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The Y terminal on furnace is just a place too land wires from thermostat and outdoor unit Y1 is first stage of cooling and Y2 is second stage. On a call for cooling thermostat switches power from Rc too terminal G and Y1 which tells indoor blower and first stage outdoor unit too start.
The humidifier can be wired from 24 vac from furnace or from seperate transformer either way its controlled by humidistat and sail switch or current switch which proves blower is on before humidifier is allowed too operate
upload pictures of terminal blocks for further help