If you mean your aprilaire humidifier isn't working then open the cover and remove the 4 screws on the fan motor mount inside and check for voltage at the transformer input 120 volts and output 25.5 volts a.c., then check the control board wherever you have it located on the furnace or upstairs and make sure yhe humidity level is set high enough to make the unit run(you will hear the water solenoid click on the furnace below the aprilaire unitwhere the water line is if its working correctly),then check the humidistat on the outside of the building as they can go bad
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If you mean your aprilaire humidifier isn't working then open the cover and remove the 4 screws on the fan motor mount inside and check for voltage at the transformer input 120 volts and output 25.5 volts a.c., then check the control board wherever you have it located on the furnace or upstairs and make sure yhe humidity level is set high enough to make the unit run(you will hear the water solenoid click on the furnace below the aprilaire unitwhere the water line is if its working correctly),then check the humidistat on the outside of the building as they can go bad