Product review (5 stars): Caution - Get the right voltage solenoid
If you have a standard Aprilaire Model 110 humidifier, this is the WRONG solenoid for you.  This is a 24 volt solenoid.  The 110 humidifier requires a 120 volt solenoid (Aprilaire 4005 solenoid).  The correct solenoid will likely be more expensive - but the wrong solenoid will not work - it will smoke and possibly burn.  If in doubt, get a professional to order and install the correct solenoid.