytic converter or some other technique (say, a secondary burn chamber) to reduce exhaust gasses.
[1]: https://en.wikipedia.org/wiki/Masonry_heater [2]: https://www.epa.gov/burnwise/epa-certified-wood-stoves
In this case the Russians just used a heck of a lot of bricks. It’s a solution suitable for Siberia…where You don’t want to trust the fate of your winter nights to something with a catalytic converter!
Or worse yet, a wood stove with firmware! ( Unless it’s damn simple. )
The problem with masonry heaters is that they require the house to be built alongside the masonry heater (e.g. you need to run the thermal mass inside the house so it can convect heat into the house). The thermal mass required can also be very expensive and/or labor intensive to make. Moreover cleaning a masonry heater and maintaining it can be a large pain depending on the size of the thermal mass.
Catalytic converters have been in widespread use for automobiles since the '70s (that's ~50 years) so they're pretty proven technology now. Secondary burn chambers are even older (early 1900s I believe.)
The hi-tech solution, is to have a computer control your heating, and let it take account of inside/outside temperature, presence/absence of people (per building and per room), predictions of weather/energy-cost/carbon-intensity/arrival times and so on.
Smart thermostats already do most of this e.g. learning how long it takes to get to a specific temperature with your heater, your insulation, your weather, your schedule, time-based tariffs. I'm not sure if they record this in any human readable exportable format though.
Too true. I call it "Thermal inertia".
The problem with heating up a wall and hoping it releases the heat back into your home is that the wall tends to radiate heat on both sides (internal and external), while you are only heating it from one side (internal). In effect, you are literally wasting half the heat you attempted to store into the wall.
With the type of houses I've lived in (concrete/brick + plaster) the external cladding (plaster) does nothing to insulate the bricks/blocks from the outside. If you want to retain more of the heat you "stored" in the walls (to be released internally) you need to put on a thermally insulated cladding.
This is less healthful for me than propane in terms of indoor air pollution. But it is more healthful in terms of exercise and fire prevention. I've accepted that compromise with the help of an air purifier that stays on all winter. Also, when I can get all of my fuel within a few hundred yards of my front door, most of the extraction and transportation costs go away.
And there's something else that's very important to me: When I burn propane, I micromanage the thermostat to save money. I wear heavier clothes all winter and end up tolerating being more cold and uncomfortable. With wood fuel being so much more economical, and surfing the edge of comfort less practical, I just keep the woodstove going all day and cool the house at mid day by opening some windows ... getting fresh air that I just don't get when using propane. So the house is mostly warmer all day than with propane, and I'm wearing less and generally more comfortable.
It's a pretty easy choice for me. I'd like to keep it up as long as I'm physically able.
I live in a semi rural heavily forested area where wood heat is an option. I definitely notice the drop in air quality when winter comes around. Part of the problem is that for whatever reason wood smoke from stoves tends to hang out at ground level and cover the area rather than rising away. I don’t get any benefit from wood heat, but I do suffer the costs…
The fire gets going very hot, burns off all of the stuff that comes out of wood, on purpose, to turn into heat, which heats the masonry, which will stay warm for a very long time. The dome also radiates heat for a very long time. The final outlet, out of the house, expels tepid, wet, clean air; contains only CO2 as a byproduct, iirc.
> Wood can be burned without too much air pollution, but then the temperature has to be high enough: 1100 to 1200 degrees Celsius. In that case, 99 percent of the wood is converted to CO2 and water vapour, almost without smoke. A metal wood stove, however, only reaches a temperature of 650 to 700 degrees, with an incomplete wood combustion as a result.
CO2 is a critical problem.
Though obviously this isn't the case if the wood comes from badly-managed forests that are not replanted. And there are other effects that can make wood fuel carbon-positive:
https://www.smithsonianmag.com/smart-news/epa-declares-burni...
Contrasted with burning coal, oil, or gas, preferred.
(Woodfuel may still be burned unsustainably, that's a separate question.)
Noted FWIW in TFA:
[U]nlike gas and oil, wood is a renewable and CO2-neutral fuel (the CO2 that is produced by the burning of wood was taken out of the atmosphere by the tree during the years before). The problem is that wood stoves are not very efficient, and extremely polluting.
Our heating costs dropped so significantly that the Gas company called, concerned that we weren't running our Natural Gas fired furnace and that we might need financial assistance.
We heated this way for many years after he died. I took up the responsibility of cutting down trees, splitting wood; and keeping the house warm. It is A LOT of work, but worth it if you have access to the trees/woodlots.
In the future (if this housing crisis ever ends), I will be adding a Rocket Mass Heater to my home. There is nothing in the world that compares to the feel of a roaring fire on a cold winter. One year the power went out in a very large geographic region that affected a large portion of the population. No electricity meant that no furnaces were running; so we had some elderly neighbors and family move in with us. They could have easily died without our wood-stove running. We even used it to cook on!
