Direct-gain passive solar systems rely on south-facing windows to bring solar energy directly into a house. That sunlight is absorbed by materials in the house (the floor, walls, furniture, etc.), which warm up, store some of that heat, and re-radiate it back into the room, warming the space.

One way of looking at this is that the house itself serves as the solar collector and heat-storage system. There are no fans or pumps to move heated air or water around. The systems are silent, trouble-free, and easy to maintain--by washing windows and, in some cases, opening and closing window blinds to adjust incoming solar gain.

The thermal storage function is most effective with high-mass materials, such as tile or concrete floors, fireplaces with brick or stone facings, and tinted plaster walls. These materials keep the space from getting too hot during the day, and they continue radiating warmth into the living space in the evening.

The key to success with direct-gain passive solar heating is to provide the right amount of south-facing glass area and to couple that glass with adequate thermal mass. If too much glazing is installed, the space will overheat on sunny days. The better insulated the house, the less glazing can be installed before overheating becomes a concern.

For complete article: http://www.buildinggreen.com/live/index.cfm/2011/2/22/DirectGain-Passive...