Product review (5 stars): A must for powered furnace humidifers
I have installed two of the Honeywell HE-360 humidifiers in my home, and use this relay to trigger the system to run only when the heat is activated in my furnace.  An humidistat also makes sure the humidifier only runs when humidification is needed, but this relay ensures that the powered humidifier runs only when the furnace blower is running in heat mode.  If your thermostat doesn't control a humidifier directly (and most do not, unless you spend $200 or more for such a system,) you must have a means to control power to the humidifier.

Without being too technical, most modern furnaces have blowers that operate at different speeds depending upon whether they are heating or cooling.  This relay is placed on the wire that powers the furnace blower when in heat mode.  The relay senses the power running through the wire, and closes a low voltage switch that drives a relay on the humidifier, turning it on.  It's very simple to install if you follow the directions and have some electrical knowledge or skill.

I've had one of these installed for 6 years, it works great with no problems.