Is duct cleaning necessary?
Many wonder if they should have their heating and cooling ducts cleaned so they can improve their home's indoor air quality. But There hasn't been any hard evidence to show that duct cleaning improves indoor air quality or prevents health problems, so we don't routinely recommend it. Exceptions would be if you find mold growth inside the ducts (first eliminate the moisture that caused the mold), the ducts have become home to rodents or insects, or the ducts are clogged by debris (usually from a remodeling project). To check for these problems, take off the register covers and shine a light down the duct as far as you can see.
If you decide to have the ducts cleaned, be prepared to spend $350 to $1,000 for a thorough job that includes cleaning the entire heating and cooling system. Be wary of companies that offer “specials” for $100 or less, since they usually don't do a complete job. And take “before” and “after” looks at your ducts to make sure you see a difference.