Abstract: A supply ventilation system controller for locating at an interior of a building. The controller comprises circuitry for receiving a signal representative of exterior air temperature and circuitry for receiving a signal representative of exterior relative humidity. The controller also comprises circuitry for establishing at least a first range of acceptable temperature and circuitry for establishing at least a first range of acceptable relative humidity. The circuitry for providing an enable/disable signal to a fan is responsive to at least one of: (i) the signal representative of exterior air temperature indicating satisfaction of the first range of acceptable temperature; or (ii) the signal representative of exterior relative humidity indicating satisfaction of the first range of acceptable relative humidity.