The AprilAire Model 4655 2-Wire Manual Humidifier Control (Humidistat) works with your AprilAire Whole Home Humidifier to accurately maintain the relative humidity in your home. Manual Mode allows you to adjust the humidity setting yourself with a turn of the dial - a tried and true method of simple humidifier control. Utilizing a mechanical filament to measure changes in humidity, this humidistat provides a dry-contact closure that is compatible with AprilAire Whole House Humidifiers past and present, such as the following Series: 400, 500, 600, 700 Evaporative Humidifiers, and 800 Steam Humidifier. Can be mounted on an interior wall (approximately 5 feet off the floor), or duct mounted on the furnace return air plenum or duct, at least 6 inches upstream of the humidifier, humidifier bypass ductwork (if applicable) or fresh air intake ductwork (if applicable). Includes a template for cutting the correct size duct opening of 4.15625 inches H x 2.4375 inches W. Model 4655 is accurate to within +/-5% RH and also includes a gasket to help reduce air leakage between the duct and the control housing (gasket is not needed when wall mounting). AprilAire Whole Home Humidifier is sold separately.