Hopefully you are cleaning your dryer’s lint trap EVERY time you start a load. Even when you do, some lint gets by and travels to the duct system. Experts recommend cleaning dryer vents and ducts at least once a year.
Annually, across the country firefighters respond to around 14,630 home fires caused by clothes dryers, according to the National Fire Protection Association.
Approximately one-third of these fires is caused by an accumulation of lint. Not cleaning out your dryer vent can lead to a drop in performance caused by constricting a dryer’s ductwork and possibly set you up for a dangerous situation that’s far more serious.
Also, you may consider replacing your ducts if they are plastic or foil accordian-style ducts which are risky because they can sag, allowing lint to build up at low points and trapping lint in their ridges. A smooth walled or flexible metal duct is a better option.
I utilize a specialized duct cleaning system which includes a spinning brush on the end of up to 24 feet of flexible rod to remove lint from the sides of ducting and suck it up into my vacuum.
Check out this informative article from Consumer Reports: https://www.consumerreports.org/clothes-dryer/how-to-prevent-dryer-fires/