I'll add if permitted an advice, besides the above Show's technical description. Let the contact state of the thermostat controlling the following sequence: stop the fan, close fresh air damper, close the valve on the protected coil, start a small recirculation pump installed on the preheat...
Billy,
the humidity sensor, as CB and Quark explained, controls the steam amount on the supply duct with regards on the second sensor installed on the return, for proper humidification adjustable setpoint and limits.
If you are talking about a humidifier based on applying an electrical current...
Pavlik,
if you are talking about the Airflow CCV cabinet units, then yes, I do know about it, and the most important thing is that are a reliable units. I'm using it in some computer rooms in a redundant configuration, one in line and one stand-by, with no ducts applied and under-floor...