A method that requires a buffer thermal mass between the sun and the living space to be heated, where the thermal mass buffer can be a structure, a wall system, an absorption device, and/or another space. In an indirect gain system, the thermal mass acts as a collector, absorber, and distributor of the solar energy. Thermal distribution is accomplished by conduction. There are three main types of indirect gain systems: thermal storage wall, roof water, and sunrooms. See also:
Passive solar design