Since water can't just run to the humidifier all the time, the water supply flow needs to be controlled. This is done by a water inlet orifice and an inlet valve assembly controlled by an electric solenoid. The solenoid is controlled by a humidistat (sort of like a thermostat for humidity).
When the humidistat says to increase humidity, the solenoid opens and supplies the water feed tube. When the humidistat says the humidity level is good or when the furnace shuts down, the solenoid shuts off, stopping water flow to the humidifier.
Through months of seasonal non-use, these parts can become seized or clogged and prevent the humidifier from working properly. Let's take a detailed look at the water inlet valve assembly in the next section.