Natural convection in the attic heat store

David Delaney, Ottawa, November 2004

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.

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 lower layers.

These two regimes of flow are illustrated below. A calculation of the convective forces in flow regime 1 may be found here.Air from the heater warmer than top layer of stones
Air from the heater cooler than top layer of stones