The Aprilaire 4040 24-Volt solenoid valve allows water to come into your humidifier. This valve can be used on all Aprilaire humidifiers except for commercial units and residential models 800, 330, 445, 110, 112, 350 and 360. Other parts that may be needed with this part are the, No 4004 and the orifice, part number 4231, 4232, 4007 and 4021 depending on which Model humidifier that you have. We recommend that any wiring on your humidifier be completed by a professional contractor.