HVAC Fan set to ON position also turns on A/C unit
The problem is that "all G wires are connected together"   

According to the Aprilaire 500 installation guide the Green wire should go from the thermostat to the G on the Humidifier. 

Then from the Humidifier Gi output to G on the Furnace. 

This facilitates the "Test" feature for the humidifier which turns the Fan on during the test and also allows the Humidifier to run in Fan "ON" and Fan "Circulate" modes.