Are there different air duct cleaning methods? Which one actually works?

There are a few different methods to clean air ducts. Rotobrush, Ram air, and Negative air.

• Rotobrush is a vacuum that looks like a shopvac with a brush attached to the end of the vacuum hose. The idea is to guide the brush into the ducts while the vacuum is picking up agitated dust on the way. The problem with rotobrush is that the vacuum hose is too thick to follow narrow air ducts, therefore it can only reach a few feet into the ductwork and get stuck at a turn. Low price of this machine is the reason why so many companies use it, but there is no guarantee that all corners and hard to get spots will be cleaned within a ductwork.
• Ramair is even less expensive to perform. The concept is to turn on your hvac system to “fan only” and when the air is circulating a technician will go inside each return vent with a brush to dislodge the dust. The system air pressure will collect dust from return ducts at the filter. Supply ducts will push the dust out so a shopvac is used to collect dust from supply vents. Unfortunately there are companies that use this method. It will cause major indoor air poluttion and clog up coils and air handler components.
• Negative Air method involves large costly equipment. A negative air machine is hooked up to the system and only then technicians will use various brushes and tools to dislodge the dust stuck to duct walls. Once loose, dust will get sucked into the machine. No dust will blow in the house and the furnace components will be clean. This is the only duct cleaning method recommended by NADCA.