I have an older 110 humidifier. It was never wired to turn on when the furnace does. Its wired to turn on by itself. Right now the wire coming from the unit is split between the control and the power coming from the transformer. Can anyone help me.

Some of the older systems had a place to wire a humidifier so it would come on in heating mode only and when the blower was on.

Sorry I should have mentioned I have a Goodman unit that is only 3 yrs old. I don't see an obvious place to wire it up. Nor do I know if I'm suppose to use the exiting transformer that its externally hooked up to

Move the wire from the transformer to "W" (low voltage thermostat wire) on the control board, It will let your humidifier run only when the heat is calling and only if the humidistat is closed.

I think the OP answers sound good, but I think you should check it out with the manufacturer first. Here's the Aprilaire Humidifier site, they have specs and advice. If all else fails shoot an e-mail to the service department.