Glenn Herbert

If the chimney top is only 2' above the roof at its exit, I think you will benefit from raising it. Common US building code is for the top of a chimney to be at least 2' higher than any roof within 10', and 3' higher than the roof at its exit point.

A 7" batch box might be okay with a 6" chimney as long as it is cleanly routed. I don't think 7" insulated chimney pipe is easy to find. It is certainly uncommon and you will most likely not find any on Craigslist. I would test an extension with some cheap 6" ducting to see if it makes enough of a difference before spending a lot of money. If the system is fully dried and still has draft issues, then going to a larger chimney is warranted.

It looks like a Kuma stove, if it is fairly recent at least, will be much more efficient than an old box stove, which is Paul's baseline for 90% reduction in wood use. So half or a quarter of your previous experience would be a decent result.

What is the batch box system size? It looks like the (8"?) exhaust pipe may be connected to a standard 6" insulated chimney. For a J-tube system, that can work fine when you have otherwise good draft, but according to Peter a batch box is not so tolerant.

Adding a length of insulated chimney above the roofline, aside from avoiding downdrafts from air currents, may also improve the system draft with a taller overall chimney.

Bricks or cobbles which allow free airflow through the bed could indeed be a viable thermal battery. I was contemplating a cobble bed for heat storage when I started planning my own house out of architecture school in the 1980s, as there was information available even then on designs for that.


Either a bench or a taller bell could be arranged as a cobble bed for direct thermal extraction and storage. A bell is not inherently at a higher temperature than the channel in a ducted bench, though that may often be the case. The pebble bed as used at Wheaton Labs is an indirect medium, as the heat radiates, convects and conducts from the ductwork and then is absorbed by the pebbles.

While steam can certainly power many things, Nancy was talking about a running stream of water...

The "sofa-like structure" is referred to as a bench, and is a primary part of the heat storage and distribution. It could be possible to get the surface of a mass bench too hot for comfort in spots, which is why there are guidelines for how much cob thickness there should be over the ducts - more near the combustion core and less at the end of the duct run.

You would not want to use sand in this, because sand has millions of tiny air spaces and slows down heat absorption and transmission. Think of a beach on a sunny afternoon - the top is burning hot, but dig down a few inches and it is cool. Cob, stone, brick, anything noncombustible and dense will work for the thermal mass.

The J-tube combustion core with ducts running through a mass to capture heat is the original RMH. There are a few important alternatives developed since, mainly the batch box which allows faster burning and more concentrated heat delivery to the mass, and the "bell" or stratification chamber, essentially a hollow masonry box which absorbs heat efficiently with very little friction allowing easier draft. Many other details are possible if you want to get into the fine points.

I doubt that it would be worthwhile to try to extract electrical energy from the RMH mass. I expect there are more efficient and effective ways to charge batteries.

I don't know about South Carolina clays, but I know there are some Georgia clays that can be used for pottery straight out of the ground (a rare situation). If your clay is similarly pure, you would probably want at least 2 or 3 times as much sand as clay to make good cob. Having a truckload of sand delivered would be sensible if you intend to do large-scale work.

A good kiln can be made straight from cob, no need to make individual bricks and build a shape. I know there are Southern potters who use "groundhog kilns" tunneled into the earth which can fire pottery as high as cone 12 (higher than even much porcelain). My local clay (which is somewhere around 10-20% clay, 10-20% silt, and lots of sand and gravel) is perfect for cob, and can be used as is for earthenware kilns. The clay vitrifies around cone 6 (electric stoneware temps) and melts at cone 10 (traditional stoneware temps).

I have a peephole in the side of my bell at the level of the riser top, and I see the flames mostly ending a bit below the top of the riser, occasionally reaching above the top by a few inches. I have never observed reignition above the top of the riser.

Fox James has made glass-topped systems, and could report what he has seen.

This isn't the original video I saw, but it covers the same info on the same house.

I think your large thin tiles would work excellently for the oven floor, as long as you add a diffuser below to spread out the direct flame from the heat riser. Something that makes the flames go in all directions under the floor, and able to circulate back under the middle between diffuser and floor, would most likely be ideal. Also, it would be better for the tiles to not have a single hot spot in the middle but be evenly exposed to heat.

My exhaust circulates around the oven space before going down a bit to enter the front chamber which opens to the chimney. The doorway between the chambers is sized according to traditional wood-fired oven door proportions and heights.