Installing AprilAire Humidifier Models 400, 500, and 600 

AprilAire Humidity Control
Healthy Humidity plays a key role in overall wellness. With an AprilAire whole-home humidifier, you can help keep your family happy, healthy, and comfortable in your home for years to come.
Learn how to install the AprilAire Model 400, 500, and 600 in your home in this tutorial. Please note that installation by an HVAC Professional is always recommended.
Contact us with questions at 1-800-334-6011 or visit aprilaire.com/contact for more information.
Edit: At 2:25, the white wires shown go to a 120V circuit. The two red wires would get attached to the terminal posts on top of the transformer pictured.