Bathroom fan not clearing steam? When to repair or replace it

If the mirror stays fogged long after a shower, the bathroom fan isn’t moving enough air. Most of the time the cause is a dirty fan, a duct that is crushed, too long or blocked, or a fan that is simply too small for the room. Hold a single square of toilet paper against the grille while the fan runs. If the fan can’t hold it in place, it needs cleaning, a duct repair or a replacement.

Why a bathroom fan matters in an Ontario winter

A hot shower puts a lot of water into the air. When the fan doesn’t take it outside, it settles on the coldest surfaces in the room. That means the mirror, the window, the ceiling corners and the outside wall. Over a winter that shows up as peeling paint, black spots in the caulking and mildew on the ceiling. It also adds to the condensation on your windows in the rest of the house.

Five things to check before you replace it

1. The grille and fan blades. Turn the power off at the breaker, take the grille down and wash it. Vacuum the dust off the blades and the housing. A thick layer of dust can cut the airflow a lot.
2. The damper. Behind the fan there is a small flap that opens when the fan runs. If it is stuck closed, the fan spins but moves almost nothing.
3. Where the duct goes. The duct has to end outside, at a wall or roof cap. A fan that blows into the attic or the space between floors moves the moisture somewhere you can’t see it, where it soaks insulation and can freeze on the roof boards.
4. The duct itself. Long runs, sharp bends and flexible duct that has been crushed or has sagged into a low spot all slow the air down. Water can collect in a sag and drip back into the bathroom.
5. The size of the fan. Fans are rated in CFM, cubic feet of air per minute. A common rule from the Home Ventilating Institute is at least 1 CFM per square foot of floor for bathrooms up to 100 square feet. A 50 CFM fan in a large ensuite with a soaker tub won’t keep up.

When it is time to replace the fan

• The motor hums, rattles or squeals, or starts slowly.
• The fan is undersized for the room, even after cleaning.
• It is old and loud enough that nobody wants to turn it on.
• The housing is rusted or the damper is broken.

When we replace a fan, we look at noise as well as airflow. Fans are rated in sones, and a lower number means a quieter fan. A quiet fan is one people actually run. A timer switch or a humidity-sensing switch helps too, because the fan keeps running after you leave the room.

Can you do it yourself?

Cleaning the grille and the blades is an easy job. Swapping a fan for one of the same size, in the same opening, on the existing wiring, is something some handy homeowners do. Turn the power off at the breaker first. Anything that needs new wiring, a bigger hole in the ceiling, a new duct run through the attic or a new exterior cap is a bigger job. In Ontario, electrical work done for someone else has to be done by a licensed electrical contractor.

We can fix it in one visit

Exhaust fan repair and replacement is one of the jobs we do most often. Book a bathroom exhaust fan replacement online and pick the date. We check the duct and the exterior cap while we are there. If the kitchen has the same problem, read our post on range hood venting.

Bathroom fan questions

How long should I run the bathroom fan after a shower?

About 20 minutes, until the mirror is clear. A timer switch makes that automatic.

Why is my bathroom fan so loud?

Usually a worn motor, a loose housing or dust on the blades. Older fans were also built louder. If cleaning doesn’t help, a new fan with a low sone rating will be much quieter.

Can a bathroom fan vent into the attic?

No. It has to vent outside through a wall or roof cap. Warm, wet air in the attic leads to wet insulation, mould and frost on the roof boards.

What size bathroom fan do I need?

For bathrooms up to 100 square feet, the Home Ventilating Institute’s rule of thumb is 1 CFM per square foot of floor. A 7 by 9 foot bathroom needs at least 63 CFM, so a 70 or 80 CFM fan. Larger bathrooms are sized by their fixtures.