Mass Sizing and Pipe placement for Greenhouse RMH

Our greenhouse rmh is coming along well; we're about to begin building the bench. We're have an 8" exhaust and are looking to build a 3' x 15' bench to start seedlings on. Using cob at a weight of 100 lbs/ cu ft we calculated we needed the bench to be just under 3 feet high to weigh in at 5 tons. Does this seem correct? Most other systems i've come across don't seem to have such a large heat storage.

The second question is regarding the placement of the pipe if the thermal storage is 3 x 15 x 3. I'm thinking to place it fairly deep in the bench, but since this is in a greenhouse and I'd like to capture as much heat as possible in the bench and not in the soil itself, I am wondering about how to set the cob and rocks underneath the pipe so that most of the heat will move up into the bench and not be drawn into the soil. Is this a legitimate concern that I should try to work with or can I throw down a layer of small rocks for drainage, then 6 - 10" of cob and rocks for the pipe to sit on and then build the rest of the bench and expect the heat to more or less rise? Should I place less rocks and more cob under the bench since the cob doesn't transfer heat as fast as the rocks?

thanks!
-Anthony

most are in houses and that means you want a bit more oomph out of the exhaust. the depth of cob above the duct needs to be at least 6 inches below the surface.

what do you mean "you want a bit more oomph out of the exhaust"? Do you mean you want to attempt to capture as much heat as possible, leading to building such a large mass and that you don't see it necessary to build such a large mass in a greenhouse?

The depth of the cob above the duct will certainly be 6 inches, probably closer to 12 inches or more. How much is too much and how even should the amounts of cob below and above the pipe be?

I think that is my basic question, how much do I need to worry about cob and rocks underneath to keep the heat from moving into the ground? Will the majority of it move up? This is a question for me because it is not the hot air/exhaust, but the hot air/exhaust heating the metal pipe (which conducts heat much better and would even out quickly) and then via conduction moving out into the cob.

Heat only goes up because of convection, this means there must be something that reduces in density when it gets hot. Air will do. When the heat source is in a solid, the heat travels by conduction, and provided the conductivity is the same throughout the mass, the heat will travel at the same speed in all directions as long as the temperature difference between the source and the outside is the same, or more correctly the temperature gradient is the same. To stop the heat in any direction e.g. down, you will need an insulator. The material will depend on what temperature you are expecting. Low temps could be done with polyurethane foam, higher temps with pearlite/clay mix.

EDIT: There is a little problem with the theory that heat travels at the same rate in all directions in a solid. Because of convection, the temperature at the top of the exhaust tube will be higher than it is at the bottom, so that would cause the heat to travel up faster than it goes down.