The Fire Risk
Clothes dryer fires are a well-documented category and the usual cause is simple: lint accumulation and a restricted vent.
Short answer
A restricted vent means the dryer cannot exhaust, so the heat stays in the machine and the duct. Lint is highly combustible and it accumulates where the air slows — at elbows, in crushed flexible duct, and at the hood. Annual cleaning of the full duct run and the machine interior is the measure that matters.
Why it happens
Lint passes the machine filter continuously, however well the filter is cleaned.
It accumulates where the air slows: at elbows, at transitions, in crushed flexible duct, and at the exterior hood.
A restricted vent means the dryer runs longer and hotter on every load.
The heating element or the gas burner cycles more and the duct temperature rises.
Lint is a fine, dry, combustible material sitting in a hot duct.
And the machine interior itself accumulates lint around the heating element and the drum, which is the part nobody cleans.
Flexible foil duct
The ribbed foil duct supplied with many installations is the worst option: the ribs catch lint, it crushes behind the machine, and it melts rather than contains. Rigid metal duct with the minimum number of elbows, and a short semi-rigid connector only where the machine has to be pulled out, is the arrangement to have.
The warning signs
Drying times getting longer, which is the single most useful indicator.
The machine or the laundry room noticeably hot after a load.
A burning or hot-lint smell.
Lint visible around the machine, around the duct connection, or at the exterior hood.
The exterior damper not opening, or no airflow felt at the hood with the dryer running.
And a machine that shuts off mid-cycle, which on many dryers is a thermal cutout doing its job and is a clear warning.
What reduces it
Clean the full duct run annually, from the machine to the hood, including the elbows.
Clean the lint filter every load, and wash it occasionally because fabric softener residue blinds the mesh.
Clean behind and under the machine, and have the machine interior cleaned periodically, which is where the lint around the element is.
Replace flexible foil duct with rigid metal.
Keep the run as short and straight as the layout allows.
And never run the dryer overnight or when out of the house, which is a free measure that addresses the consequence rather than the cause.
Common questions
How often should a dryer vent be cleaned?
Annually, and more often with a long run or heavy use.
What is the best warning sign?
Drying times getting longer. That is also when the dryer runs hottest.
Is foil duct a problem?
Yes. It catches lint, crushes and melts. Use rigid metal.