Fragrance systems are provided that can be used in conjunction with heating, ventilation, and air-conditioning systems. The fragrance systems can inject a fragrance into an airstream that is supplied to a space such as a room of a building or a vehicle cabin. In some embodiments, the speed a fan of the fragrance system can be automatically modulated in a pre-determined relationship to the speed of a fan of the HVAC system. Accordingly, a desired level of fragrance can be automatically maintained as desired within the airstream that is supplied to the space, even when the HVAC fan speed or other conditions fluctuate.
