Product review (4 stars): Great product, just over priced
Great product, just over priced.

Many people replace this part unnecessarily.  Hard water deposits are the main cause for the lack of water flow in a humidifier.

I found this to be the case with my humidifier's solenoid valve.  I had to disassemble my solenoid valve and orifice for cleaning (as I had no water flow) and the solenoid valve was not able to open after reassembly. The O-ring seal in the solenoid valve had flattened out.  I replaced the O-ring and everything worked great.

Many people should remove their humidifier valves/orifice assembly in the spring and soak them (do not soak the electrical coil) in CLR or white vinegar.  There is a strainer in the orifice assembly that needs to be cleaned out too.

If none of the above steps work, check to make sure that your solenoid valve is getting power to it (mine is 24VAC, others might be 120VAC).  If there is power present to the coil, place a screwdriver tip near the end of the coil.  You should feel a vibration (magnetic field) that will tell you that the valve is trying to open.  If you don't feel a vibration then turn off the power and unwire your solenoid vavle.  Use an electrical meter set on resistance scale (ohms) and check to see if the coil is open or shorted by placing one of the meter leads on each end of the coil wires.  If the coil is good you should get an Ohm reading in the 30-80 Ohms range (for my model).