Permsteading.com

So I finally got around to running a second experiment. This time I opted for a 6" system with various sorts of linings.

First, the heat riser. I cut another batt of the Roxul into 9 pieces, each 23" long, each with a trapezoidal cross section and a short face of 3". Easy with an appropriate cutting guide. After cutting three pieces in half, I had the parts for a hexagonal riser close to 3' long. My wife mixed some powdered clay, water and furnace cement, achieving a "light buttercream consistancy," and spread it on two sides of each riser segment. It was a piece of cake, for a baker. I may eventually go into more detail, but this is something that, as I expected, did not work out well in use, so the details aren't worth much attention.

We assembled the iced segments around a 6" diameter hardware cloth core inside a 13" diameter concrete reinforcing wire frame. The pieces had fit okay in a dry fit, but with the icing on the core was too big for the pieces. We ended up spreading the problem around, leading to irregular gaps between the segments, then backfilling with various things. What the heck, its something expected to fail anyway. Here it is, set over the woodstove and drying for a few days.

I will have to get back to this later. Off to the airport...

I will skip most of the boring details about the burn chamber build. Basically it's a 6" square channel of Roxul, this time with some crw framing and a liner made of Fiberock. This material is a backer-underlayment made of gypsum and cellulose fibers. The fibers burn out at fireplace temperatures, and so the material becomes like weak chalk, but the gypsum doesn't actually decompose up to about 2400˚F, so what the heck, let's see what happens.



The base is another piece of Roxul, with the wire frame resting on a couple pieces of scrap 2x4. You will see the connection between the burn chamber and the heat riser later.

Really cool experiment Nutcase. Looks like fun to me.

It took a while to get the system going. I think the chimney was very leaky, plus it was partially obstructed at the top by the hardware cloth core frame, especially before the paper layer burnt off. I eventually got to a peak temp of 480˚F on the barrel top, with no smoking, but it wasn't so easy. Substantially more wood went in than you see in the previous picture. Along the way, I noticed that the wood framing the base had begun smoldering. My wife wasn't willing to just let it go to see if they would actually catch fire.

Anyway, the fire burnt down and I took the barrel off, unfortunately mauling the chimney in the process.



Yes, I know that using galvanized hardware cloth is problematic. I won't do it again, I promise.

I will do a more thorough postmortem on the heat riser at a later date. What is already clear is that the clay layer was fragile, and that the chimney per se didn't get hot enough to melt.

In this photo you can see the connector between the burn chamber and the heat riser, a 6" coffee can. The fit was gappy to begin with. The color changes give some idea about the temperatures. The riser was basically held up by those four free ends of the crw you see sticking up in the picture.



Hmmm. Why is the left sidewall flopped over like that?

Here's a view with most of the top off. The Fiberock has sagged and cracked to varying extents. The top was cracked into two pieces, but still basically in place. Can't really see it in this picture, but the bottom has broken on the right side where combustion air was getting channeled. This was venting combustion air under the bottom, which is much of why the 2x4s started to smolder.



There are things to be learned just from contemplating the pattern of color changes in the Roxul sidewalls.

Here's a final look at the base. The base does seem to have sagged a bit. It's possible that my wife's precautionary soaking of the smoldering 2x4s may have been a factor. Note also that the insulation thickness was at a minimum (1") on the sides of the base where the 2x4s came under the wire frame. However, my impression is that when the system comes up to temperature, radiation dominates convection for heat flow, so all sides come to a similar temperature.

I wonder if that side of the liner was laying in the burn tunnel while it was burning? That would explain the low temps.

I don't know what part of the world you are in Nutcase, but that soil looks nice and moist. You could pack mud around a similar backer board type mold and have yourself a little testbed core that would hold up to many experiments with heat risers and other configurations. Just a thought, I've had a lot of fun messing around in the yard with quick'n'dirty mud stoves. Plus, nothing to catch on fire and worry the wife. Not to say this isn't really cool. I'm loving seeing the burn pattern as it telegraphs through the insulation batts. Really good info to be gleaned from that, I think.