The Aprilaire maintenance kit 4793 includes everything you need to make your Aprilaire Model 550, 550A and 558 like new.
This 550 Series kit includes one each of the following items:
Aprilaire Flow-Thru humidifiers have a solenoid with a small strainer filter in the body. These strainers should be inspected and cleaned when replacing the water panel. If the strainer becomes clogged the water pressure will tear it and pieces will jam the solenoid or loose pieces will block the orifice.