Passive systems have two basic types such as integral collector storage passive systems and thermosyphon systems. First type system works very well in areas where temperatures rarely fall below freezing and in households with significant daytime and evening hot water needs.
Thermosyphon systems reliable but have a heavy storage tank. When warm water rises as cooler water sinks water flows through the system. Conventional storage water heaters usually provide backup as a part of the solar system package for cloudy days and times of increased demand. Back up heating usually provided by electricity or gas.
Passive heating systems are simple and need no electricity to operate so most of the systems installed worldwide are passive.