Rocket mass heater

I just built a rocket mass heater in my shop. Im trying to get ideas for a dry fill for the mass. Does anyone know if crushed stone will be good, it packs very well. They use it for driveways up here? I want to build a box out of cement board around the exhaust and fill it?

Responses to this thread may be slow because there is a very similar thread already which discusses Paul's portable RMH, using pebbles. So try to find that, and read that first, it may answer your questions. With regard to concrete board, it tends to be brittle. Maybe use it as a lining, but I suspect it will need to be supported by 2-bys. Rocks, gravel, pebbles, sand, etc, all exert a lot of sideways pressure/weight.

Lots of points of contact between gravel pieces = good, but tight airspaces between pieces = bad. Gravel can work, though only half as efficiently as cob. Sand does not work well, but acts as an insulator due to the millions of tiny airspaces that are effectively immobile. See Paul's thread for full details.

So with stone dust being easy to pack would that be better than sand?

Someone who has actually used stone dust will have to answer that one. If it has really good contact between all the particles with few airspaces, it might work. Do you have access to rocks? Using those for most of the mass with just enough stone dust to make good contact all around might work best. That is assuming you have some reason for not wanting to make a solid fill like cob. Cob and rocks will make the best mass.

Thanks Glen. I don't want a cob fill as I may have to modify this set up and there is 3 feet of snow and the frost level where I live is about 4 feet deep. So, I would have to buy the clay to make the cob and I would have to fill buckets of water in the house and truck them down to my shop. If I can find a dry fill that can prove this set up will perform to my needs I can refill it with cob in the summer. My thinking is that if stone is the best material then stone dust that packs hard would have little air so it should work almost as good?

A time constraint that you don't want to wait for clay to be accessible is reasonable. In that case, I say to try the stone dust, and include some rocks if you can. Ideally you would put a few different kinds or mixes of fill in different sections of a bench mass (with the duct doubling back so each section gets a balanced average amount of possible heat), and see how the different sections perform. This would be valuable information to share with the whole community.

Alan, i would say, forget the stone dust.

Sand is prety much that, stone dust, only bigger. You will always have the air pockets, would it be sand or stone dust, and we know sand doesn't work.

So, i've been saying this since i got that sand was no good:

Make the airspace between the rocks big enough, so convection can hapen.

I'm always saying, from fist sized to head sized rocks. There, convection can happen.

Even better, make a bell instead of a flue, with rocks inside, so you have the mass in direct contact with the hot gases.

If stone dust can pack so tightly that it can approach solid, it might be useable. But I defer to actual experience with it in this application. Yes, if you can get large gravel or rocks that will allow airflow between them, that would probably be best for a temporary build. Without an airtight container, rocks inside a bell won't work, though. I think the duct is the easiest solution for this case.

Glen, daft question, why rocks inside a "bell" won't work?

I think I will make a couple boxes out of plywood and fill one with packed stone dust and one with stone dust and rocks then keep checking them with a I.R. thermometer. Thanks for the reply's

One more question. I was just reading the Rocket mass heater book, and it say's you can use un screened clay soil for the bench with less sand. Am I correct in assuming the sand is strictly for preventing cracking? If so how would dry clay soil packed in a box around the exhaust work? Doesn't the water in the clay just make it pack harder or does it cause a catalytic reaction like in cement?

Max, rocks inside a bell should work fine, but you need an airtight masonry or metal bell for that, which is not what the OP has in mind to build.

Alan, the sand in cob is to reduce shrinkage and thus cracking, as well as add mass (stone being more dense than clay). Packing dry clay would leave lots of microscopic airspaces which would be insulating. Wetting it just enough to pack tightly will increase particle-to-particle contact, but there is not a setting reaction. The wetting just allows all the particles to deform so they pack as tightly as possible. Working or kneading the clayish soil after wetting will get the particles in the best possible contact.