>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...

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.

There are two properties which are interesting: (1) the heat capacity and (2) the heat conductivity.

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.

They have hydronic registers that have very dense copper/aluminum fins and are powered by fans. An extremely small unit can heat a huge area; they are by far the most space efficient system that exists. They are also very fuel efficient.

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.

Couldn't you combine both systems relatively easily to get the best of both worlds?

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.

Re: latency, a story... The landlords for a house I lived in once had installed some German European-style electric rads which contained a pile of ceramic bricks. They were far more efficient than regular electric baseboards for obvious reasons. The problem was they just weren't up to the job of handling a Canadian winter. Not because they couldn't keep the house warm enough but because we are prone to extreme temperature swings. From +16C to -20C in 24H is entirely conceivable and happened one winter while we were there. We froze for hours while waiting for the bricks to heat.

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.

Yes, even a metal stove can be efficient…it does not have to be made of stone. The efficiency increase of a proper designed wood stove over a fireplace is due to two factors

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.

> The same is true of radiant floor heating in a slab. This can make your home difficult to adapt to temperature swings.

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.

> 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.

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.

I live in a very rural area where wood burning for heat makes a lot of sense. We have vastly too much deadwood on the ground and it is a serious fire hazard. There is simply no other economic way to dispose of it than to burn it. Most of it gets burned in vast piles by the county and forest services. From my lot, I divert a few cords per year of that to my woodstove. Either way it is going up in smoke. This way, for an acceptable amount of labor, I heat my house and save about $1,500 a year in propane charges.

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'm from Northern Ontario, and I agree that wood smoke can be killer, especially below -30°C (-22°F) when the smoke stays near the ground. Oven stoves, according to the article, address this. The section "Complete combustion", is all about this issue:

> 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.

Modern heating is not nearly as cheap as wood heat, at least around me. The problem is not wood burning as much as it is burning unseasoned wood. There should be strict regulations for firewood sellers ensuring wood is seasoned properly but what happens is every year some guy with a truck cuts a bunch of wood and sells it with very little seasoning. This creates the smoke you are experiencing. I can heat my home for about 2 months for $300 where any other heat would cost significantly more. Not just heat my house but heat it to the point I can walk around in my underwear. I've never been able to afford to do that with any other type of heat. Furthermore I have 2 hot water tanks. The first one is connected to the second and city water enters the first tank and the first tank is not powered. It does run to my fireplace through copper pipe and a copper coil at the stove. So for the price I pay for wood I get excess heat and hot water. No other heat can come close.

Purely out of curiosity (I've never permanently lived somewhere where woodburning stoves were essential for home heating): is this because people are over-burning wood to compensate for the slow warming-up time, or because the stoves themselves are inefficient, or something else?

Manually operated wood furnaces are also terrible with regards to particle emissions. The article acknowledges that and claims that oven stoves do not have this problem because they can reach higher temperatures. But that ignores the fact that there is a time period until the fire has reached the right temperature during which the particle emissions are very high (particle emissions from wood furnaces are scary high and dwarf emissions from cars). If the whole neighborhood is heated by these things, then you have several hours of the day with smoke and particles, even if everyone operates their ovens correctly so they reach the correct temperatures for a clean burn.

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.

I don't know if this is right. We're talking about thermodynamic heaters, like Peltier elements, correct? Those need electricity to work and the question then becomes: where does the electricity come from? Burning wood, if it can be done efficiently, and like the article says, is almost neutral in terms of carbon emissions because you're only realeasing carbon that is part of the atmospheric carbon cycle, circulating between the atmosphere and the biosphere.

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.

To go to the opposite direction from the low tech starting point: You could have both a micro-chp setup to genetate both heat and electricity from the fuel, leaving the option to use the electricity on a heat pump or other household demands.

And heat pumps can be fit into existing systems.

This is less of a concern with modern batch-rocket stoves that also burn fine particles, once hot, after a few minutes, they burn without smoke.

This sounds awesome.

Well, what does "energetic output" mean? If I understand the core point of the article correctly, radiant heat only heats the surface of objects not the air, so it will require less heat for the same effect. I don't how to put a number on this.

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.

this article lost me when I saw those numbers. It also treats heat like some kind of magic and not based on thermodynamics. heat is heat, no matter how quickly or slowly it spreads. A big thermal mass will hold heat, but will also take longer to heat in the first place. You're spending the same number of BTUs in the long run.

That seems awesome!