An autovent's a device for maintaining a greenhouse or conservatory within a range of temperatures. The basic principle's that as greenhouse heats above ambient the air inside becomes lighter, the vent opens when a certain temperature's reached and lets the hot air out - drawing cooler air in from outside. The force to open the vent's provided a number of ways, the most common being through thermal expansion and by electric motors. Thermal expansion based actuators use either the differential ex… (More on Autovent)