The Odd, Little Happy

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How Long Should You Run the Bathroom Fan?

Posted by theBarefoot on July 6, 2013


Other than removing the occasional odors, your bathroom fan’s main job is to remove moisture. So how long does it take to turn over the air in your bathroom? Well, that requires math, so bear with me. I’ll do all the solving for X. You need to know 2 things: the volume of your bathroom and how much air your fan can move. Unfortunately, here in the USA we don’t yet use the metric system, so all these examples will be in Imperial.

To determine your bathroom’s volume, measure the length and width. If necessary, measure the height, but most houses have 8′ ceilings standard. A standard bathtub is usually 5′ long, so if your tub is the full length of one side of the room, you’ve saved measuring that, too. If you don’t know your fan’s capacity, you’ll have to remove the cover to read the specs. Most contractor-installed fans are cheap, 40 or 50 cubic feet per minute (CFM) models.

The equation to determine how long it takes to turn over the air is: cubic feet divided by cubic feet per minute = minutes, ft3/CFM = mins.

So if you have a 40 CFM fan and a small bathroom, say 5′ X 5′ X 8′ or 200′3, the fan will turn over the air in 5 minutes (200/40=5). If you have a larger bathroom, say 10′ x 10′ X 8′ (800′3, with a larger 70 CFM fan, you’ll need to run your fan for 11 and a 1/2 minutes (800/70=11.4).

These numbers are probably longer than it takes to brush your teeth and comb your hair at which point you shut everything off and leave. If that’s the case, you’re turning your fan off too soon to evacuate the moisture and moisture damages houses. But don’t leave the fan on all day either. That just pulls out your air-conditioned air and runs up your cooling/heating bill. If you don’t have the time or patience to run your fan for the required amount of time, consider putting the fan on a timer. A timer switch will run $20-$50 and could pay for itself in mildew and/or mold damage. Be sure to get a switch with the correct ratings. Most standard light switches are rated 15-amp.

So now that you know how long it takes to remove the moisture from your bathroom, it’s time to make sure that moisture is getting out of your house. One of the most common corners cut by contractors is not venting the bathroom fan properly. Here’s where the fun begins. Let’s go into the attic. Don’t worry. You probably won’t have to walk around up there. With a good flashlight, you’ll be able to see what you need from the access area.

Shine your light on the area above your bathroom and look for an exhaust hose. It will be either plastic like your dryer exhaust or aluminum. The hose should lead from the fan to the soffit area (edge of your attic) or through the roof. If you don’t see a hose or it’s attached to the fan, but just laying loose inside the attic, you have a problem that needs attention. Improperly vented bathroom fans are just pumping the moisture into your attic. (Did I mention how bad moisture is for your house?) If the fan has been venting into the attic for a long time, inspect the roof panels in the attic for black discoloration. This indicates mold.

Correcting your exhaust problem may be as simple as connecting a $9 hose and extending it to the soffit. However, most will require external vents like your dryer, but that’s beyond the scope of this article.

Keep chasing the odd, little happy and happy DIYing.

P.S. You can find a review of the timer switch I installed in my own bathroom on the Lowe’s website.

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5 Responses to “How Long Should You Run the Bathroom Fan?”

  1. Thomas Forthe said

    It was legal (when I last worked there) to vent one into a ventilated attic space out west, but there was no humidity to worry about. Here in Florida that would be a very bad thing….
    Nice to see a homeowner do it right. :)

  2. @TheBarefoot thanks for the input! My bathroom fan is set to off if the lights are off. So as soon as the one uses the bathroom gets out it’s off. I think you just give the a good idea on what to improve.

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