Some one has the humidifier hooked up to the R and W on the furnace.
1. it should be hooked up to its own transformer or to a proper low voltage terminal on the circuit board(beware there are high voltage HUM terminals) markd HUM and the other wire to C or the common.
2. You could hook the wire to W or white and C Common but I would rather see a seperate transformer since there is a possiblity this could cause solenoid chattering