. If you connect the humdifier directly to the blower motor, the humidifer will be fed with different voltages when the blower operates at different s peeds.
idifer. A not so bad way to deal with it is to connect the humidifier to t he HIGH speed tap. Then the humidifier will see full line voltage when the blower is on high and it will see LOWER line voltage when the blower is on low. This is not too bad usually. The worst thing you can do is connect the humidifer to a low speed tap. Then the voltage to the humdifier may g et above 120 when the blower is on high speed, which can be very bad for th e humidifer.
and use a meter to check the voltage at the humidifer over all modes of ope ration.
I don't know about your furnace or blower. But mine and I think most moder n ones of the type we're talking about, the AC comes into the main furnace control board on a pair of wires that gets connected to the house 120V AC. The ECM blower motor is driven by a wiring harness connected between the bl ower motor and that main control board. Between the incoming AC and the ECM blower is the guts of the control board including CPU and power control electonics to drive the ECM motor. IDK how exactly one figures out what is going on in there and simply taps into a PC board. And even if you do, what's there surely isn't just 120V, because the controller can make the motor spin at whatever continously variable speed it wants and can somehow even sense the load on the motor to gauge the airflow.
If it's done the way you say, I'd like to see an example of an install manual for a humidifier that says so.