whole house humidifier 24v question? help

OK gurus of HVAC if you're out there. I've gone round and round with this and have had 2 different hvac guys fail. I need 24v to switch humi on WITH furnace (obvious to me but not the DB that did our new system), and can't figure out how to get it off furnace board. can I just use the same terminals on my t stat that run furnace?

edit: ok my t stat wires get 24v when the blower is off. WTH?

 

One wire to the "C" or Common, the other wire to the "W" on the furnace control board.

 

I would use a 120/24 volt transformer. I would hook up the 120 side to the eac terminal on the board and then run the 24 volt side thru a humidistat and then to the solenoid valve. This way the transformer is only energized when the blower comes on.