[walterbell]

Previously posted in a sub-thread, https://diy.stackexchange.com/questions/133503/installable-s...

  Feolite 

    specific heat = 920.0 J·kg−1−1·°C−1,
    density = 3,900 kg·m−3,
    thermal conductivity = 2.1 W·m−1·°C−1.
    maximum operating temperature 1000 °C.

  Water 

    specific heat = 4184 J·kg−1·°C−1
    density 1,000 kg·m−3,
    thermal conductivity = 0.591 W·m−1·°C−1.
    maximum operating temperature <100 °C unpressurised.

[zasdffaa]

Indeed, thanks. FTR "Feolite is a type of iron oxide sintered into building blocks, which are then used for heat storage" which is exactly the stuff.

That's an astonishing difference in capacity.