The AC plug is for the induction fan and the 24v is for the water solenoid. Did you ever get this all working the way you wanted to?
Personally I have a hub system variable based on the attached calculation. I average out the humidity sensors in my home then turn the humidifier on if the averaged indoor RH drops below the Humidistat variable value. I use the OpenWeather driver, but you can also use a weather station or outdoor temperature sensor for a local solution
