Bathroom Exhaust Fan Not Working, Noisy or Weak? How to Fix, Size and Vent It
Most bathroom exhaust fan problems are a seized motor, a stuck damper, a blocked or disconnected duct, or a fan that was never sized or vented properly. Here is how to diagnose yours, when to replace the motor or the whole fan, how many CFM you need, and where the fan must vent.
Still stuck after this guide? You can ask an HVAC technician online — see pricing, expert credentials and common questions first.
The Short Answer
If your bathroom exhaust fan is not working, check the breaker and any GFCI outlet first, then pull the grille and see whether the blades spin freely by hand. A fan that hums but will not turn has a seized motor that usually needs replacing. A fan that runs but does not pull air has a stuck damper, a crushed or disconnected duct, or a blocked outside cap. Rattling usually comes from a loose grille, housing or damper.
A bathroom fan must vent to the outdoors, never into the attic. For sizing, the Home Ventilating Institute (HVI) recommends about 1 CFM per square foot of floor for bathrooms up to 100 square feet, and running the fan for about 20 minutes after a shower.
Why Is My Bathroom Exhaust Fan Not Working?
| What you notice | Most likely cause | What to do |
|---|---|---|
| Completely dead, no sound | Tripped breaker or GFCI, failed wall switch, timer or humidity sensor, motor unplugged inside the housing, or a burned-out motor | Reset the breaker and any GFCI (see how to reset a GFCI). Check that the motor's plug is seated in the housing. Then test the motor. |
| Hums but the blades do not turn, or start slowly | Dust-packed or dry, seized motor bearings | Power off and spin the wheel by hand. If it is stiff, clean it. If it stays stiff or keeps stalling, replace the motor. |
| Runs, but the mirror still fogs and tissue will not stick to the grille | Backdraft damper stuck shut, duct crushed, kinked or disconnected in the attic, outside cap blocked by lint, a nest or snow, or the fan is too small | Check the damper, then follow the duct to the outside and look at the exterior cap |
| Rattling or vibration | Loose grille springs, loose housing screws, damper flapping, or a cracked blower wheel | Tighten the housing, bend the grille springs for a snug fit, clean or replace the wheel |
| Grinding, squealing or a loud whine | Worn motor bearings | Replace the motor or the fan. Oiling usually buys little time on sealed bath fan motors. |
| Water dripping from the grille | Moist air condensing inside an uninsulated duct in a cold attic, a duct that slopes back toward the fan, or a fan venting into the attic | Use insulated duct, slope it toward the outside, keep the run short, and make sure it actually ends outdoors |
| Fan runs by itself | Humidity sensor or timer doing its job, or a faulty control | Check the control settings in the manual before assuming a wiring problem |
How Do You Test and Clean a Bathroom Fan?
- Turn off the breaker for the fan. A wall switch alone is not enough, because you will be touching the motor.
- Remove the grille. Pull it down an inch or two and squeeze the two wire springs to release it.
- Vacuum the grille, housing and blower wheel with a brush attachment. Wash the grille in warm soapy water.
- Spin the wheel by hand. It should coast freely. Stiff, gritty or wobbly means the motor is on its way out.
- Check the damper. The flap where the duct connects should open easily and fall closed. Free it if it is stuck with dust or paint.
- Do the tissue test after restoring power: hold a single square of toilet paper to the grille. A working fan holds it in place.
- If suction is weak, inspect the duct. From the attic, check that the duct is connected at both ends, is not crushed or sagging into a low loop, and ends at a roof or wall cap. Outside, make sure the cap's flap opens when the fan runs.
Cleaning the grille and housing once or twice a year keeps a fan quieter and helps the motor last.
Should You Replace the Motor or the Whole Fan?
| Option | When it makes sense | How hard |
|---|---|---|
| Replacement motor or motor-and-wheel assembly | The housing is sound and the maker still sells the part for your model | Easy from below. Find the model number on the label inside the housing and order the matching part. |
| Upgrade or retrofit kit | Several fan makers sell kits that fit into existing housings of certain brands and sizes, often quieter than the original | Easy to moderate from below. Check the kit's compatibility list against your housing. |
| Whole new fan | The housing is rusty or damaged, the fan is too small or too loud for the room, or you want a humidity sensor or light | Moderate. Often easiest with attic access. A larger housing may mean enlarging the ceiling hole and reconnecting the duct. |
Basic steps for a full replacement: turn off the breaker, remove the grille and motor, disconnect the wiring inside the housing's junction box (note which wire went where), detach the duct, unscrew the housing from the joist, and install the new housing at the same or better duct size. Connect the duct with foil tape and a clamp, make the wire connections inside the new junction box including the ground, then reinstall the motor and grille. If you are adding a light, a heater or a humidity sensor, the wiring and the circuit's capacity change; that is a good time to check with an electrician.
A fan installed over a tub or shower must be listed for that location, and many makers require GFCI protection there. The label and instructions will say so.
What Size Bathroom Exhaust Fan Do I Need?
Fans are rated in CFM (cubic feet of air per minute). HVI, the industry group that certifies fan performance, gives two methods:
- Bathrooms up to 100 square feet: about 1 CFM per square foot of floor area. An 8 by 5 foot bathroom is 40 square feet, so 40 CFM or more.
