Active systems are more complex. The solar radiation that passes through the glazing falls on a collector plate. The plate can be a duct with air flowing through or a plate with loops of piping carrying water or refrigerant that is warmed and pumped to the space to be heated or to a storage facility. If air is used as the medium, a large bin of rocks or a layer of rocks beneath the floor of the house will absorb the heat to be stored. If water is the medium, it will be pumped to a tank, and can be circulated to a conventional radiator system when heat is needed. If a refrigerant is collecting the heat, it will be follow the same design as a heat pump system. Water will be heated in an exchanger and stored for later use. The refrigerant system eliminates the need for anti-freeze protection and drain down that a water system requires. With additional piping and a heat exchanger, all solar heating systems can produce domestic hot water. The earliest solar systems were for domestic hot water supply, then expanded to meet the heating demands of the home or to heat swimming pools.
An active solar air system consists of four possible air cycles.
- Solar heated air to house
- Solar heated air to storage
- Storage to house
- Backup heat to house
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