Why this one is different
Most home maintenance you can put off. This one has a number attached to it: the US Fire Administration attributes roughly 2,900 residential building fires a year to clothes dryers, and the single largest cause — around a third of them — is failure to clean.
Lint is not dust. It is dry, fine cotton fibre packed into a warm metal tube with a heating element at one end.
The five signs
1. Clothes need a second cycle. The most common and most ignored sign. A restricted vent means moist air cannot leave, so the drum stays humid and the machine runs again. If a normal load has gone from 45 minutes to 90, the duct is the first place to look.
2. The laundry room gets hot and damp. The heat and moisture that should be leaving through the wall are staying in the room instead.
3. The top of the dryer is hot to the touch at the end of a cycle. The machine is having to work against back pressure.
4. No visible airflow at the exterior hood. Go outside while the dryer is running. You should feel a strong, warm push of air and see the flapper open. A weak trickle, or a flapper that stays shut, means the run is blocked.
5. A burning smell. Stop using it. This is lint contacting the heating element, and it is the last warning you get.
The five-minute check you can do yourself
Run the dryer on a normal heat cycle and go outside to the vent hood. Hold the back of your hand about six inches away. You want a firm, steady flow — the kind that would push a loose sheet of paper away.
Then look at the hood itself. Lint packed around the flapper, or a bird's nest behind it, is extremely common and often the entire problem.
What a proper cleaning covers
The whole run, not the first three feet. A rotating brush on a flexible rod goes the full length of the duct from the machine to the exterior hood, with a vacuum pulling at the same time.
The other half of the job is inspection: crushed ducting behind the machine, flexible foil hose where rigid metal should be, disconnected joints venting into a wall cavity, and runs that are simply too long for the appliance. Those are the faults that make lint build up again three months later.
Ask for airflow readings before and after. A cleaning that does not change the number did not achieve anything.