A: Most old furnace humidifiers no longer function due to the hard water we put through them. Most have a float system that is on an arm. When the float is down it opens a water valve to fill the tray. When the water comes up sometimes the float gets stuck and water begins to overflow the tray. Most furnace plenums have rusty stains on them from just this problem. If you feel you need to humidify your home then you will need to learn to keep the humidifier clean and operable. In my opinion I don’t think very many homes need a humidifier in the first place. If the humidifier is not properly cared for it could cause all kinds of problems including mold growth.