posted 7 years ago
I would add that there is one situation where an underfloor mass could work: If the mass is the floor and not beneath a wood-framed floor.
You could either build the fire core into a recess in the floor, like a little sitting alcove, and run the mass from there, or if you have good draft in a tall chimney, you may be able to drop the flue below floor level from a standard core layout. You would need to have the floor surface be some sort of masonry - tile, brick, stone, earth... You would also need to insulate well beneath the mass so it isn't trying to heat the whole earth below the house.