Indirect Gain


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.

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