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Why is my bathroom vent fan not exhausting enough air?

Monday, June 25, 2018

If it seems like it takes forever for your vent fan to clear out the steam or smells from the bathroom, here’s our list of a five possible scenarios that would cause the problem:

  1. Sometimes a vent fan has been installed with no duct to the exterior, and it simply vents directly out of the fan assembly into the attic. That alone would not cause reduced ventilation, but if an insulation contractor blew in a thick layer of loose insulation over the entire attic and your fan is buried under it, that would explain the problem. This actually happens pretty often in older houses. Check in the attic or have it examined by a professional.
  2. The exhaust duct could be crimped or collapsed. Again, check in the attic.
  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.

    Also, 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?  

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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? 

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. 

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