AirKing Lasko B-Air Blue-dri

The 50 CFM Exhaust Fan Dilemma

We will admit it; most people are not staying up late at night thinking about their exhaust fans. For most homeowners, it is that thing in the bathroom that makes a lot of noise. The mindset is because it is making noise; it must be working – right.

Unfortunately, that might not be the case. The most popular fan size sold is a builder grade 50 CFM exhaust fan. These models are typically low cost which makes them attractive but they are loud and might not be doing the job they are intended for. Some of the issues come from the fact that they are installed in rooms that are bigger than their intended use. In an ideal situation (which never exists) a 50 CFM fan is good for a room about 50 square feet. In a real-world setting, it is probably closer to being able to handle a room between 35 and 40 square feet. If you have one of these installed in your bathroom, you probably notice things like the mirrors fog over when you shower and mildew seems to build up quickly. The good news is there are a few things that you can do. These vary from inexpensive to a bit of an investment.

  1. Get on top of the situation. If you are buying a new construction home, talk to the builder about what they are installing. You might need to push a little, but it will be worth it. Sometimes you will hear things like “we put this fan in all our homes” or “we’ve been using this one for years”. That might be true but a little upfront work will save you money and headaches later. Many builders are also realizing that the old standby is not getting the job done and are already upgrading the exhaust fans. Also, make sure they are using a minimum of 4 inch ducting – 6 inch is even better and they are sizing the fan correctly. It is okay to oversize the fan a little. When we talk about a fan being able to handle a room X square feet – that is a minimum, not the maximum size.
  2. Replace the existing one. Replacing an exhaust fan is a somewhat easy DIY project as long as you can access it from the attic. One issue that typically happens is a mismatched duct size. Most builder grade 50 CFM exhaust fans use 3 inch ducting. You will want to change that to 4 inch at a minimum. You can install a 4 inch to 3 inch reducer but that will decrease the airflow of the unit and cause the sound level to be higher. Definitely better to change the ducting if you can. If you are not sure about doing it yourself, licensed electricians or handyman can typically do this in an hour or two. To have a properly sized and functioning exhaust fan it will be worth the costs.
  3. Let it run. If replacing the fan is not an option, extending the run time of the fan is the next best thing. Instead of turning the fan off as soon as you are done in the bathroom, allow it to run. The run time will be dependent on a bunch of factors. It might need to run for 20 to 60 minutes or more. Remembering to come back and turn it off might not be a practical solution. Installing a timer switch can be a good solution. An even better one is to install a wall mounted humidity sensor like the Air King DH55. These replace the wall switch and can usually be installed very easily (again, if you are not sure how to do this, contact a licensed electrician). It is also recommended you leave the bathroom door open to allow as much air exchange as possible to help reduce the humidity.
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