Although it uses the same materials and design principles as a direct gain system, an indirect gain system positions the thermal mass - such as rock or contained liquid - between the sun and the space to be heated.
The sun's heat is collected and trapped in a narrow space between the window and the thick masonry wall (thermal mass) after it passes through the windows. This heats the air, which rises and spills into the room through vents at the top of the wall. Cooled air then moves to take its place from vents at bottom of the wall. The heated air circulates throughout the room by convection. The thermal mass continues to absorb and store heat to radiate back into the room after the sun has gone. Dampers can be placed in the vents to prevent warm air from escaping through them at night.
During the summer months, the process is reversed. The thermal mass is prevented from receiving direct sunlight while absorbing the heat in the room, helping to keep the temperature cooler.
This indirect gain system can also be used with flat-plate collectors, normally associated with an active solar system. The collectors are always set below the thermal mass storage tanks or bins to take advantage of the natural movement of heat - warm air rising and cool air sinking,
Sunlight is absorbed by the collector and the heat transferred to the thermal mass. As the air in the collector and thermal mass warms up, it rises and enters the house through ducts and vents. As the air cools, it falls, entering return ducts that carry it back to be reheated. This system is sometimes referred to as a thermosiphon but it's just a fancy name for convection.
The Trombe wall also is an indirect solar gain system developed in 1956 by Jacques Michel, an architect and Felix Trombe, a scientist. The Trombe wall, despite some variations, remains a black-faced masonry or concrete south-facing wall, a foot to 16 inches thick, that acts as a solar collector. As with all indirect solar systems, the wall is separated from the glazing by an air space.
All day the natural thermosiphon process (remember convection) keeps warm air in the room and because it takes from 6 to 8 hours for the heat to pass completely through the thermal mass, the radiant heat from the wall itself warms the room at night.