Aprilaire humidifier (Model 550)

Is that 50 = 50% humidity? What is the current indoor humidity?

That's the way it should work. 35 and

50 are relative humidity, most likely. So when set to 50 (%) it turns on and add moisture because the humidity is less than 50% and greater than 35% where it didn't operate. It's probably a good idea to get a humidity gauge with a dial to see what the actually humidity is in the house. That will also give you an idea of the accuracy of the humidistat. At 25 degrees outside, I usually add very little humidity with the Aprilaire set to about 30%. But a lot has to do with the home insulation and how much heat you are adding. Before good quality window, my Aprilaire ran most of the winter. Now, it only runs when the outside temperature goes below 20 degrees, and pretty intermittently at that.

I'm sure you are right, that must be the humidity level. I don't know the actual level since I have no gauge. I knew the humidifier had quit working when I started getting "zapped" when I touched objects in the house!