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Passive space heating is what happens in a car on a sunny summer daythe car gets hot inside. In buildings, air is circulated past a solar heat surface and through the building by convectionless dense, warm air tends to rise while the denser, cooler air moves downward. No mechanical equipment is needed for passive solar heating.

Passive solar space heating takes advantage of the warmth from the sun through design features, such as large, south-facing windows and materials in the floors and/or walls that absorb warmth during the day and release it at night when the heat is needed most. Sunspaces and greenhouses are good examples of passive systems for solar space heating.

Passive solar systems usually have one of these designs:

1. Direct gainThis is the simplest system. It stores and slowly releases heat energy collected from the sun shining directly into the building and warming up the materials (tile or concrete). It is important to make sure the space does not become overheated.
2. Indirect gainThis is similar to direct gain in that it uses materials to hold, store, and release heat. This material is generally located between the sun and the living space, usually in the wall.
3. Isolated gainThis collects solar energy separately from the primary living area (a sunroom attached to a house can collect warmer air that flows through the rest of the house).

Active heating systems require a collector to absorb the solar radiation. Fans or pumps are used to circulate the heated air or the heat-absorbing fluid. These systems often include some type of energy storage system.

There are two basic types of active solar heating systems. These are categorized based on the type of fluid (liquid or air) that is heated in the energy collectors. The collector is the device in which the fluid is heated by the sun. Liquid-based systems heat water or an antifreeze solution in a hydronic collector. Air-based systems heat air in an air collector. Both of these systems collect and absorb solar radiation, transferring solar heat to the interior space or to a storage system, where the heat is then distributed. If the system cannot provide adequate heating, an auxiliary or backup system provides additional heat.

Liquid systems are used more often when storage is included and are well suited for radiant heating systems, boilers with hot water radiators, and absorption heat pumps and coolers. Both liquid and air systems can adequately supplement forced air systems.

Active solar space heating systems are comprised of collectors that absorb solar radiation combined with electric fans or pumps to distribute the solar heat. These systems also have an energy-storage system that provides heat when the sun is not shining.

Another type of active solar space heating system, the medium temperature solar collector, is generally used for solar space heating. These systems operate in much the same way as indirect solar water heating systems but have a larger collector area, larger storage units, and much more complex control systems. They are usually configured to provide solar water heating and can provide between 30 and 70 percent of residential heating requirements. All active solar space heating systems require more sophisticated design, installation, and maintenance techniques than passive systems.