A method and apparatus for maintaining an equilibrium of temperature and moisture between grain stored in a grain bin and the surrounding atmospheric air. A grain temperature sensor and an air temperature sensor are connected to a comparator circuit which compares the relative temperatures. When a predetermined temperature differential between the grain and the atmospheric air is exceeded, the comparator circuit switches on an activating circuit which, in turn, completes an electrical circuit to the fan motor. When the fan is running, outside air is circulated through the grain to bring the temperature and moisture of the grain and the outside air back into an approximately equilibrium condition, plus or minus an allowable temperature differential. When the temperature differential between the grain and the atmospheric air is reached, the fan motor is turned off. When the temperature of the atmospheric air rises above a predetermined level, a cut-out device will disable the actuating circuit to thereby prevent outside air from being circulated through the grain. Under these conditions, the grain may be held in a cured state for an almost indefinite length of time with no deterioration of the grain.
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