Each time the bathroom is used, it generates humidity and odors that need to be exhausted outside to maintain a healthy indoor environment. There are a couple of possible reasons your fan is not doing its job well enough:

   The fan is not on long enough to clear the air. Ordinarily, the bathroom light and exhaust fans are on separate switches, but most bathroom fans are only rated to move 50 cubic feet of air per minute. It takes a while for a full air change and, if there is an ongoing moisture problem, you have a choice of three ways to get more air exhausted from the bathroom:

1. Tie the circuit for the exhaust fan to the same switch as the lighting for the bathroom. This is the cheapest solution and guarantees that when the bathroom is being used the fan is on, but will not keep the fan operating as long as the next two solutions. 
2. Replace the on/off switch for the bathroom exhaust fan with a timer switch, like the one shown below. You can adjust it to continue running for several minutes after you leave the room.
3. The newest technology is a fan control switch that incorporates a humidistat device that doesn’t turn the fan off until the humidity and temperature in the room have reached an acceptable level. A photo of one made by Panasonic® is below and, depending on the brand, this hi-tech solution will set you back $40 to $60.

    There is an air flow obstruction, either at the intake gap under the door or the exhaust to the exterior. 

    See our blog posts How can I check to see if my bathroom exhaust fan is actually working and moving air? and Can a bathroom exhaust fan dump air (discharge/terminate) in the attic? and What are the requirements for bathroom ventilation? to learn more. 

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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.