I'm guessing it's outside of the economic envelope in which people are burning wood anyways, but the article makes it sound like combusting at the "right" temperature essentially solves that problem. But I suppose at that point you might as well just burn something cleaner and retain the other advantageous parts of the design.
Of course, since each rmh is custom built, you need to actually test the exhaust of every one to ensure it works as intended.
Mine is pretty swell, albeit a tad annoying to keep going if we leave for a few days and the whole mass cools. With that said, it uses about 1/10th the wood of a new high efficiency wood stove we use in a different building.
RMHs are very interesting to me, but unfortunately due to their niche status, they're currently very hard to get insured.
(PS. I would love to see a picture of your RMH, is possible!)
Reading that article, I am curious as to why they say that rmh are less efficient than tile stoves. The exhaust temp on mine is around 130-150 degrees F- cooler than a fresh cup of coffee. And yet, I get pretty complete combustion- so all of the heat from the wood is captured inside my house.
I recently saw a claim that rmh can produce less CO2 for the same heat than gas furnaces, though that was just someone on the internet, I would definitely want more research to claim that.
There's no coal consumption to speak of. There's diesel fuel consumption, though that is largely concentrated on transport corridors (I-80, I-580, US-101).
Wildfire emissions all but certainly swamp woodstove usage.
That said, yes, a small number of poorly-tuned fireplaces can create a large amount of smoke. Where the intent is actual useful heating, and not fireplace-as-decoration, thats' a solvable problem.
Look on page 7
39% Wood Smoke 12 Cooking 12 Other Mobile Sources 12 Road Dust 7 On-Road Motor Vehicles 6 Combustion: Stationary Sources 5 Industrial/Commercial Processes 3 Construction and Farming Dust 2 Animal Waste 1 Wind-Blown Dust 1 Wildfires
Wildfires are basically non-existent in the winter, although the Colorado fire currently doesn't bode well for future California winters.
My understanding is that here in NYC the single largest (non-industrial?) source of air pollution is heating oil. The city banned the two worst heating oils back in 2015, but with a long phase-out period[1]. I would have expected the situation to be the same in most other cities, although SF's weather certainly doesn't justify oil burning the way NYC's does :-)
[1]: https://www.ny-engineers.com/blog/how-nyc-is-phasing-out-hea...
Furthermore, heat pumps are just so much more efficient, as they actually move several times the amount of energy as heat as they consume as electricity. Combined with floor heating - which gives a much larger surface area for heat dissipation than these oven stoves can ever give you - you can further lower the temperature of the water, raising efficiency even more. Floor heating is also way more comfortable as it gives an even distribution of heat throughout the house, and you don't get cold feet.
Quantitatively, of course planet Earth has infinite warmth to give compared to what we could possibly extract but I have no idea what the numbers are actually like.
You are right that extracting too much heat from the ground will make the ground around the exchanger too cold, which will make the process less efficient and have negative environmental effects, which means it is illegal in many places. Smaller systems like for homes are usually very shallow and will regain enough heat from the air and sun over the course of the year. Larger systems can be problematic. The common solution to this is to use the ground as a thermal battery (or install a thermal battery in the ground, using it for insulation). Then you can use your heat pump to extract heat in the cold season and in the warm season you can cool the building and move that heat back underground. This has the extra benefit of being even more efficient, since you are basically just trading heat and cold from season to season.
Where I live, the outside temperature goes from -40C to +40C at the extremes (though those are rare and the 'normal maxes' are more like -30 to +30). In F that's -40 to +104F.
But in kelvin, which actually measures the amount of energy in the air, that's 233K to 313K. It's not a small difference in the amount of energy, but neither number is anywhere close to zero.
It's a common meme with heat pumps that they "stop working under X temp" because it's too cold, but the actual, immediately practical issue is moisture. Ice is a great insulator, so when the outside part of your heat pump is covered with ice it no longer works well. Presumably the same might be true of ground source heat pumps and frost, though I know less about that. Obviously there are other factors like the efficiency and operating range of the coolant used and such, but you hit this moisture barrier before those.
There are ways around that (I installed an air source heat pump this year and it runs in reverse (ie. as an air conditioner) periodically to defrost the outside part. I've used it successfully to heat my home down to about -20 outside before it no longer spends any real time in heating mode. It wasn't cheap though, and is almost certainly not cost-effective even over a very long time period. That's not my priority though.
I looked into this, since I'm getting an air-to-water heat pump installed next month, and considered a ground heat pump. Ground heat pumps require a large area of pipes, roughly 3x the area of the house. Less than that, and I was told that I'd risk creating permafrost around the pipes, greatly reducing the efficiency of the pump.
Regarding cost, my heat pump will replace a gas heater, and should actually cut my yearly expenses to almost 1/3. If that is actually how things are going to work out (and assuming the electricity price surges in Europe are temporary) then it'll pay itself back in 8-10 years. Granted, I live in Denmark where most of the price of gas and electricity is environment taxes, and those are cut for the electricity that is spent by a heat pump, thus making it more cost effective than if those taxes weren't there.
