Natural thermosiphoning of solar-heated air is important for many passive solar applications. It is also important to prevent reverse thermosiphoning at night, often with the use of backdraft dampers. Commonly used plastic film dampers may restrict air flow. Therefore, this project developed a novel backdraft damper that utilizes bimetal coils to actuate a conventional metal damper. Two prototype dampers were tested in the upper and lower vents of a thermosiphoning air panel. The upper damper opened fully on sunny days, but partial opening of the lower damper reduced collector efficiency by 19%. A single upper damper should minimize reverse thermosiphoning.