The Autoflo 40W humidifier is a whole house power style humidifier. This model uses a fan inside it to draw air across the humidifier pad and back into the furnace plenum. Water is recirculated from a water tray in the base of the unit to the top of the humidifier filter. It is important to replace the humidifier filter at least once per year with a new Autoflo 40EP humidifier filter available on this page. This humidifier is old and obsolete. Many parts are no longer available. We recommend the Autoflo 250 or the Autoflo SC15 to replace it. You will need to do a little modification to use either one.