Bathroom Exhaust Fan Not Working? Clean It or Replace It
A weak or dead bathroom fan traps moisture and grows mold. Test it, clean the buildup, fix common causes, or swap the unit — with CFM sizing and a step-by-step guide.
Your bathroom fan has the most thankless job in the house: every shower, it's supposed to pull a roomful of warm, wet air out before that moisture settles into your walls, ceiling, grout, and window frames. When it stops doing that — quietly, because almost nobody notices a fan that's only half working — the humidity has nowhere to go, and within weeks you start seeing the signs: peeling paint, black speckling in the corners, a mirror that won't clear, that faint musty smell that no amount of cleaning fixes.
The good news: a fan that "isn't working" is dead far less often than it's just dirty. Most of the time this is a free, fifteen-minute fix. This guide shows you how to tell which problem you actually have, clean a fan back to full strength, fix the common causes, and know when it's genuinely time for a new unit.
First, figure out which problem you have
"Not working" covers four very different faults, and the fix for each is different. Spend two minutes here and you'll save yourself from buying a fan you didn't need — or cleaning one that's actually dead.
| What you observe | What it usually means | First move |
|---|---|---|
| Spins, but weak or no suction | Dust on the blades (most common) | Deep-clean it |
| Spins fine but room stays humid | Clogged/crushed duct, or venting into attic | Inspect the duct run |
| Hums but won't spin | Seized or failing motor | Replace the motor/unit |
| Dead silent, no movement | No power — switch, wiring, or motor | Check switch & breaker |
The tissue test (do this first)
Turn the fan on and hold a single square of toilet paper flat against the grille. A fan in good health pulls the sheet up and holds it with no hands. If it flutters and falls, or only barely clings, the fan isn't moving enough air — and nine times out of ten that's dust, not a dead motor. This one test stops most people from over-diagnosing.
How to clean a bathroom exhaust fan (the fix for most cases)
Bathroom air is humid, and humid air makes dust sticky. Over months it bakes onto the blades and housing as a gummy film that a quick vacuum won't touch — and a coated blade is heavy, unbalanced, and slow. Cleaning is the highest-value thing you can do, and it costs nothing.
Clean it in 15 minutes
Free, twice a year, restores most fans
- Switch the fan off (and the breaker, if removing the cover)
- Pinch the spring clips behind the grille and lower it out
- Wash the grille in warm soapy water; let it dry
- Vacuum dust off the blades and inside the housing
- Wipe the blades with a damp cloth — this removes the sticky film a vacuum leaves
- Snap the grille back, restore power, re-test with the tissue
Don't do this
Common mistakes that cause damage or hide the problem
- Don't spray water or cleaner into the motor
- Don't pull the grille with the breaker live if you're reaching inside
- Don't skip the damp wipe — vacuuming alone leaves the slowing film behind
- Don't assume it's dead before you've cleaned and checked the duct
- Don't ignore a fan that vents into the attic — that's a mold source, not a quirk
A clean fan that passes the tissue test is fixed — you're done. If it still won't pull air, the problem is downstream of the fan: the duct.
Check the duct — including where it actually goes
A perfectly good fan can't move air through a blocked or badly routed duct. With the cover off, look behind the fan housing for:
- A crushed or kinked flex duct — the corrugated hose is easily pinched against a joist or stepped on in the attic.
- A lint and dust clog inside the duct, or a backdraft damper stuck shut with debris so air can't push past it.
- A disconnected joint that's been blowing humid air straight into the wall or attic cavity.
- Too long a run with too many bends — the DOE recommends the most direct route to the outside with as few elbows as possible.
And the big one, worth checking even if everything else is fine: where does the duct terminate? A startling number of bathroom fans empty into the attic or crawlspace instead of outside. That warm, moist air hits cold roof sheathing, condenses, and quietly grows the kind of attic mold and wood rot that turns into a four-figure repair. The U.S. Department of Energy's guidance is unambiguous — a bath fan must vent to the outdoors, not into an attic, crawlspace, or between floors. If yours doesn't, extending the duct to a proper roof or wall cap is the single most valuable fix in this whole guide. (Related: attic condensation and how to prevent mold.)
