The HVAC ducts are flexible and they have lots of nooks and crannies. It would be impossible to remove the dust and dirt in the ducts with the type of rotating brush that they use. Also, the brush can puncture the flexible duct material, which would cause a leak and allow conditioned air to escape into the attic space. And, you would not know the leak exists until you start to experience problems. I would never allow a contractor to clean out the HVAC duct system.
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