Just before Christmas we finished the heat storage bench (the thermal mass) of our stove. At some point in the whole process I started to wonder just how much cob was in it (beyond a lot), and how much the heater weighs.
The tractor that we use to move dirt around has a one yard bucket. This means that when the bucket is full it contains one cubic yard of material. We’ve built a screen that we put over the tractor bucket that allows us to screen material down to less than 1/2″ aggregate. We shovel material through the screen and shake the larger material off. What we are left with in the bucket is clay or sand that is ready-to-use for cob.
For the bench we have used two buckets of sand and two buckets of clay (our clay is actually dirt with about 20% clay by volume), or 4 cubic yards of material in total. I am going to assume that one cubic foot of material weighs about 75 pounds. This means that a cubic yard of material weighs 2025 lbs, and there is over 8000 lbs of material in our heater. This number does not include all of the rock we put in the bench or the bricks that form the core of the stove.
Given all of this and the fact that there is still more cob to go onto the bench, a safe estimate of the weight of our heater is over 10,000 lbs. This seems like a good start for heat storage! Our hope is that the wall that the stove is built next to will also store heat from the stove. The wall itself easily approaches 30,000 lbs of compacted dirt, and should store a lot of the heat from the stove as it is gradually warmed up.
I would add a bit more to that weight according to a web site visited sand compacted weight about 105pound per cubic foot and 95pound for dry earth! so I would add a good 25% to your weight estimate!
By how is the stove working for you so far? how log do you burn to get decent heat? How do you get the ash out, it seem like in front you have a trap for it, but it look like it is below the floor level?
anyway always a pleasure to read your progress.
I think my numbers are still good … I measured the volumes of sand and dirt loaded into the bucket of our backhoe. We screen this material as it goes into the bucket so it is definitely not compacted at this point!
To date we have not had a lot of experience with the stove. We did some day long burns while we were drying the stove out. We got most of the moisture out of the cob, but it still needs some more drying.
The bench warmed up to the point that it was impossible to sit on it for extended periods of time. We still need to add cob to the top of the bench to give us 6″ of cob between the top of the pipe and the top of the bench. The extra cob should spread the heat at the top of the bench to a comfortable level … we will see!
If the stove is completely cold it takes a bit of work to re-light it and get it drafting properly … not unlike a masonry stove. If the stove is still warm it lights up right away … again like a masonry stove. (If the stove was lit the previous day there seems to be no problem starting it in the morning).
When we had the stove lit for a couple of days we saw the temperature in the earthship slowly go up. We got it up to around 10 degrees C from an average of 3 degrees C. However, an earthship is all about thermal mass and I think it is going to take a bit more time to get all that mass a little warmer …
Your estimate for the mass is probably good. My rocket mass has 27 feet of duct and my estimate of the mass was around 5 to 6 metric tons.
For your hope that the wall next to the RMH might store some of that heat. From my own experience, over 11 inches of cob around the pipe, you don’t feel that much heat. Of course, if the heat has no where else to go, in the long run it will be warmer over 11 inches of cob. In a earthship like yours, a RMH will surely help to keep the eat in your walls.
About your problem to light up the stove. We had those same problem here in the first two months of operation. I think, it’s mostely because of the moisture. After those two month we rarely had problem, and when we had some it was because it was to warm and humid outside (over 10 degrees C).
We are noticing the same thing … it is getting easier to light.