This duct-mounted, whole house, small bypass evaporative humidifier is designed for tight homes under 3,600 square feet. 

The model 500/500M bypass humidifier is ideal for smaller homes. The unit delivers 12 gallons of moisture per day. Model 500/500M humidifiers can be plumbed to hot water to improve efficiency in heat pump applications. The 500 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 500M models; for applications where an automatic control is not practical.