Water, which comes from a small valve attached to the household water supply. It travels through a small tube to the medium inside the humidifier.
A medium--a pad, screen, mesh, or filter. If the medium is a pad, it's mounted to a cylinder and a small motor rotates it through a small reservoir of water. If it's a mesh or screen type, the water flows through a trough at the top of the screen to distribute the water evenly across it.
A fan--often the furnace blower fan--that blows across the damp medium, to cause evaporation.
A valve or float that allows water to flow either into a reservoir or across the medium and then into a drain, when called for by the humidistat.
A humidistat that determines how much moisture to add to the air, and turns the humidifier on and off in conjunction with the furnace blower.
When the humidifier is running (usually when the furnace is running and the humidistat is calling for more humidity), here's what happens:
The water flows either into a reservoir (in which a pad mounted to a cylinder turns) or through a screen or wick, then straight into a drain.
The fan blows air across the damp medium, which evaporates some of the water into the air. The warmer the air, the more water evaporates--so the humidifier is often mounted close to the furnace in the warm-air ducting.
The water stops flowing and the humidifier shuts off when either the humidistat or the furnace shuts off.
