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September 2021 EnergyWise Tip: Bathroom Fans

September 07, 2021


September 2021 EnergyWiseSM Tip: Bathroom Fans

By: Energy Efficiency Program Manager Cory Fuehrer

Walk into your bathroom, turn on the light and fan, and the first thing you think about is energy efficiency, right? Well, probably not. However, your bathroom’s exhaust system could be letting you down. Many homes have bathroom fans that:

  •  are too noisy
  •  move little air
  •  are not energy efficient
  •  may cause backdraft

So, how can you avoid these pitfalls? First, identify what size of fan you need. Fan size is usually rated in the amount of air it can move in terms of cubic feet per minute (CFM). Most experts recommend eight air changes per hour for bathrooms. Determine your bathroom’s volume by calculating cubic feet. You can do this by multiplying length by width by ceiling height. Take the cubic feet and divide by 60, which is the number of minutes in an hour. Now multiply by eight, which is the targeted number of air changes. For example, a 10’x8’ bathroom with an 8’ high ceiling would need 85 CFM. When shopping, round up to the nearest size.

Next, choose the quietest, most energy-efficient fan in the size range required. Most fan labels have Home Ventilating Institute (HVI) ratings so you can compare noise levels, as well as their energy efficiency. Fan noise is rated in "sones." The lower the sone rating, the quieter the fan. Efficiency can be compared by how many CFM of air a fan moves per watt of electricity the fan requires. The best fans have sound ratings of 0.5 sones or less and move about 2½ CFM of air per watt. For added assurance of quality and efficiency, look for the ENERGY STAR® label.

Third, select low-resistance (smooth) exhaust ducting. Seal the joints and insulate sections that run through unheated spaces. This will help maintain the fan’s air volume rating while reducing the amount of heat gained or lost while the fan is not operating. Undersized or droopy flex ducting and ineffective or dirty backdraft dampers and exhaust louvers can cut rated airflow by more than 50%. Also, duct the exhaust air to where it will not cause moisture damage. Many times, this requires ducting to the outdoors.

Remember, if you have combustion appliances, such as natural gas or propane water heaters or gas furnaces or fireplaces, backdraft may be a concern. Because fans can potentially create a negative pressure in your living space, they may cause the combustion appliance exhaust to back up into the indoor environment. Not only should you ensure this will not happen by installing sealed-combustion appliances, but it is always a good idea to have a working carbon monoxide detector in use for an extra layer of safety.

Fifth, install proper controls. Bathroom fans connected to light switches start running when the light is turned on. Often, users turn the light "off" before all the moisture is exhausted after a bath or shower. Meanwhile, use of a separate fan toggle switch often leaves the fan running longer than necessary. Instead, use a timer switch with a maximum of 60 minutes. This should keep the fan running for at least 10 minutes after you leave the room to remove excess humidity.

Following these simple steps will help you save energy and confirm installation of a quality, energy efficient bathroom fan that will provide you years of service. For other ideas on how you

can become more EnergyWiseSM, visit with your local electric utility or go to www.nppd.com.

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