The air is drawn from the cellar into the rooms when a thermostat triggers it. The fan needs to be powered. The heat storage lasts for many weeks, and even in winter Finland there is is enough sunlight to keep the heat storage topped up. The experimental house that an architect friend built used the same system, though he said the cellar volume was not quite enough for the room volume. But it still managed to keep a base temperature of 10 - 14 C. The top heat came from a miserly Norwegian wood stove during the day and evening - but in the mornings the bedrooms would be down to the base temperature.

That makes you shiver like my childhood. A warm breakfast fire downstairs was a definite incentive to get out of bed ;-)

You can't be me, I'm taken