Rocket mass heater with sand battery?

Does anyone have any recommendations or sources that show a large battery bank being used or is that a little over redundant? My home is going to be built into the side of a hill, I’m contemplating doing a daylight walk out basement, and since my home is in the side of a hill, why not dig the hill back, insulate a 10x10x8 area, and run the exhaust through it? Would that be over kill? It would be awesome to get the battery up to 200-300 degrees and be able to leave the house without it freezing for a week or two.

There are a few threads that discuss using sand as a battery. It probably is as effective as the pebble style, but an 800 cubic foot mass would have a time warming up to past 200 without a monster stove. My 8" batch rocket runs through a cob mass of 2x4x8 (64 cubic feet) and heats up to 115 max when burned once per day. I suppose you could burn throughout the day, but I think the heat might not be able to transmit quick enough to allow adequate storage without a huge loss of efficiency going out the chimney.

Possibly overkill on size: If warmed up to max temp, my stove will keep my 900 square feet quite warm for over a week in sub zero temps. Plus, being partially bermed, your geothermal gain might prevent freezing.

Daniel Ray wrote:There are a few threads that discuss using sand as a battery. It probably is as effective as the pebble style, but an 800 cubic foot mass would have a time warming up to past 200 without a monster stove. My 8" batch rocket runs through a cob mass of 2x4x8 (64 cubic feet) and heats up to 115 max when burned once per day. I suppose you could burn throughout the day, but I think the heat might not be able to transmit quick enough to allow adequate storage without a huge loss of efficiency going out the chimney.

Possibly overkill on size: If warmed up to max temp, my stove will keep my 900 square feet quite warm for over a week in sub zero temps. Plus, being partially bermed, your geothermal gain might prevent freezing.

Thanks Daniel, what if I hooked up a radiator jacket around the flu within the sand using glycol? This may all be super overkill, but if it’s doable and feasible, it would be really cool to heat the home passively while I’m away for extended periods of time. I live in Alaska 🥶

Jacob Zivotic wrote:My home is going to be built into the side of a hill,

Have you researched wofati eco building?



If you could incorporate some of the ideas from that you might find that you don't need much heat at all to keep it from freezing as the whole mass around the home will absorb heat during the summer and release it gradually over the winter.