The Dryer Vent In Winter
A dryer puts several litres of warm water vapour down a pipe to the outside, and in a Toronto January the far end of that pipe is well below freezing.
Short answer
Moist exhaust air condenses in the cold end of the duct and at the hood, and then freezes. The ice traps lint, the restriction gets worse, and eventually the dryer cannot exhaust at all. The fixes are a short straight run, a properly working damper, insulation on any duct in an unheated space, and keeping the hood clear.
Why it ices up
The exhaust is warm and very moist, and the last part of the duct and the hood are at outdoor temperature.
Moisture condenses there, mixes with lint, and freezes into a plug.
A long duct run, a crushed flexible section or several elbows all slow the air and give it more time to cool and condense.
A damper that does not close lets cold air back up the duct between loads, which chills the duct further.
A damper that does not open fully restricts the flow.
And an exterior hood screened with fine mesh collects lint and ice together, which is why fine mesh on a dryer hood is the wrong detail.
Longer drying times are the warning
A load that used to take forty minutes and now takes ninety is a restricted vent, and in winter that restriction is frequently ice. It is also the point at which the dryer is running much hotter than it should, which is the fire risk. Treat a change in drying time as the symptom rather than as the machine getting old.
What to change
Shorten and straighten the run as much as the layout allows, and use rigid metal rather than flexible foil.
Insulate any section running through an unheated space — a garage, a crawlspace, a cold basement — which reduces condensation substantially.
Fit a proper hood with a large damper flap and no fine mesh screen.
Keep the hood clear of snow, which in Toronto means checking it after a storm and after shovelling near it.
Clean the duct annually, and more often with a long run or a heavily used laundry.
And check the damper opens and closes, which is a two-second look from outside with the dryer running.
What not to do
Do not vent into the garage, the crawlspace, the attic or the basement to avoid the cold. That puts litres of water into the building every load.
Do not fit a lint trap box in place of venting outside, which is a last resort for a situation with no exterior route and is not a solution.
Do not use flexible foil duct, which collects lint and crushes.
Do not screen the hood with fine mesh.
Do not use a longer run than necessary to reach a tidier exit point.
And do not ignore the drying time, which is the only early warning there is.
Common questions
Why does my dryer vent freeze?
Moist exhaust condenses at the cold end and freezes with lint into a plug.
What is the first sign?
Longer drying times. That is also when the dryer is running hotter than it should.
Can I vent into the garage instead?
No. That puts litres of water into the building every load.