Thermosiphon systems rely on the natural convection of warm water rising to circulate water through the collectors and to the tank, which is located above the collector. As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the tank's cooler water below flows down pipes to the bottom of the collector, causing circulation throughout the system.

Features:
• flat plate solar collector(s) and mounts
• piping to connect the collector with the water tank
• oftentimes, a specialized thermosiphon water tank
• controllers and valves
Conclusion: Thermosiphon systems are typically less expensive than active, pump-driven systems, and they’re more reliable than stand-alone integral collector systems. They should be connected to and backed up with electric or gas water heaters. The most cold-resistant models can only be used in areas that never experience ambient air temperatures below 20 degrees Fahrenheit for more than 18 hours.