Although it was not a home a/c duct, I did have a fire yesterday in a large ducting system in a very large commercial vehicle repair facility. There are several rubberized type flexible ducts (one located aay each vehicle lift) that connects into the vehicles exhaust pipe that attaches to a large metal duct at ceiling level. I have had a few of these where there are baffles within the metal duct where combustible debris will collect. This is primarily due to the failure or absence of a metal mesh screen used to prevent the debris from getting pulled into the ductwork. The fires I have seen usually never occur within the flexible duct itself but usually where the first restriction is (within the metal duct at ceiling level). I have also found that routine "cleaning" by an outside source is not always perfomed adequately. Large amounts of debris have been found right after the duct work was "cleaned". Just an FYI.

Anthony Tester