convection in the attic heat store
During sunny periods hot air moves from the solar air heater into
the attic heat store, through the stone bed thermal mass, and back into
the air heater. The fan operates only when the living space
below requires heat. The exchange of air between the stone bed
and the solar air heater is driven almost entirely by the difference of
density between hot air and cool air. This note describes that
air exchange when the fan is not running.
David Delaney, Ottawa, November 2004
There are at least two regimes of convective air motion in
the attic heat store, 1) when the air entering the attic heat store
from the air heater is at least as warm as the top layer of stones in
the stone bed, and 2) when the air entering the attic heat store is
cooler than the air in the top layer of stones, but is warmer than
These two regimes of flow are illustrated below. A calculation of the convective forces in flow
regime 1 may be found here.