Wick Filter replacement for SPT humidifier SU-9210 (set of 2). Through normal use waterborne minerals and sediment become trapped in the filter. The harder the water the greater the amount of minerals in the water and the more frequent filter needs to be replaced. The filter should be replaced when the filter’s surface area is thoroughly discolored or when water usage is significantly reduced. With normal daily usage the lifespan of the filter is approximately 3 months.