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Q: I was reading your article about the properties of different materials used in wood stoves, and it occured to me: why not take the soapstone idea to it's logical conclusion: small, intense, short duration fires and store the heat in a truly massive pile of masonry to radiate into a dwelling?
So I did a little research, and it seems Europeans have been using this basic idea for ages.
These huge European heaters might not be so good for quickly heating a room, but they would tend to avoid creosote build up in the chimney, or at least I'd think so. No one would try to smolder a fire, there'd be no point.
With the right design one of these babies should still be radiating heat in the morning or evening from a evening or morning fire respectively.
Would you need the re-burn feature of modern stoves? Or if you put enough air/draft through the thing to really zing the fire along could you avoid that complexity?
One owner/builder of these heaters claims you can put your bare hand over the exhaust and not get hurt. I'd not want to try that with 325+ deg. wood stove exhaust.
I realize this is not something that can easily be retrofitted into a home like a wood stove, but is there any reason to think that it would not be more efficient?
Your idea has indeed been around for centuries. The concept is to heat up a large thermal mass with a hot, short-duration fire, then warm the house for hours, as the stored heat slowly radiates from the mass. An elaborately channeled flue system is often incorporated to extract the maximum heat from the exhaust (hence the cool flue temperatures you heard about). The Germans call these beasties "Kakelofens": they are more commonly known in the US as "Russian Fireplaces" or "Masonry Heaters".
You are correct, these are not easy to retrofit: to operate efficiently, no part of the masonry heater can be exposed to the outdoors until it exits the roof, so the house is basically built around them.
Since the fire that heats the thermal mass in a masonry heater is uncontrolled, an approximate 35-1 air/fuel ratio is maintained in the firebox throughout the short-duration burn: this makes for a relatively clean burning fire, so masonry heaters are exempt from EPA emissions regulations.
Perhaps the ultimate example of a masonry heater is the Tulikivi, or "Finnish Fireplace," which can be ordered in kit form, and, like our Hearthstone soapstone wood stoves, uses nature's best heat storage material, soapstone.
The drawback to masonry heaters is lack of controllability: if the day happens to warm up after you've had your morning fire, you might be spending the afternoon out in the yard! Thus, masonry heaters are best implemented in climate zones where it gets cold and stays cold all Winter.
As to efficiency claims, an important thing to remember is, there's no free lunch. X amount of fuelwood contains X amount of heat value. Even if your masonry heater is properly designed and built, it will burn about the same amount of wood you'd burn in a controlled combustion wood stove to heat the same house.
The Chimney Sweep, Inc.