The Aprilaire 4655 manual humidifier control is designed for low voltage (24v) control of humidifiers in central heating and air conditioning systems. This humidistat can be wall or duct mounted. Includes plenum cutout template (4 5/32" X 2 7/16"). For duct mount, locate at least 6" upstream of humidifier, humidifier bypass ductwork, or fresh air intake ductwork.