A user-programmable monitoring and dispensing system for controlling the dispensing of water vapor and various other media into an HVAC air stream in residential or commercial structures. The various media to be dispensed are preferably water-soluble, and mixed with the system water supply to be dispensed with the water vapor added to the HVAC air stream. These materials may be fragrances or aromas, intended to produce an aesthetic effect, or they can be agents capable of pesticidal, bacteriacidal, fungicidal or sporacidal effect for use as acute or prophylactic treatment for infestation.