I have an Aprilaire hooked up to my furnace also, but it only comes on
when heat calls, not when the fan is running by itself. I just checked
the wiring on my furnace and the humidifier is hooked up to the W and
C terminals on the circuit board, so you can check if thats correct on
your furnace. You might have your humidifier wired up wrong and thats
why it comes on even if you only have the fan running.