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.