What is a rocket mass heater?

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 link or ▶ this video and link 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.

Quick Video about the rocket mass heater. I'm guessing the cost to heat with wood, would be even lower if your house was built around this. ▶ this video

The rocket mass heater is the modern replacement for this. ▶ this video Uses about 1/10th the fuel, and fuel can be wood unusable in other stoves. Very little exhaust / smoke. Super efficient.

You can do a hybrid approach with the hot metal burn chamber for wood gas and earthen materials to slowly radiate the heat. This is called a rocket mass heater. The exhaust temperature is usually below 100C and often smokeless (a complete burn). Using a fan to extract heat from the final vertical stack (pipe around the exhaust with air running by) can give you an exhaust temperature of around 40C. Here's a quick 3 minute video that explains the concept; there's other YouTube videos that go into greater details if you wish to research this topic more. ▶ this video

Best fireplace in terms of efficiency is a rocket mass heater. It burns the wood gas in a secondary burn chamber and then routes the exhaust through a lot of thermal mass. One 2 hour fire a day will usually heat 2000 sq feet of house. 3m video explains the concept; longer videos will better explain it ▶ this video

I wonder what the efficiency is compared to a modern "rocket stove" build [1] Mass rocket stoves are _incredibly_ efficient. So much so that I've always wondered why more similar designs weren't used in times when the primary heat sources were wood, etc. 1 - ▶ this video (skip to 1:50 if you want to see a full one and not the ms paint diagrams)