If you can hear the fan when you turn it on, but it seems like it’s always stinky and humid in there, here’s a simple test you can do to check and make sure it’s working. Close the bathroom door, tear off a strip of toilet paper about a foot long and lay it gently along and over the gap between the door and floor inside the bathroom. Then turn on the fan. The tissue should immediately be sucked away from the door by air flow from the fan.

    If not, the air flow must be reduced or plugged at one end or the other, and there are a few things that could be causing the lack of air flow:

1) The opening under the door (undercut) is not high enough or obstructed by a thick floor covering on the one side. A bathroom exhaust fan pulls air from under the door, and also from the air conditioning supply air register (vent) in most newer homes. But some older homes have no supply register in the bathroom, so the undercut of the bathroom becomes the sole source of incoming air to replace the air that is exhausted by the fan.

    Check the gap on both sides of the door. The problem may be plush carpet that virtually seals the bathroom door from the outside or an undecut that’s too small. Normal undercut is 3/4” to 1”. An extremely narrow gap under the door, will still cause the tissue to move but limit the cubic feet of air exchange.

2) There is an obstruction to the exhaust termination of the fan duct in the attic. This is more difficult to check. Does a strip of toilet paper over the ceiling intake of the exhaust fan get sucked onto the grille when the fan is turned on, and can you feel air flow with you hand over it? The most common defect causing a termination problem is that the exhaust fan was never connected to a duct that runs to a roof or soffit vent and, at some point, the top of the exhaust fan in the floor of the attic got covered over by insulation. We see this pretty often. Also, the termination could be clogged or the duct crushed by someone crawling over it in the attic. Investigating for these issues may require crawling in the attic or getting up on the roof, and might be best left for a construction professional.

3) The exhaust fan could be big enough for the size of the bathroom but, because of an extremely long run of the duct to a distant roof or soffit vent, the rated output of the fan in cubic feet per minute (cfm) might be significantly reduced due to the static pressure (resistance to air flow) of the long duct run. A fan with a higher cfm rating may be the answer.

4) The fan could be too small for the size of the bathroom. The building codes require a minimum 50 cfm rating for switched bathroom exhaust fans, which is also the smallest size available at home improvement stores. It should be adequate for the average 5-foot by 8-foot bathroom, based on the rule of thumb of 1 cfm per square foot of a bathroom with an 8-foot ceiling. But larger bathrooms, and especially the glamour master baths with high ceilings and both a shower and spa tub require much more cfm than the one for one calculation, and often a fan at both ends of the room, to effectively clear the air.

5) The fan motor is at the end of its lifespan and producing reduced air flow. Vent fans usually start to get noisy when their performance deteriorates and they are about to die, so that would be a clue.

    And, we almost forgot, but there is a third possibility: the exhaust fan works fine but doesn’t get left on long enough. There are three solutions for that problem you can find at our blog post My bathroom is stinky and humid even though it has an exhaust fan. What can I do to fix it? 

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.