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The short answer
Lint that builds up in a tumble dryer, its filter or its exhaust duct blocks the flow of air, makes the dryer overheat and can start a fire. Clean the lint filter before or after every load, and clean the vent duct once a year, or sooner if clothes take longer than normal to dry. If you have a gas dryer, or the duct is out of reach, have a professional do it.
The warning signs the US safety agencies describe:
Clothes still damp at the end of a normal drying cycle
Drying taking longer than it used to
Little or no exhaust air coming out of the outside vent while the dryer runs
Outside vent flap that stays shut when the dryer is on
Why it happens
The US Consumer Product Safety Commission explains the mechanism in its safety alert Overheated Clothes Dryers Can Cause Fires: fires can occur when lint builds up in the dryer or in the exhaust duct, because lint can block the flow of air, cause excessive heat build-up and result in a fire in some dryers.
The National Fire Protection Association's Clothes Dryer Safety tip sheet states that the leading cause of home clothes dryer fires is failure to clean them, and the US Fire Administration's Clothes Dryer Fire Safety leaflet says you are at higher risk of a dryer fire if you do not clean your lint filter and dryer vents.
The type of duct matters. The CPSC says most manufacturers specify rigid or corrugated semi-rigid metal duct, which provides maximum airflow, while flexible plastic or foil duct traps lint more easily and is more prone to kinks and crushing that greatly reduce airflow.
How to fix it
- Read your dryer's manual first. The NFPA asks you to follow the manufacturer's operating instructions. Use the manual for how to switch the dryer off and disconnect it safely before you move it or touch the duct. If you have a gas dryer, do not disconnect or move it yourself: the NFPA says gas dryers should be inspected by a qualified professional to make sure the gas line and connection are intact and free of leaks.
- Clean the lint filter every load. The USFA and NFPA say never to run the dryer without a lint filter. The NFPA says to clean it before or after each load; the USFA says before and after each cycle. The NFPA also says to remove lint that has collected around the drum.
- Clean behind the dryer. The USFA and CPSC both point to the back of the dryer as a place where lint builds up. Keep the area around the dryer clear: the NFPA lists boxes, cleaning products and clothing as things that can burn.
- Inspect the duct. The USFA says to check the venting system behind the dryer to make sure it is not damaged, crushed or restricted.
- Clear the duct. The CPSC says that to remove a blockage in the exhaust path it may be necessary to disconnect the exhaust duct from the dryer. The NFPA says to clean lint out of the vent pipe once a year, or more often if drying takes longer than normal, or to have a dryer lint removal service do it for you. If the duct runs through a wall, a ceiling or to an outlet you cannot reach from the ground, use a service rather than a ladder.
- Reconnect before use. The CPSC reminds you to reconnect the ducting to the dryer and to the outside vent before using the dryer again.
- Replace plastic or foil duct. Swap accordion-type plastic or foil duct for rigid or corrugated semi-rigid metal duct, as the CPSC recommends, or have the dryer installer do it. The USFA and NFPA both advise having your dryer installed and serviced by a professional.
- Leave the inside to a service person. The CPSC recommends having a qualified service person clean the interior of the dryer chassis periodically. Do not open the dryer casing yourself.
Check that it worked
With everything reconnected, run the dryer and check the outside vent. The CPSC says exhaust air should be escaping while the dryer runs; if it is not, the vent or duct may still be blocked. The USFA and NFPA add that the outdoor vent flap or covering should open when the dryer is operating, and the NFPA says to make sure it is not covered by snow. Over the next few loads, drying times should return to normal.
Habits that lower the risk
The NFPA tip sheet adds a few habits that go with a clean vent:
- Do not overload the dryer.
- Turn the dryer off if you leave home or when you go to bed.
- Clothes that have been in contact with flammable substances such as petrol or paint thinner should be laid outside to dry first, then washed and dried as usual.
The CPSC adds that clothes soiled with cooking oils, cleaning agents, finishing oils or stains need special care: wash them more than once if possible and preferably hang them to dry. If you do use the dryer, choose the lowest heat and a cycle with a cool-down period, and do not leave the dried clothes sitting in the dryer or piled in a laundry basket.
This guidance applies to a dryer that vents its exhaust air through a duct. Error codes and faults specific to your dryer brand are covered by Appliances, our sister publication. A clean filter and a clear, metal duct are the two things all three safety bodies return to; when the job means gas fittings, the inside of the machine or climbing to reach a vent, it belongs to a professional.
Questions about this page
How often should a dryer vent be cleaned?
The NFPA says to clean lint out of the vent pipe once a year, or more often if clothes are taking longer than normal to dry, or to have a dryer lint removal service do it for you. The lint filter itself should be cleaned before or after every load.
What are the signs of a blocked dryer vent?
The US CPSC says clothes still damp at the end of a normal cycle, or drying that takes longer than usual, may mean the lint screen or the exhaust duct is blocked. Another check is whether exhaust air comes out of the outside vent while the dryer runs.
Is a plastic or foil dryer hose safe?
The CPSC advises replacing plastic or foil accordion-type ducting with rigid or corrugated semi-rigid metal duct, which most manufacturers specify. It says the flexible plastic or foil type traps lint more easily and kinks or crushes more readily.