Product review (5 stars): All you need
If you Aprilaire humidifier is not working, you can bet it because the solenoid is not working. I have replaced the solenoid two or three times over the last twenty or so years of owning my Aprilaire unit. This is the only moving part on the entire system, and if you replace the solenoid, you'll be back in business. The only other thing it may be is the water supply to the unit is clogged (those darn saddle valves) But you already checked that, right?