We live in northern Michigan in a passive solar home. We put 17-gallon steel drums filled with water, painted flat black, in the house for thermal mass to store excess heat and even out temperature swings.
This worked well enough that I stacked two more water barrels on each side of my free-standing woodstove. They store excess heat energy when I burn a fire and then release heat into the house long after the fire goes out. This summer I’m planning to replace my masonry hearth with a water-barrel hearth, too.