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True Trade Path

Diagnostic guide

Bathroom Fan Not Venting

A bathroom exhaust fan that runs but does not move air — moisture stays in the room and mold follows within weeks.

System
Ventilation
Causes covered
7
You can check
3 without a technician

Stop and read this first

  • Turn off power at the breaker before removing the fan grille or working on wiring.
  • A fan venting into an attic is a mold and structural rot hazard — correct it promptly.
  • Use a GFCI-protected circuit for bathroom fans — required by NEC in most jurisdictions.

Work through these in order

Ordered from most to least likely. Working top down means you rule out the cheap, common causes before paying anyone to look at the expensive, rare ones.

  1. Clogged with dust and lint

    CommonYou can check
    How to check
    Fan runs but little airflow at the grille. Cover has not been cleaned in years.
    The fix
    Turn off power, remove the grille, and vacuum the fan housing and blades. Clean the duct opening.

  2. Disconnected or crushed duct

    CommonYou can check
    How to check
    Fan runs but no air exits the exterior vent cap. Duct in the attic is disconnected or flattened.
    The fix
    Reconnect the duct with foil tape (not cloth duct tape) and support it so it does not sag. Replace crushed flex duct ($20 to $50 in materials).

  3. Blocked exterior vent cap

    CommonYou can check
    How to check
    Exterior cap is painted shut, blocked by insulation, or covered by a bird nest.
    The fix
    Clear the cap and verify the damper opens freely. Install a pest screen if birds are nesting.

  4. Duct too long or too many bends

    PossibleNeeds a pro
    How to check
    Fan is weak despite being clean. Duct run exceeds 25 feet or has more than three 90-degree bends.
    The fix
    Shorten the duct run or replace with rigid metal duct. Each bend reduces airflow by 10 to 25%.

  5. Undersized fan for the room

    PossibleNeeds a pro
    How to check
    Fan is rated below 50 CFM and the bathroom is larger than 50 square feet.
    The fix
    Replace with a fan rated at 1 CFM per square foot of bathroom floor area. Installation runs $150 to $300.

  6. Failed fan motor

    PossibleNeeds a pro
    How to check
    Fan hums but blades do not spin, or spins very slowly. Unit is over 10 years old.
    The fix
    Replace the fan unit ($50 to $200 for the fan, $150 to $300 installed). Motor-only replacement is rarely economical.

  7. Venting into attic instead of outside

    UncommonNeeds a pro
    How to check
    Duct terminates in the attic space rather than through the roof or soffit. Attic insulation is damp above the bathroom.
    The fix
    Extend the duct to an exterior termination. Venting into an attic causes mold and rot — this is a code violation in every U.S. jurisdiction.

Once you know what it is

Step one

Know what it should cost

Get the range before you call, so the first number you hear is not the only number you have.

Step two

Decide repair or replace

On equipment near end of life, the repair is usually the first of several.

Step three

Check the quote

Compare against the typical range and confirm the terms before you sign.

Ceiling Fan Installation cost guideRepair or replace calculatorEstimate repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms