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Bathroom fan runs but won't clear steam/moisture

A fan that hums away without clearing the mirror is annoying, but it's almost always a cleaning or a duct fix — not a sign the whole thing needs tearing out.

tuneWhat are you noticing?
Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.
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In Canada, start with the cheapest fix. Confirm the fan is genuinely weak by holding a single square of toilet paper to the grille — it should stick firmly. Then pop the cover off: if the grille and the little fan wheel behind it are furred with grey dust and lint, that alone can choke a fan down to almost nothing. A vacuum and a damp cloth often restore most of the airflow in ten minutes.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order
Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

  1. With the fan running, hold a single square of toilet paper to the grille. It should stick firmly and stay put. If it flutters or falls, airflow is weak — keep going.
  2. Pull the grille cover down (most squeeze or pinch off two spring clips) and look at the underside of the grille and the fan wheel behind it. A grey, felted coat of dust and lint is the usual culprit.
  3. Wipe or vacuum a small patch clean and run the fan again with the toilet-paper test. A noticeable jump in pull confirms it was mostly dirt — clean the rest and re-check.
buildNo tools schedule10 min

handymanHow to fix it

Take the grille off, vacuum and wipe the grille and fan wheel clean of dust and lint, and put it back. No parts, no cost beyond a rag.

  1. Turn the fan off. Pull the grille down — most cases pinch two spring clips together to release; it stays tethered by the clips.
  2. Vacuum the grille and the fan wheel with a brush attachment, then wipe both with a damp cloth. A soft toothbrush gets the caked lint out from between the blades.
  3. If the wheel is thick with grime, unplug the fan motor at its little in-housing plug (if fitted) and wash the wheel separately; let it fully dry before it goes back.
  4. Clip the grille back on, run the fan, and re-do the toilet-paper test. Firm stick means you've got the airflow back.
schedule20 min savings$0–5
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Cleaning a fan grille, re-connecting or replacing exhaust duct, and clearing an exterior vent cap are routine homeowner maintenance in every province — no permit or licence.

visibilityHow to check

  1. Find the exterior cap for the bath fan — a small hood with a flapper on an outside wall or the roof, roughly in line with the bathroom.
  2. From the ground or another safe vantage, have someone run the fan while you watch the flapper. It should swing open with the airflow and fall shut when the fan stops.
  3. A flapper that never opens, or a hood you can see is packed with lint, leaves, or a bird or wasp nest, is the blockage. In deep winter a damper can also freeze shut with frost.
buildNo tools schedule10 min

handymanHow to fix it

warning Ladders and roofs are the real risk hereThe blockage is harmless; the height is not. A wall cap you can reach from a stable ladder is a fine DIY job — footed on level ground, someone steadying it, not overreaching. A roof cap, a steep or icy roof, or anything above the first storey is where people get badly hurt: that's a job for a roofer or ventilation contractor, and it's a quick one for them. Also treat old bird droppings and nests as a mild health hazard — wear a mask and gloves and don't stir up the dust.

Clear the cap of lint, debris, and any nest, free or thaw the flapper so it swings, and replace the cap if the damper is broken. If the cap is on the roof, this is often better left to a pro.

  1. Reach the cap safely (see the gate before you go up a ladder or onto the roof). Pull out lint, leaves, and any bird or wasp nest by hand — gloves and a mask on, and check a bird nest is inactive first.
  2. Work the flapper by hand. It should swing freely and fall shut on its own. Clean grime off its hinge; if it's frozen, let it thaw rather than forcing it.
  3. If the damper is cracked, seized, or missing, swap the whole cap for a matching one — they're inexpensive and clip or screw on.
  4. Run the fan and confirm the flapper now opens with the airflow and closes after.
schedule30 min savings$0–40
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Cleaning a fan grille, re-connecting or replacing exhaust duct, and clearing an exterior vent cap are routine homeowner maintenance in every province — no permit or licence.

visibilityHow to check

warning Move carefully in the atticAttics are unforgiving: step only on the joists or a laid board, never the ceiling drywall between them, and mind low roof nails and wiring. Wear a dust mask and watch for old vermiculite insulation, which can contain asbestos — if you see it, back out and get advice before disturbing it. If the attic is cramped, unfloored, or you're not sure-footed up there, this inspection is a fair one to hand to a ventilation contractor.
  1. If you can get into the attic safely, find the fan housing from above and follow its duct with a flashlight.
  2. Look for the duct pulled off the housing collar, kinked, pinched flat under insulation, sagging into a low U that traps water, or run so long and looping that a small fan can't push through it.
  3. Confirm where it ends: it must terminate at an exterior wall or roof cap, never blowing loose into the attic. A duct dumping into a Canadian attic in January turns to frost, then a spring-thaw "roof leak."
  4. A duct that's loose, crushed, kinked, or dead-ending in the attic is the cause — re-seating and straightening it, or shortening the run, restores the airflow.
buildflashlight schedule15 min

handymanHow to fix it

Re-connect the duct to the fan and the cap, cut the run as short and straight as practical, support it so it can't sag or crush, and seal the joints with foil tape and clamps.

