You can do simple maintenance to keep it clean and operating in peak condition. Start by replacing the furnace air filters as directed–often, this is once every month or every three. Next, clean your air vents once a month, as they can help you see how much dirty air filters through. If you notice them becoming dirtier, you should have your ducts cleaned. With flexible ducting being more susceptible to tears, you’ll want to hire a pro to do the work for you. 

Whether air duct cleaning is worth the cost depends on your specific situation. While it hasn't been proven to prevent health issues, it can be beneficial for improving indoor air quality by reducing contaminants, which is especially important for households with members who have asthma or allergies. Cleaning is generally considered worthwhile if you notice visible mold growth, signs of a pest infestation, excess dust, or debris blowing from the vents. It is also recommended after a major home renovation. Additionally, if your ducts are dirty enough to leak air, cleaning and sealing them can improve your HVAC system's efficiency, ensuring the heated or cooled air reaches your living spaces and potentially leading to long-term savings.

You’ll want to spot-clean your air vents once every month. Spot-clean them by using a handheld vacuum, a brush attachment, or a microfiber cloth dipped in warm, soapy water. Make sure you trace over the corners and crevices, which trap dust. Once or twice a year, you should also deep clean. You can remove the vents and soak them in warm, soapy water for a half hour before cleaning, drying, and returning them.

Dirty air filters are the likely culprit. You should swap them out every month or at least every quarter. If you’ve kept on top of those filters and still notice a rapid accumulation of dirt, it could indicate that the ductwork requires a thorough cleaning. It’s also probably time to schedule that HVAC system inspection.