I work for a small resi company who offers duct cleaning and it's a joke as far as I'm concerned. All we do is seal off all the registers, hook up an old Hepa 2500 vac to one of the supply ducts, while the vac is running we clean the registers one by one, maybe blow some air through the duct. It only costs about 200 and takes a couple hours but I'm pretty sure it does absolutely nothing. Not to mention the Hepa 2500 is huge and bulky and we have to bring it in the house, the vac hose (about 8" diameter) is a PITA to hook up. In my opinion you'd get much better draw from a 4" hose and it would be easier to hook up. Either way the service is a joke and I wish we didn't even offer it. Of course I understand a more legitimate duct clean would probably take a whole day and cost a lot more.