In the photo above, the furnace thermostat is calling for heat and the humidistat is calling for more humidity (I will show you how to test for that next). The humidistat will only send voltage to the solenoid when the furnace is running and moving air.
You can see the multimeter is showing approximately 24 volts AC current to the solenoid. If you see this voltage being displayed and the solenoid is not allowing water to flow to the humidifier, then the solenoid is defective and needs to be replaced.
Now let’s review how to test the humidistat.