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In a drum-style furnace humidifier, a rotating drum covered in an absorbent pad picks up water from tray. The moist conditions can lead to mold growth. If you have mold in the humidifier, it could release a musty smell into the air. Mold could also spread from the humidifier into your air ducts, according to "The Everything Fix-It Book." You can clean the pad in the humidifier by removing it and soaking it in a combination of water and vinegar. You can use the same mixture to clean out the tray that holds the water. If mold growth is severe or if it has entered your air ducts, consult a professional and do not turn your furnace on until after the mold has been removed. You can prevent mold from growing in your furnace humidifier by cleaning the pad every month and replacing it at the end of every winter. Maintaining the proper water level in the tray will reduce the risk of excessive moisture building up, which reduces the risk of mold, according to "Black & Decker Complete Home Repair."
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Read more: Furnace Humidifiers & Mold | eHow.com http://www.ehow.com/facts_7161493_furnace-humidifiers-mold.html#ixzz2Da3toDhF