Right now though the cost to heat my house with gas and the cost to heat it with the heat pump at the marginal temperatures is about the same. I still have an operational and relatively new gas water heater though or I'd be looking into turning off the gas for 2/3 of the year and that would save a lot of money since there's a lot of fixed costs on my bill for even having gas at all.
With deeper boreholes you're extracting heat from radioactivity decay.
In both cases the rate at which you can extract that heat depends in the heat transfer rate in the ground around the coils/boreholes.
With a heat pump, if the electricity comes from burning fossil fuel then you're contributing to greenhouse gas emissions (all those carbon gasses are released in a much higher rate than they were stored and, more importantly, than what the environment can keep up with). If the electricity comes from renewable sources, then it's a real saving, but how probable is that right now? If you can burn wood at 80-90% efficiency, as the article claims, then that's a net gain right now.
Personally I also don't like the reliance on a complicated mechanism that can't be easily made by hand. I don't like that because it means the technology is harder to adopt. I'm talking about heat pumps. By the sound of it an "oven stove" is much easier to make and maintain, if they had them back in the middle ages. So they can be used by many more people for whom the price of electricity is higher than the price of scrap wood. For a technology to make a difference it doesn't just need to be efficient, it must also be affordable and useable by everyone.
Technically electricity is not required, although most of these units, in practice, use electricity, converted to mechanical energy, to power compressors.
Heat pumps are much more efficient than e.g. electric baseboard heating because the energy consumed is used to move heat, rather than to generate heat. The efficiency difference drops in "very" cold climates (ambient temperatures well below freezing) and in those climates the heat pump usually is backed up by a direct heating method such as resistance electric heating, gas furnace, et cetera.
There are two things that make them much more efficient in terms of GHG than other kinds of heating:
- Using central electricity generation means using more efficient combustion. Where I live most heating is done by gas and it is extremely inefficient compared to even a gas power plant. This is why we use central electricity generation to begin with; everyone running their own generator would be extremely bad for the environment. Even if it was 1:1 generated energy input to heat it would still be better for the environment for me to be using that generated electricity than directly buring gas in my home. Also as you allude to but dismiss, much of the electricity cost can be offset by using solar or wind directly at site, and it's likely that eventually most central generation will switch off GHG emitters and if/when it does you will automatically go with it.
- Heat pumps with a refrigerant cycle don't use 1:1 energy input to heat produced. They're moving energy, not producing it as burning, resistive, or even thermoelectric do. So long as conditions are right, heat pumps only use a fraction of the energy they bring into your home. So the efficiency gains are compounding here.
At any rate, it's simply not feasible to convert all heating to wood burning, even if it is efficient. As the article points out the last time this was done they basically caused intense deforestation and likely there were climate effects from that. The idea that now, with several billion more people on the planet, we can do it again but get it right this time is absurd. We will never be able to plant trees on the scale necessary to do this, never mind the transportation costs (in terms of both environmental and monetary) of shipping a bunch of wood around to homes all over the place to keep them heated.
At any rate, we're likely to start needing tech like heat pumps a lot more as climate change accelerates. There are parts of the planet that are likely to become basically unliveable without AC soon, and as the heat dome last year showed even 'cold' areas are likely to start experiencing heat levels they're unable to really manage (up here in the north we build homes to hold heat in. Guess what that's like in a heat dome).
Such as the lack of a convective air circulation system.
That’s not an advantage.
So you cook in the evening so that you have hot water for bathing before going to sleep and the house is warm through the night.
It also has warm shelves near the ceiling used for drying clothes/shoes/fruits/mushrooms.
Very efficient, but it means the whole house is build around it.
> The energetic output of an oven stove is 80 to 90 percent, compared to 40 to 50 percent for metal stoves or central heating appliances
Energy.gov
> Old furnaces: 56% to 70%
> Mid efficiency furnaces: 80% to 83%
> High efficiency furnaces: 90% to 98.5%
https://www.energy.gov/energysaver/furnaces-and-boilers
EDIT: We can cut the article some slack as ducting (as typically done) does account for losses which are highly variable.
I think the energy.gov article you mentioned primarily talks about heat leaking outside:
> An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere.
[1] https://www.unforgettablefirellc.com/kimberly-wood-stove/
https://www.youtube.com/watch?v=fwCz8Ris79g
Uses about 1/10th the fuel, and fuel can be wood unusable in other stoves.
Very little exhaust / smoke.
Super efficient.
I've experienced radiant floor heating powered by a wood stove, on my trip to Europe, it was very nice. Not sure how it worked.
Ash buildup could still be a problem, but would take years to build up enough to be a problem, although it likely would still need to be cleaned out at some point, although a vacuum with a proper filter could be an option or have access points built in for that eventuality.
Plugging https://www.uzume.fr (site in French) an engineer and indie stove maker that makes neat products starting at €4000 for a stove kit that you build yourself.
The article is from 2008, I wonder what the author refers to here. Possibly using more environmentally sustainable energy sources? Or using less energy in general?