Kacheloffen -ever heard of it?

[D L Bahler]

I'm sure we are all familiar with the basic concept of a thermal mass heater -using a wood fire to heat up a masonry firebox and then slowly release that heat over a long period of time.

Have yall ever heard of the Kacheloffen? It's a type of German wood heater using this principle -only they've had about 1000 years to work out the details. These things are incredible, and really bring the idea of efficiency to new heights. Some say a good oven can heat a house on just 2 armfulls of wood a day -one first thing in the morning, and one before you go to bed.

The brilliance of this system is that the masonry firebox lets you burn the wood at extremely high temperatures, burning off everything flammable so emissions consist only of H20 and CO2. But instead of letting the heat escape up the chimney, the hot air is first run through a series of twisting passages, giving more masonry to hold on to the heat.

The walls of the oven are fairly thick and filled with sand. This way the firebox can get extremely hot, but the heat will very slowly make its way out into the room -outside surfaces are usually warm to the touch, not hot.

Just radiant, the effective comfortable range of one of these things is about a 20 foot radius. But with some manner of circulation and proper design, they can easily be used to heat an entire house.

[beenthere]

Are you going in the business of producing them? What would a tiled stove like this cost to have installed?
What would they do that storing the heat in water wouldn't do?

[D L Bahler]

I've thought about producing them, and would install them for folks if they asked. But there is so much that goes into the proper design of one... They have to be sized right for your house, etc. and they are big hunks of masonry so you have to have a very stout floor to support them.

Costs would vary over a tremendous range, depending on the area you want to heat, how long you want it to hold its heat, and how fancy you want it.

The big disadvantage to storing heat in water is that the water 1: has to be pumped to a radiator, losing heat along the way, 2: loses heat to radiation in the tank and 3: hot water can be dangerous. Also water lines can and therefore will leak.
Also when you heat a circulating water system, that cools you fire down which equals inefficient fuel consumption and excess emissions.
When wood burns, it undergoes 2 chemical changes. We all know about oxidization, but before that can happen the compounds in wood have to be broken down into things that will react with oxygen. So the first thing that happens is pyrolosis , where wood breaks down into coal and a number of tars and other volatile compounds that will burn.
If you have a too cold fire, most of the compounds go up in smoke (literally) and all you burn is the coal. If you see white smoke coming out of your chimney, that's all potential fuel that you are dumping into the atmosphere. even CO is flammable, so if your putting CO out, you are not burning your fuel efficiently.
The only problem is to burn all these chemicals -tar, methane, hydrogen, and a few others- you have to have a fire that would turn steel red -not good for a steel firebox. But if it's in a big hunk of masonry, it's all good.
The theory behind these is that you can have a short, hot fire that burns ALL of your wood's fuel potential, and store that tremendous heat to be slowly released into your home.

[r.man]

Old tech but doesn't have many of the benefits of an owb. I was under the impression that a masonry stove had a fireplace like chimney so it wouldn't be possible in my house but I think that it would certainly be desirable in many new designs. 

[boilerman101]

Interesting, probalby works, but very old school technology. Evidently you aren't up on todays gasification owf. My Eclassic water jacket is insulated with sprayed on urethane insulation, no seams or gaps. Near zero heat loss. Snow sits on roof until sun melts it off. Urethane insulated PEX delivery lines lose less than 1 degree of water temp over a 100 ft run. Gasification process burns clean and extremely stingy on wood. You can see the process at the following link:    http://www.youtube.com/watch?v=rwL01iW36MA     
Dirt, mess, fire all stays outside.

[Dan_Shade]

I've heard of these designs, but I've wondered how one would clean the curvey pipes.

[D L Bahler]

Yes it is 'old school' technology.

I wonder, boilerman, what temperature are the exhaust gasses from your wood boilers?

With a well built exhaust line, you can put your face over the exhaust and feel gently warm, moist air coming out.

As for the chimney, it can use a regular stovepipe. No need for a masonry chimney. But again, it would be pretty heavy.

One advantage this has over outdoor boilers is that there is no need to go outside...

I am familiar with gassification, and how it works. The idea here is similar, only using a method to get the same results that requires no mechanical or electronic control.

As for cleaning the curvy pipes it's really quite simple, there are only 2 locations that debris will gather -just beyond the firebox and at the bottom of the flue. So just give yourself access to these 2 points and that is all you need to worry about.

Another big advantage is that you don't have to have a fire going all the time, just or 3 relatively small fires per day.
It's a good low-maintenance system.

[Dan_Shade]

how much creosote forms if the exhaust is that cool while coming out?

I am a big fan of the high efficiancy stoves.  I have a secondary air fireplace in my house.  I can keep the place warm with a small amount of wood.  Less splitting makes me a happy camper!

[boilerman101]

I still don't get your advantages of that indoor concrete unit and I don't know what the stack temp coming out of my Eclassic is. Never climbed a ladder up 15 feet and put my face over it. Fire is not always burning either. When water temp setting is met, air shuts off to firebox and it looks like the fire has gone out inside the firebox after just a short period. I start just 1 fire at the beginning of each season and load mine just one time per day at even at coldest winter temps. Yes I load it outside, but I go outside everyday anyway. My insurance company does not charge me a premium fee for burning wood indoors either and burning outdoors does not bother my wives asthma. Heat is delivered evenly and thermostatically throughout my house through existing duct work, not just radiating from one location. Heats my domestic water too. Seems like owf has a lot of advantages, but I guess whatever floats your boat.

[DeerMeadowFarm]

My experience with the German masonry heaters have all been very positive. My company is German owned and I have spent a lot of time there over the years. My favorite hotel/pub has one and the tile work coming up through the pub sections is amazing. There is a corner seat at the Stamdish(sp?) table (this is where you sit if you come in alone or with a friend, it is usually a large table where the locals will wander in to sit and have a beer or two) that I always tried to grab; the tile work formed an inside corner there so you'd have heated tile along your back and sides when you sat diagonally into the corner....nice and toasty!