I can't find a lot of info regarding when to, or if you should clean your ductwork. Some info states to have it done if you spot mold in the duct but otherwise, it doesn't conclusively minimize dust or improve air quality. I do not have mold, but when I moved into my home a couple of years ago, the returns had approximately a 1/4 of inch of dust lining the inside of the boot. Pretty gross. I vacuumed those clean. We seem to accumulate dust in the home a little faster, and in quantity, than I would consider "normal". I change the furnace filter every 90 days as stated on the filter. Any thoughts or recommendations?