What is a dryer vent?
Updated August 2026
A dryer vent is the duct that carries hot, moist air and lint from the back of the dryer to a hood on the outside of the house. In nearly every home it is 4-inch round duct, and code wants smooth-walled rigid metal for the concealed part of the run, with a short listed transition hose bridging the machine to the wall. Everything the dryer pulls out of your clothes, several pounds of water and a steady stream of fiber, has to finish outdoors.
Where it is
Start behind the dryer. A flexible transition hose leaves the exhaust collar low on the back panel and enters a wall or floor boot, and from there the duct travels through a joist bay, a crawlspace, or an attic to an exterior hood with a flap or louvers. To find the outside end, run a hot cycle and walk the perimeter with your hand out. You want a firm push of warm, damp air and a flap standing wide open. A flap that barely lifts means the run is restricted somewhere between the two ends. Roof terminations are common in townhouses and are the hardest to keep clear.
What it does
A healthy duct moves roughly 150–200 cubic feet of air per minute, and every year of lint film on its walls shaves that number down. A restricted vent makes the dryer run two and three cycles for one load, bakes the laundry room, and eventually trips the thermal fuse, a $15–40 part attached to a $150–250 service call. The larger stake is fire. Failure to clean is the leading cause of home clothes-dryer fires in U.S. fire data, behind roughly a third of them, because lint is fine dry fuel parked in a hot air stream. The running cost of ignoring the duct shows up long before that, in wasted electricity and dryers that quit years early.
Maintenance
Clean the full run at least once a year, and every 6 months if the duct is over 20 feet, turns more than two corners, or serves a household with pets and kids. A rotary brush kit costs $30–50 and handles most straight runs; a pro with a vacuum and a camera charges $100–200 and earns it on roof caps and long attic runs. Replace the transition hose once it is crushed, kinked, or foil, since that hose has its own interval. Clearing the lint trap every load and vacuuming its housing twice a year keeps the duct loading slower, and the dryer vent maintenance checklist puts all three on one calendar.
What it costs to replace
A replacement dryer vent runs $150–500 to rebuild a short run in rigid metal duct with a new exterior hood, against $100–200 for a professional cleaning of the duct you already have.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
How do I know if my dryer vent is clogged?
Time a normal load. If cotton towels need a second cycle, or come out hot and still damp, airflow is the suspect. Other tells: the laundry room feels humid, the cabinet is hot to the touch, lint gathers around the outside hood, and the flap out there never opens while the machine runs. A burning smell moves this to the front of the list. Stop drying and clean the duct before the next load.
How long can a dryer vent be?
Residential code caps the concealed duct at 35 feet of developed length, and each 90-degree elbow subtracts 5 feet from that budget while a 45 costs 2.5 feet. A path with three 90s is down to 20 usable feet of straight duct. Some dryer manufacturers publish a longer allowance for their own blowers, so read the manual before assuming you are over. Past the limit, the answers are a shorter route, a straighter route, or a booster fan.
Can I use flexible foil or plastic duct?
Not inside the wall. Codes require smooth rigid metal for concealed dryer ducts because ribbed foil catches lint in every corrugation, sags between supports, and in the vinyl version actually burns. The one flexible piece allowed is a listed transition hose between the dryer and the wall boot, limited to 8 feet, never concealed, and never crushed flat behind the machine. Pulling the dryer out 4 inches from the wall protects that hose more than anything else you can do.
Can a dryer vent into the garage or an indoor lint box?
No. A full load releases a couple of gallons of water, and an indoor vent kit dumps that moisture plus fine lint straight into the room. In a garage, attic, or crawlspace that becomes a mold and rot problem within a season. With a gas dryer it is also a carbon monoxide problem, because the exhaust carries combustion gases. Every dryer duct has to terminate outdoors, with the hood clear of windows, soffit intakes, and walkways.