No Water Flow to Honeywell Enviracaire HE225A1006 Humidifier

SOURCE: Aprilaire 760 Humidifier water continues to run

You need to determine whether the problem is in the solenoid or in the furnace electronics. You could put a voltmeter on the solenoid connection to see if the furnace is still holding the solenoid open after the blower stops. This will tell you whether it is the solenoid or not.

SOURCE: Water flow through humidifier

The solenoid that turns on the water to the unit should not change the water flow, so you should get the same amount of water that flows into the solenoid. If you want less water, you can partially close the valve at the water source.

SOURCE: Continous ware flow on Honeywell HE220A Humidifier

I am having the same problem. I have had my humidifier for about 5 years now and I just started having the problem where the water continuously flows through the solenoid. I took apart the solenoid by removing the fittings from both sides. When I did that, I noticed that there was a hole in the metal that allowed me to see through to the other side. Apparently, it had corroded and and broken through, allowing the water to pass through the solenoid without really going through the valve within the solenoid. There are several sites that sell replacements for about $70, which is not cheap, but cheaper than buing a new humidifier for $120. Here is one site I found: http://www.iaqsource.com/product.php/honeywell/32001639-002/?product=112783&category=1142

SOURCE: cleaning a furnace humidifier steel filter?

Most water panels are generally replaced once a year on flow through humidifiers.

SOURCE: wiring problem

Sounds like you may have put 120v through a 24v solenoid and humidistat.
Is the HUM terminal on the furnace 120v or 24v?
If it is a 120v terminal you will need a 120x24v transformer and a new solenoid and maybe a new humidistat.