"It hums but won't spin" or "it's totally dead"
If cleaning and the duct check don't apply because the fan won't run at all, the diagnosis splits cleanly:
- Humming with no rotation means power is reaching the motor but the motor is seized. With the power off, try spinning the blade by hand — a stiff or stuck blade confirms it. Seized motors don't recover; plan to replace the motor cartridge or the whole unit.
- Total silence means power isn't getting there. Confirm the wall switch works (a failed fan/light combo switch is common), check the breaker hasn't tripped, and — only if you're comfortable and the power is off — verify the wiring connection at the fan. If power is present and the motor still won't turn, the motor has failed.
Because bathroom outlets and circuits are GFCI-protected wet locations, also rule out a tripped protector: if the fan died alongside an outlet, see why a GFCI keeps tripping and how to test a GFCI outlet before assuming the fan itself is at fault. While you're in there, it's worth a quick look at nearby outlets and cords for damage.
Repair or replace? And what to buy
Replacement is cheap, fast, and a real upgrade — modern fans are far quieter and often add a light, humidity sensor, or built-in timer. Here's the honest math:
| Task | How often | DIY cost | Pro cost | Prevents |
|---|---|---|---|---|
| Deep-clean the existing fan | Every 6 months | $0 | — | 90% of weak-fan complaints |
| Replace the motor cartridge only | As needed | $15–30 | $120–200 | Buying a whole new unit |
| Replace the entire fan unit | Every 10+ yrs | $30–120 | $150–400 | Mold, noise, weak airflow |
| Add a timer or humidity-sensing switch | Once | $20–40 | $120–200 | Lingering post-shower moisture |
| Extend a duct that vents into the attic | Once | $30–60 | $200–500 | Attic mold & sheathing rot |
Clean first — most fans come back for free. Replace when a clean fan with a clear duct still won't pull air, when the motor is seized or humming, when it's painfully loud, or when it's simply undersized.
Sizing a new fan: CFM and sones
Two numbers matter, and buying for them is the difference between a fan you run and one you never touch:
| Spec | What it means | Target |
|---|---|---|
| CFM (airflow) | Cubic feet of air moved per minute | 1 CFM per sq ft, 50 CFM minimum (≤100 sq ft) |
| Sones (noise) | How loud the fan is | ≤1.5 sones = quiet; 4+ = loud |
For a bathroom up to ~100 sq ft, multiply length × width and round up — a 7×9 (63 sq ft) room wants at least a 70 CFM fan. Bigger bathrooms are sized by fixture: roughly 50 CFM each for the toilet, shower, and tub, plus 100 CFM for a jetted tub. Look for the HVI-certified rating on the box, and don't ignore the sone number — a quiet fan is one you'll actually leave running, and running it is the whole point.
The 20-minute rule: Leave the fan on for about 20 minutes after you finish a shower, not just during it. That's how long it takes to clear the humidity that's still hanging in the air and condensing inside the duct. A cheap timer or humidity-sensing switch does this for you automatically — one of the best small upgrades you can make against window condensation and bathroom mold.
A working fan is mold prevention you can automate
A bathroom exhaust fan is the cheapest mold-prevention device in your home — but only if it's actually moving air, venting outside, and running long enough after each shower. All three quietly fail over time, and none of them announce it. The fan still spins, so you assume it's fine, while the room slowly gets damper.
The fix is a routine, not a one-time repair: a twice-a-year cleaning, a periodic suction test, and a glance at where the duct goes. Build your free Owner Tools manual and we'll put the bath-fan cleaning right into your moisture-prevention schedule, alongside the other small habits — gutter checks, humidity control, and seasonal energy-and-air sealing — that keep mold from ever getting started. Sorted into what's critical, what saves money, and what can wait. No login, no address required.