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The thermosiphon collector consists of clear, corrugated polycarbonate panels fastened to 2-by-6-inch studs. The clear panels admit sunlight, and an absorber suspended inside the collector captures the sun’s heat energy. The air around the absorber warms, expands and rises, creating a convection current. Vents at the top and bottom of the collector allow air to circulate through it. Cool air enters the lower vents from the interior, is heated by the absorber, rises to the upper vents and returns to the interior. Air circulation continues as long as the sun shines on the collector.

At night, airflow reverses as air in the collector cools to outside temperatures. Simple flapper valves on the top vents stop this reverse circulation and keep the heat inside.

To estimate the size of a FreeHeat Solar Collector array that would work for you, divide the number of square feet you want to heat by 10. For example, a well-insulated 1200 square foot house would need 120 square feet of collector surface.