The theory of a thermosiphon is that it uses ambient temperature differentials to slowly move air in a pipe between the head space and the bottom of the silo. Active recirculation uses a fan in a sealed system to circulate the air from the bottom to the headspace of the silo. The application chamber where phosphine is applied must be large and open to the grain so it can disperse if the recirculation systems stops, otherwise explosive gas concentrations can be reached. For more information see ‘Fumigating with phosphine, other fumigants and controlled atmospheres