We are crazy!! We bought a 15K duct cleaning machine. It puts the ductwork under negative presssure. We have an IntellaHepa Vac that hooks up at the furnace. We have a Honda compressor system that has the whip that can go up to 100', the compressor system will agitate the dust in the ducts and then the IntellaHepa Vac will suck all the dust that has been agitated. We clean the vents and the return airs. It's an intense experience and topped off with a sanitization fog spray to end the process.