Never call a HVAC business in for ductwork cleaning. For starters, the tools HVAC contractors use in ductwork cleaning are not that advanced. Basically what they use is a giant scrub brush. They then push the brush through the air vents and take a guess on how clean they made it. The HVAC professionals then track dirt all over the home repeating this process in every area that has ductwork. Their guess is as good as yours if the ductwork is actually clean or not. What was horrible is the one time I got ductwork cleaning, the HVAC contractor ruined my HVAC duct. He got really into his scrubbing and the brush slipped. I heard it bang against my ductwork and a loud crack echoed out of it. After some inspecting, the HVAC contractor and I deduced there was a new crack in the air duct. He did not have his tool for ductwork sealing with him. So he wanted to just leave the mess he created. To add more to this, the HVAC professional wanted to be paid for ductwork cleaning. I refused to pay for a service that was done improperly. I was not going to pay a ton of cash when the guy busted up my air ducts. I ended up calling the owner of the HVAC company and screaming at him over the phone. I did not have to pay for the duct cleaning, but I did have to pay for duct sealing. I vowed to never again mess with the ductwork after that whole experience.