A few things: Clay weighs significantly more than sand. For the same volume of material, clay will store significantly more thermal energy. The big question is

A few things: Clay weighs significantly more than sand. For the same volume of material, clay will store significantly more thermal energy. The big question is- is the hot air on a closed loop or an open loop? I don't know of too many fans comfortable cycling 600C air, which means that you need an open loop of air (fresh air > fan > heater > sand > exhaust). If the sand isn't drawing heat off of the hot air fast enough, the heat won't actually "sink" in through the sand. If we are talking a closed loop of air (fan > heater > sand > fan) then the sand will get fully heated. My reference to "residential" is the aformentioned rocket stove mass heater- see https://richsoil.com/rocket-stove-mass-heater.jsp https://richsoil.com/rocket-stove-mass-heater.jsp or https://www.youtube.com/watch?v=fwCz8Ris79g https://www.youtube.com/watch?v=fwCz8Ris79g and https://www.youtube.com/watch?v=8ptwncPImuo https://www.youtube.com/watch?v=8ptwncPImuo if you prefer videos. These types of things typically stay warm for about 24 hours, so they're burned once a day, maybe once every other day depending on how the house is insulated. Since these are all wood fired, they must be open loop, and sand is almost universally acknowledged as a terrible idea to use for thermal storage.