What are the two basic types of active solar heating systems?

There are two basic types of active solar heating systems. One is liquid-based system that heats water or an antifreeze solution in a “hydronic” collector. The other one is air-based systems heat air in an “air collector.”

Even though these two systems differ in their fluid types, both of them collect and absorb solar radiation, then transfer the solar heat directly to the interior space or to a storage system, from which the heat is distributed.

Usually, in home heating, these systems might not be able to provide enough space heating, an auxiliary or back-up system provides the additional heat. Often time, the liquid system is more used when storage is included, and is better suited for radiant heating systems, boilers with hot water radiators, and even absorption heat pumps and coolers. If you have forced air systems, both air and liquid systems are great supplements.

Source: EERE