We recently had service call where one 6" flex closest to the air handler on top of the metal supply plenum appears to have caught fire ( soot on ceiling around vent) and completely burned no signs of it the vinyl lining, discolored the insulation and melted the Mylar coating. Equipment is an American Standard air handler with electric heat 8.0 kw on a four ton, (second floor in Houston) doesn't need much heat. Variable speed blower comes on consistently and temp rise not excessive' plenum temps in the 90s. Duct goes to a bathroom. Checked for wiring touching, in almost twenty yrs in the Hvac world never seen a duct burn.