The Rotobrush is an absolute JOKE!!! I had one for the first year I did ducts. Not only did it
not get the ducts clean it also broke beyond repair in the first year.
The Rotobrush wont even get into many ducts because of its inch and one half diameter, some duct
openings are only an inch. So you will find yourself turning away work.
The correct way to clean air ducts is by putting the system under negative air pressure then whipping, brushing
and air washing each individual duct line and trunk lines.
Check out Nikkro systems, Heat seal, Air care or Myers vacuum for a real duct cleaning set up.
You can get a starter system for around the amount you where going to waste on a Rotobrush.
The best way to do it is with a 15,000 cfm truck mounted system, but then your getting into it big time.