The clothes dryer is the appliance that starts house fires for a documented, mechanical reason: it heats air, fills that air with lint, and pushes it through a duct that narrows as lint accumulates, until airflow fails, heat builds, and the fuel is already packed in the pipe. The dryer vent cleaning fire hazard is thus an airflow story with a fire ending, and the prevention is duct hygiene plus a few construction rules. This reference covers the duct done right, the cleaning rhythm, and the symptoms that say now.
The mechanism: lint plus heat minus airflow
Every load sheds lint, the screen catches most, and the rest rides the exhaust into the duct, snagging at ridges, joints, sags, and the exterior flap. As the path narrows, airflow drops; as airflow drops, drying slows and temperatures inside the machine climb; and the failure modes converge, thermal fuses tripping (the takes-forever dryer that eventually stops heating at all), heating elements glowing against lint-fouled housings, and, in the statistics fire services publish yearly, ignition, with the duct’s packed lint carrying the fire. The same restriction economics run the money meter too: a choked vent doubles cycle times and bills long before it burns anything, so the efficiency symptom is the safety warning, the appliance world’s version of the furnace filter’s airflow sermon.
The duct done right
Material: rigid smooth-wall metal duct for the concealed run, joints fitted in the airflow’s direction and taped with foil tape, never screwed (screw points inside the duct are lint combs), with a short semi-rigid metal transition connecting dryer to wall. The ribbed foil and vinyl flex hoses of hardware-store tradition are the fire literature’s recurring exhibit, lint-catching, crushable behind the machine, and vinyl’s melting point adding insult; they belong nowhere in the system. Route: as short and straight as the house allows, every elbow and foot of length taxing the blower (manufacturers publish maximum equivalent lengths, and long runs earn boost fans designed for lint), no sags that pool lint and condensation, and the behind-the-dryer crush, the machine shoved back onto its own hose, checked every time the unit moves. Termination: outdoors, always, through a hooded outlet with a working damper flap, never a screen (screens mat into felt), and never into attics, crawl spaces, garages, or chimneys, indoor terminations delivering hot lint and gallons of water vapor per load to exactly the cold surfaces and mold economies this site catalogs. Gas dryers raise the stakes a step: their exhaust includes combustion products, making a blocked or indoor-terminated vent a carbon monoxide path as well as a fire one.
The cleaning rhythm
Every load: the lint screen, and periodically a wash for it, since dryer-sheet residue films the mesh into an invisible restriction (the water-beading test on a screen tells). Yearly, the full path: dryer pulled, transition hose off and cleaned, the run brushed end to end with a rotary vent-brush kit on a drill (the consumer kits handle typical runs well) or a service visit for long and complex ducts, the blower housing and screen cavity inside the machine vacuumed (lint packs the cabinet too, around the very heat source), and the exterior hood’s flap cleared and swinging free. The audit: with the dryer running, the outside flap should stand open on a firm stream of warm air, and a limp flap, weak flow, or lint beard at the hood grades the system on the spot.
The symptoms that say now
Clothes taking a second cycle is the headline symptom and the earliest; the supporting cast: the machine and its room running hot, moisture and lint collecting around the door seal, burning smells, thermal shutdowns mid-cycle, and the exterior evidence above. Any of them moves cleaning off the calendar and into the week, and a dryer that has tripped its thermal fuse has already written the warning in a part. The habit layer finishes the program: no dryer running while the house sleeps or empties (the fire services’ standing plea), the area around the machine kept lint- and box-free, and washer hoses inspected on the same annual pull-out, the laundry’s two famous failures sharing one maintenance visit.
The bottom line
Lint is fuel, the duct is where it collects, and airflow is the whole game: rigid metal, short and straight, taped not screwed, terminated outdoors through a flap and never a screen. Clean the screen every load, brush the run every year, and read a slow dryer as the fire-safety instrument it is, because the second cycle is the system asking politely before it stops asking.