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April 1, 2010
Vol. 5, Issue 4

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Hi , happy building!


This Month's Topic:

     Will Your New Bath Exhaust Fans Really Work?

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Will Your New Bath Exhaust Fans Really Work?

In previous ezines I’ve talked about how important it is to keep moisture from penetrating the building enclosure. What many people don’t realize, however, is that it’s just as important to remove excess moisture from inside the house. One of the biggest moisture-generators, of course, is the bathroom. Fortunately, there are a variety of efficient bath fans available to deal with this moisture problem. Unfortunately, due to poor installation procedures, even the best-designed fan can be virtually useless.

Selection: Panasonic and Broan are two manufacturers of effective and extremely quiet Energy Star® -rated bath ventilation fans. Although these products are more expensive than some of their counterparts, it’s worth it to get a fan that actually has the power to do the job without sounding like a train running through your house.

Selecting good fans, however, is only the first step. How would you feel if you paid for these superior products only to have improper installation, incorrect sizing, or lack of protection during construction reduce their performance or render them totally worthless? Sadly, this happens all too often.

In many cases the subcontractors responsible for installing exhaust fans have no understanding of or training in best installation practices. The builder may not be well-versed on the subject either, as you’ll see from some of the photo examples below. That’s why it’s important that you know what to look for during your building or remodeling project.

Duct PictureSizing: First of all, ask your builder how the fans will be or have been sized for the various baths in your home. You want to avoid the one-size-fits-all method. If your builder hasn’t already done so, make sure he or she sizes your fans specifically to fit your rooms.

Sharp curves or elbows, as well as long duct lengths, reduce the performance of a fan. Yet, as seen in the photo to the right, sometimes curves and distance can’t be avoided. The duct shown here, which is connected to the fan just off the left side of the photo, must curve around obstacles as it runs through the floor trusses to reach the exterior wall on the right.

The good news is that correct sizing of a fan can overcome these hurdles. Panasonic’s site offers a sizing tool that asks for the duct type, diameter, and length, as well as the size of the room and the number of elbows. To access this feature, visit www.Panasonic.com

Installation: Sometimes tight bends or elbows in the duct are due to sloppy installation, as shown below. If the fan in the photo on the left had been turned around, no bend would have been necessary. If an obstacle prevented the duct in the right photo from being directed straight out (the obvious fix), this fan could have been repositioned and vented to the outside further away from the fan to avoid the choking “S” curve.

                   Sloppy Fan Installation

The bottom line is that there are two main rules for proper duct installation:

  • Avoid elbows and bends whenever possible. When bends are necessary (and they often are), make the best of the situation by allowing a 2–3 foot straight run out of the fan before the first elbow. This approach allows airflow to be uniform before passing through the elbow. Conversely, an installation with a 90-degree elbow immediately after the fan exhaust port will cause air to flow back into the fan, both reducing performance and increasing noise.
  • Use a wide-radius angle (not a sharp turn) to help ensure optimum performance and minimum noise. The goal is to achieve optimal fan performance, which means aiming for a smooth, inner surface duct with the least number of elbows.

Proper InstallationThis photo to the right shows a well-thought-out installation.

For an eye-opening video showing measured air flow of a bath fan resulting from different duct materials, different duct length and size, compromised flex duct, vent caps, and turns in the duct, visit, www.Panasonic.com This video also sheds light on the best duct materials, explains when it’s important to insulate the duct, and offers advice on exterior vent cap selection.

Vent to the Outside: Be sure your exhaust fans vent to the outside, not into the attic. Why is exterior venting required by building codes in most (but not all) of the nation? Because venting into the attic can generate mold and contribute to rotting wood—unintended science experiments caused by the difference in air temperature between the attic and the vented air.

Protection: Once your bathroom exhaust fans have been installed, check to make sure they are covered and that they remain so until the house is completed.

Unprotected FanFans left unprotected during construction, like the one in this photo, are vulnerable to damage. The damper, which opens when the fan is turned on and closes when it is turned off, and/or the blower wheel can become clogged during processes such as spraying ceiling texture. At best, such construction debris can reduce performance and/or increase noise; at worst, it can choke the fan to death. For example, if a blower wheel is knocked off balance, the load on the motor is increased, shortening its life. If the damper becomes clogged but the blower wheel is still operational, the fan can sound as though it’s working, but in reality it’s not doing a thing.

You can tell if your fans are operational by performing the tissue test. Turn on a fan and hold a tissue up to it. If the tissue is held in place, the fan is providing at least some ventilation.

How can you take an active role in making sure your bathroom exhaust fans will do their job?

  • Select Energy Star-rated fans.
  • Be sure each fan is sized for the room where it will be installed.
  • Make certain all fans are vented to the outside.
  • Confirm that, once installed, all fans are protected against construction debris.
  • Don’t accept installation that involves sharp bends or elbows.

Ensuring your bathroom fans are correctly sized, installed, and protected today will result in years of useful service down the road.


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Tracy DeCarlo, author of "The Difference is in the Details" publishes the idea-packed, monthly e-zine "Tracy's Home Building Tips." If you're ready to learn how to plan a more durable, energy-efficient, and easy-to-live-in home, get your FR*EE tips now at www.DetailedSolutions.net.

Tracy DeCarlo owner of Detailed Solutions, Inc. and One Stop Green Home Certification, has more than fourteen years' experience in the residential construction industry. As a Home Building Function Analyst, Certified Green Professional, Green Certifying Agent, and Certified Aging-in-Place Specialist, Tracy addresses the functional aspects of home construction, design, and usage, including items such as energy efficiency, disaster mitigation, air quality, aging in place, green building, plumbing, electrical, lighting, and storage.

By working with homeowners to focus on their daily habits and preferences, she helps them create living spaces that support and compliment their lifestyles, while at the same time teaching techniques to manage the bottom line.

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