HE300 humidifier solinod wont close

SOURCE: HE440A Steam Power Humidifier

The most common cause of overflow is when hard water deposits build up and prevent the float from closing. You'll have to open up the unit, make sure there's no water in it, and clean out any mineral deposits.

The problem with these steam producing units is that there is no built in way to remove those deposits automatically. The water comes in, evaporates, and leaves anything else that was suspended in it behind.

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: Water keeps running to Honewell 360A Whole House Humidifier

You may need to replace the valve that is wired to the furnace that controls the water flow

SOURCE: No Water flow thru Humidifier

Loosen the brass fitting where it connects to the soleniod valve. If water comes out your saddle valve is ok. Carefully retighten this fitting. With the humidifier turned on loosen the first fitting down stream from the valve see if water comes out. if it does turn the humidifier off and see if it stops, it should. That will tell you the the valve is ok then, just clean the filter and the oriface. if it does not turn on and off check the wiring. if ok replace the valve.
Good luck

SOURCE: Water flow from humidifier never stops. Even when furnace is off.

No, it will not stop flowwing if it is the water supply. A solenoid valve opens the flow everytime there is a call for humidity, but it stops at the humidifier when there is no call, so it will leak from that point until it is fixed or tightened.