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I recently had a fan installed in a small bathroom (5x8). The mirror and windows still fog up even when I take a brief shower. A few minutes after the water stops running, the moisture does seem to clear up. Is this a problem that moisture is building up while the shower is running? It is either a 50CFM or 80CFM fan. Thanks.

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  • A Miller May 11, 2010

    General comment ... You are probably overwhelming this little fan. A little fan is better than no fan however, I dont know that there is a fan big enough to solve this. The trouble is with the temperature difference and the relative humidity. If the room were hot enough, there would be no fog.


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On the assumption that your bathroom is 8 ft high, then the volume of air in the bathroom is 5x8x8 or 320 cu ft. Even if your fan is 80 cfm it would take 4 minutes to completely exchange the air. Leaving the door slightly ajar would assist.
To overcome a similar problem, I installed a second fan with a push button 10 minute timer so the fan runs for a few minutes after I leave the bathroom.

Posted on Sep 24, 2009

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Take the room sizes to an electrical whole salers and they will calculate the air flow and size of fan required

Posted on Nov 18, 2009

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