When I got a new furnace five years ago, I kept the old humidifier. Recently I noticed that as soon as the furnace turns on, the fan in the humidifier runs, even if the water solenoid has not kicked in.
I believe that over the years a bit of moisture got in and made a conduction path on the printed circuit board. There was about 26 ohms across the open relay contact.
I built a new control unit and have it mounted externally. It's nothing more than a 24 volt transformer and a 24vac DPDT relay. Works just fine.
What I noticed is the mfg seems to have made things more complicated that they should have. The original control unit contains a 24v *DC* relay and a 24 v DC power supply.
Seems like that was unnecessary.