A thorough visual inspection is the best way to verify the cleanliness of your heating and cooling system. Some service providers use remote photography to document conditions inside ducts. All portions of the system should be visibly clean; you should not be able to detect any debris with the naked eye. Show the Post-Cleaning Consumer Checklist to the service provider before the work begins. After completing the job, ask the service provider to show you each component of your system to verify that the job was performed satisfactorily.

How exactly do you expect to kill or remove mold without it ? You can’t even clean your hvac systems coils without moisture of some type ,if you use a steam cleaner you can literally clean the whole system minus the electronics . You should do more research and possibly talk to people who do the work before posting , but don’t listen to me i only have a class A contractor license ,epa license , install /repair hvacs , rent and flip houses for a living .

Be cautious with companies that offer “whole house air duct cleaning,” urges the NADCA. The company may be using unscrupulous tactics to upsell you once they get started. Before any work begins, always clarify in writing what the job entails and what the cost will be. To protect yourself against fraud, read customer reviews and verify that your HVAC cleaning service has applicable licenses and certifications.

We bought a short sale home buit in 1987.  The home is gross to say the least. I called last week to set the appt. up once I knew we had a close date. They called to say they were running a bit late, which happens with this type of work. They did great work, cleaned up after themselves, very nice men, cleaned all the vents,fans inside and out. I will feel much better bringing my family into a clean home now.
Be very careful, and definitely consider going with a different company if you want your air ducts cleaned.  They were in & out in a hurry, which leads me to believe they kind of half a$$ed the work. They're just like a restaurant that turns tables so quickly that the service really lacks. Save your $49 or $59 for the discounted deal and put that towards the cost of a company coming out and doing an actual good job.
In addition, the black dust was found in most of the heat registers. He could rub it off on his fingers but did nothing but spray compressed air on it which didn’t make it disappear since it wasn’t loose. Also, in the register in the basement he described the black dust as being oily. The reality of that situation is that I had the AC on until the morning of the service and minutes after he snaked the hoses in my back door it started raining. The basement register had condensation from the humidity all over it. It was not oily but wet. He asked for an old towel and wiped it out. Never saw anything so black – because I never had to clean soot. That is what I have in my ducts. Soot from years of the old coal furnace.
After they left, I was feeling uneasy about the quality of work that was performed. I used a camera to get some video of the vents that they cleaned. They looked pretty good, but then I started to remember how they were cleaning them.  They only used their large, commercial vacuum on one vent.  They also did NOT clean 2 air returns under Negative Pressure like the Groupon said they would.  I did a little research and found that they should have cut open an access point in duct work in the basement and sealed up the vents with plastic to create that negative pressure.

Air duct cleaning service providers may tell you that they need to apply chemical biocide to the inside of your ducts as a means to kill bacteria (germs) and fungi (mold) and prevent future biological growth. They may also propose the application of a "sealant" to prevent dust and dirt particles from being released into the air or to seal air leaks. You should fully understand the pros and cons of permitting application of chemical biocides or sealants. While the targeted use of chemical biocides and sealants may be appropriate under specific circumstances, research has not demonstrated their effectiveness in duct cleaning or their potential adverse health effects. No chemical biocides are currently registered by EPA for use in internally-insulated air duct systems (see Should chemical biocides be applied to the inside of air ducts?).