I have two Carrier geothermal units (Climatemaster) each with a bypass humdifier. (Carrier rotating drum type) They are rusting the duct!! Am I doing something wrong? Other info: I use a Culligan water softener and each unit has an electronic air cleaner. I have them off now because the rust area is enlarging but the house is clearly dry. My little ones get a kick out of the static electricity but it's wearing thin.