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> 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.

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...

Funny how we have a post here from some company selling these and it feels like the thought is "this is new".

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

https://en.m.wikipedia.org/wiki/Masonry_heater

>Funny how we have a post here from some company selling these and it feels like the thought is "this is new".

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:

https://en.m.wikipedia.org/wiki/Masonry_heater

You can find these all over Poland and many other Eastern European countries in older houses.

I think the whole novelty is not the masonry heater part, but the rocket stove part, which helps you burn as much carbon in your firewood as possible.

The novelty is indeed the bell combined with the rocket stove. A classic old Kachelofen restricts the airflow since the channels that the fire fades are routed through are long and often small. This construction achieves the same effect of capturing almost all of the heat from the fire, but does not restrict airflow. Better airflow leads to a better combustion.

It’s a pretty ingenious modification of a known concept. As far as I can tell, this is indeed new.

Most surprising is the fact that this definition exists at all.

This is exactly how fire stoves and fireplaces were built in Europe for centuries, there is nothing new or unknown in this technique.

Yup, most old houses in Poland had this kind of stove. Some were replaced with gas stoves, some still have them (but they are now rarely used because it's not very convenient to split the wood and carry it to your house every day - much easier to use the gas stove).

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.

> I would have thought anything that can radiate heat slowly and consistently would by necessity also absorb heat slowly and consistently

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).

TL;DR different modes of thermal transfer can transfer heat at different rates.

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.

>“at least one 180 degree change in flow direction”

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 would have thought anything that can radiate heat slowly and consistently would by necessity also absorb heat slowly and consistently

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.