Before cleaning a duct/vent, you first want to identify the source of the dust and dirt. If there is an air leak or draft of some kind, that should be dealt with first so the cleaning job won't be done in vain. If your building is relatively new, and yet, your vents and duct system are very dirty, it could be the builder was running the heat/AC without filters during construction. In that case, you might just need a one-shot cleaning. If, however, there are air leaks, they have to be sealed. We can find them through a blower door test and eliminate the problem. If you have flex duct material, cleaning of dirt and debris is simply enough (with the right tools), but significant mold growth could mean it's time to toss the old and invest in the new. Duct board, metal duct work and vents, plastic, and other materials all vary in how they respond to different levels of contamination - so it takes some experience to really know if a vent or duct material can be saved on not.