Question for built-in humidifier experts

[swf]

I found a good deal on a whole house Aprilaire 600M humidifier. I need it installed on a Trane XP80 furnace. My brother-in-law is a sheet metal expert and can easily accomplish the physical installation of the unit. My problem is the electrical hook up. Wiring up the 24v transformer won't be a problem. But, do I need a current sensing relay for this unit? The instructions show to install one "if needed?" If not, how does the humidifier turn off and on with the furnace blower? How do I know if one is needed? Is this a job for an amateur or should I get a pro to do the electrical work?

[cbelt3]

I'll admit to having a pro do the setup last time.. and he got a better deal on the unit via his wholesale pricing. But then again the guy I use is one of those rare types... sole proprietor, honest as the day is long, and willing to work with you when you don't have all the money in hand.

[ADent]

My furnance had a humidifier output on the circuit board.

Usually you can also wire it up to run when thermostat calls for heat if you don't have the proper output on the furnace.