The Aprilaire 4655 Humidistat is designed for low voltage service to control humidification equipment. An increase in relative humidity expands the nylon ribbon which opens the control switch to stop operation of the humidifier. A decrease in relative humidity reverses the process and closes the control switch. Can be wall mounted or mounted on return plenum. 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.