Replace UV bulb annually. Overtime the lamps ability to emit UV light decreases. Remember UV light is invisible - the the lamp might still be glowing but not enough to effectively neutralize the bacteria on your filter

Used lamps should be handled/disposed of in accordance with federal, state and local regulations. Check with your local hazardous material recycling facility or bring your unbroken bulb to Home Depot's free, national in-store, consumer bulb recycling program at all 1,973 The Home Depot locations.