- Bathrooms over 100 square feet: add up the fixtures. 50 CFM each for a toilet, a shower and a bathtub, and 100 CFM for a jetted tub.
There is also a code floor. The International Residential Code and ASHRAE Standard 62.2 call for at least 50 CFM for a bathroom fan that runs on demand, or 20 CFM if it runs continuously.
| Bathroom | Floor area | Suggested fan |
|---|---|---|
| 5 x 8 ft hall bath | 40 sq ft | 50 CFM (the code minimum is higher than 40) |
| 8 x 10 ft bath | 80 sq ft | 80 CFM |
| 10 x 10 ft bath | 100 sq ft | 100 CFM |
| Large primary bath with toilet, shower and tub | Over 100 sq ft | 150 CFM (50 + 50 + 50) |
| Large bath with toilet, shower and jetted tub | Over 100 sq ft | 200 CFM (50 + 50 + 100) |
Noise: fans are rated in sones. Building-science guidance tied to ASHRAE 62.2 calls for 3 sones or less on a fan that runs on demand and 1 sone or less on one that runs continuously. A quieter fan is a fan people actually leave on.
Real airflow is lower than the box says when the duct is long, narrow or full of elbows. Keep the duct the same size as the fan's outlet or larger, and as short and straight as you can. An enclosed toilet room needs its own fan or an opening window.
Where Does a Bathroom Fan Have to Vent?
Outdoors. The International Residential Code requires bathroom exhaust to discharge directly outside, and it does not allow it to end in an attic, a crawl space or other spaces inside the building. Venting into the attic puts warm, wet air right where it can condense on the roof sheathing and grow mold.
- Good terminations: a roof cap or wall cap with a backdraft flap, sized to the duct.
- Not acceptable in the model code: blowing the duct at a soffit vent or ridge vent, or leaving it lying loose in the insulation. Some local codes have their own rules for soffit terminations, so check with your building department.
- Duct: smooth metal duct moves the most air; insulated flexible duct is common in attics. In a cold attic, insulate the duct to stop condensation, and slope it so any water drains toward the outside.
- Keep it away from openings: do not end the duct right next to a window, a door or an air intake.
If your fan turns out to be venting into the attic, fixing that is worth doing even if the fan itself works. Check the attic sheathing near the fan for dark staining while you are up there, and see our guide to mold in the house if you find it.
How Long Should You Run a Bathroom Fan?
HVI recommends leaving the fan on for about 20 minutes after you finish using the bathroom. Most people switch it off on the way out, which is why a timer switch or a humidity-sensing fan or control is one of the cheapest moisture fixes in a house. Black spots on the caulk or ceiling are often the first sign the fan is not running long enough or is not moving enough air; see bathroom caulk turning black.
Safety and When to Call a Pro
Do it yourself: cleaning, the tissue test, replacing a grille, a motor or a like-for-like fan on existing wiring, and reconnecting a loose duct.
Call an electrician for a new fan where there was none, adding a light or heater, a fan that trips the breaker, wiring with no ground, aluminum wiring, or a fan over a tub or shower without GFCI protection. Call a roofer or a general contractor for cutting a new roof penetration.
Always turn off the breaker before opening the fan, and watch your footing in the attic: step only on joists.
Not sure whether your fan is undersized or your duct is the problem? Send a photo of the fan label and the attic run to a home repair expert.
Notes for Canada, the UK and Australia
- Canada: bathroom fans must also exhaust to the outdoors, and the same sizing logic applies. Provincial codes set the exact rates.
- UK: extract rates are set by Approved Document F (15 litres per second for an intermittent bathroom fan). Bathrooms are a special location under the wiring regulations: a fan inside the zones around a bath or shower needs a suitable IP rating, and new circuits there fall under Part P notification in England.
- Australia: the fan connection is electrical work that must be done by a licensed electrician.
Pro Tips
Frequently Asked Questions
Why is my bathroom fan humming but not spinning?
The motor is getting power but the bearings are seized or packed with dust. Turn off the breaker and try spinning the blower wheel by hand. If cleaning does not free it, replace the motor or the fan. Leaving a stalled motor powered can overheat it.
How do I know if my bathroom fan is working?
Turn it on and hold a single square of toilet paper to the grille. A working fan holds the paper against the grille. If it falls, check the damper, the duct and the outside cap.
How many CFM do I need for my bathroom?
HVI recommends about 1 CFM per square foot for bathrooms up to 100 square feet, and at least the 50 CFM code minimum for a fan that runs on demand. For larger bathrooms, add 50 CFM each for the toilet, shower and tub, and 100 CFM for a jetted tub.
Can a bathroom fan vent into the attic?
No. The International Residential Code requires bathroom exhaust to go directly outdoors. Venting into the attic puts moist air where it condenses on the roof framing and can cause mold and rot.
Why is my bathroom fan so loud?
Rattling usually means a loose grille, housing or damper. Grinding or whining usually means worn motor bearings. A fan with a high sone rating, or one pushing air through a long or undersized duct, is loud even when healthy.
Why is water dripping from my bathroom fan?
Usually condensation. Warm, wet air cools inside an uninsulated duct in a cold attic and runs back down to the fan. Insulate the duct, slope it toward the outside, and confirm it actually ends outdoors.
How long should a bathroom fan run after a shower?
About 20 minutes, according to HVI. A timer switch or a humidity-sensing control makes that automatic.