  1. Re-seat the duct on the fan housing collar and on the exterior cap collar, and lock each end with a clamp or zip tie, then wrap the joint with foil tape (not cloth "duct tape," which dries out and lets go).
  2. Pull the run as short and straight as you can. Remove needless loops and slack; a fan pushes far better through a short, smooth path than a long, ribbed, sagging one.
  3. Support the duct with a strap or two so it can't drop into a low spot where condensation would pool and freeze, and lift it clear of anything crushing it.
  4. Insulated flex duct in the cold attic run cuts the condensation that drips back and stains the ceiling. Re-check airflow at the grille when you're done.
schedule60 min savings$15–60
Foil (aluminum) duct tapeUL 181 foil tape — do not use fabric "duct tape" on the joints Duct clamps or zip tiesTo lock the duct to the fan and cap collars Insulated flexible duct (4 in.)Match the fan's outlet size — most bath fans are 4 inch; some larger fans are 6 inch
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Cleaning a fan grille, re-connecting or replacing exhaust duct, and clearing an exterior vent cap are routine homeowner maintenance in every province — no permit or licence.

visibilityHow to check

  1. Measure the bathroom's floor area in square feet (length × width).
  2. Find the fan's airflow rating in CFM — printed on the grille, on a sticker inside the housing, or in the model's spec sheet.
  3. Compare: a common rule of thumb is roughly 1 CFM per square foot of bathroom, with about 50 CFM a typical minimum, and more for larger rooms, high ceilings, or an enclosed tub/shower. (Confirm the exact figures against current NBC 9.32 and HVI sizing guidance.)
  4. If the fan is well below what the room calls for — and the grille is clean and the duct clear — the fan is simply undersized and needs to be replaced with a larger one.
buildtape-measure schedule15 min

handymanHow to fix it

warning Kill the breaker before you touch the wiringA bath fan is wired into a 120 V circuit. Switch the breaker off and confirm the fan is dead with a voltage tester before you disconnect anything — the wall switch alone is not enough. In Quebec, homeowner electrical work is licensed-only; in Ontario an electrical permit and inspection apply; other provinces allow owner work on your own home. If the wiring is at all unclear, or your province requires it, have a licensed electrician make the connection — it's a short visit.

Replace the fan with a correctly sized, quieter unit. The swap is a hardwired 120 V connection and, on some models, a larger duct — electrical work whose legality varies by province.

  1. Choose a fan sized for the room (see the sizing check) and, ideally, a low sone rating so it's quiet enough that people actually run it.
  2. Kill power to the fan at the breaker and confirm it's dead at the fan with a tester before touching any wiring (see the gate).
  3. Fit the new housing, connect the duct — step up to the larger duct size if the new fan calls for it — and make the 120 V connections to match the old wiring.
  4. Restore power, test, and check the airflow at the grille and the flapper outside. If any part of the wiring is beyond your comfort or your province requires a licensed electrician, hand the electrical connection off.
schedule90 min savings$60–200
Replacement bathroom exhaust fanSized to the room in CFM and, ideally, a low sone rating; match or step up the duct size
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: permit + inspection required · PE: homeowner may do this · QC: licensed professionals only
Most provinces allow a homeowner to do electrical work on their own home; the mechanical/duct side is unrestricted. Confirm the local rule before publishing.

handymanHow to fix it

warning Confirm it's dead before you reach inEven to swap a plug-in motor module you'll have your hands in the fan housing, where the 120 V connections live. Turn the circuit off at the breaker and verify with a tester at the fan — not just the wall switch. If you smelled burning before this, treat the wiring as suspect and have a licensed electrician look before reusing the fan. Quebec restricts homeowner electrical work to licensed electricians; Ontario requires an electrical permit; elsewhere owner work on your own home is generally allowed.

On many fans the motor/wheel assembly plugs out and swaps in minutes; where it doesn't, replace the whole fan. Either way the housing's 120 V connection is electrical work whose legality varies by province.

  1. Kill power at the breaker and confirm the fan is dead with a tester (see the gate).
  2. Drop the grille and look for a plug-in motor/wheel module — many modern fans let you unplug and lift the whole assembly out, then drop a matching one in with no wiring.
  3. If the motor isn't a replaceable module, or the part isn't available, replace the whole fan unit — often the better value and a chance to right-size and quiet it at the same time.
  4. Restore power and test. If replacing the unit means new wiring connections and your comfort or your province's rules say so, hand the electrical work to a licensed electrician.
schedule60 min savings$30–200
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: permit + inspection required · PE: homeowner may do this · QC: licensed professionals only
Most provinces allow a homeowner to do electrical work on their own home; the mechanical/duct side is unrestricted. Confirm the local rule before publishing.
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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.