Thermal storage.

Been thinking about building a huge solar heater. Could I run a water from a solar collector to a cob bench to store the heat like a rocket stove? I think for this to work, I would need a huge collector. With a thermosiphon this would be complete passive and very appealing to me.

If you're going to collect hot water, it makes sense to store hot water,  since water holds far more energy than cob does.

https://permies.com/t/9933/energy/Water-cob-thermal-storage

That is a great idea as storing more than a couple hundred gallons of water is quite difficult and thermal losses from the tank are typically not delivered to the interior of the home. I would however recommend full electrical control, i.e. pumps, etc.

The tank belongs inside the house and therefore all energy is delivered to the home. If weight is a major concern, put it in the basement. The weight of water can be 1/5 that of cob for an equivalent amount of heat storage.