How to Upgrade to a Bigger Bath Fan

Q: Our bath fan doesn't keep moisture off the walls, even if we open a window. Do we need a bigger fan? Bill Jackson, Olyphant, Pa.Matt Tomis of Tomis Electrical Contractors replies: You shouldn't have to open a window! Maybe the fan is clogged with dust. Take off its cover and vacuum out its housing. And, if you can get to the fan's duct, make sure it isn't pinched or blocked. If that doesn't help, it's probably time for a new fan.
You want one with a cubic-feet-per-minute (cfm) rating that's high enough to pull all the air out of your bathroom in 7 minutes. To determine the fan's minimum cfm, divide your bathroom's volume by 7.5. Go with a higher cfm if the duct run exceeds 20 feet or has more than one elbow. Also, put the fan on a timer so that it runs for at least 20 minutes after you finish showering and exhausts all the moisture.
The fan I'm installing here has a built-in timer. Finally, compare sone ratings; the lower the number, the quieter the fan. It has an important job, but nobody wants to hear it working.