How often should you replace your furnace humidifier pad? Most bypass humidifier water panels should be replaced once a year, usually at the start of the heating season. Homes with hard water may need to replace them more often, because minerals crust over the pad and stop it from adding moisture. A clogged pad is one of the most common reasons a home stays dry all winter. Sherwood Mechanical explains how to spot a worn pad and keep your humidifier working. Learn when to replace your furnace humidifier pad.