The answer to 'when to clean air duct' is that air ducts should be cleaned every 3-5 years, or more frequently if you have pets, smoke indoors, or notice a significant buildup of dust and debris. Regular air duct cleaning helps improve indoor air quality, extend the life of your HVAC system, and prevent potential health issues caused by mold, mildew, or other contaminants.
The frequency of air duct cleaning depends on several factors, including the size of your home, the number of occupants, and your local climate. Homes with more occupants, pets, or located in dusty or humid environments may require more frequent cleaning. It's also a good idea to schedule an inspection if you notice any visible signs of dust, debris, or pests in your ductwork.