We clean all supply and return ductwork up to filter on furnace. I use a portable 20hp gas engine vacuum which is attached on the main trunk lines by the furnace. All air duct vents are taken off and an air compressed whip is run down each one. This agitates and breaks debris loose so that the vacuum can remove all the debris from the ductwork. The many benefits of having your air ducts cleaned is it reduces the dust, dander and other allergens that otherwise would be recirculated in the air. It helps reduce the need to dust and helps reduce flair ups of allergies and asthma. I also clean dryer vent lines. Doing regular dryer vent line cleaning helps to prevent dryer fires. It keeps drying times to a minimum, which saves gas and electricity and helps dryers to last longer I have a specially designed line that is run off of an air compressor at 175 psi. I do this from the outside of the home, so that all the debris is kept outside.