AprilAire 600/600M Whole-House Large Bypass Humidifier is a duct-mounted bypass evaporative humidifier designed for tight homes under 5,000 square feet. 

Model 600/600M bypass humidifiers deliver up to 17 gallons of moisture per day. Model 600/600M humidifiers can be plumbed to hot water to improve efficiency in heat pump applications. The 600 includes an automatic digital control that delivers the correct level of humidity based on indoor and outdoor temperatures.  A manual humidistat is provided with all 600M models; for applications where an automatic control is not practical.