How to choose the right exhaust fan-ipanergy

by Zoe Lin
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How to choose the right exhaust fan-ipanergy

Types of Bathroom Exhaust Fans

There's a difference between an exhaust fan and ventilation fan; an exhaust fan draws air out and a ventilation fan brings fresh air in. If you have to replace a bathroom exhaust fan, your best bet is to go with the type that's already in place. Four types of bathroom exhaust fans include:

  • Ceiling mount: A ceiling-mounted bathroom exhaust fan is the most popular type of bathroom exhaust fan. It works by connecting to an exhaust duct above the bathroom.
  • Wall mount: A wall-mounted fan is used when a ceiling mount is not practical. The wall mount fan vents directly outdoors without the need for exhaust ductwork.
  • Inline: An inline exhaust fan is installed in a remote location and taps into the ductwork in the attic. There's a grill installed in the bathroom ceiling connecting the system.
  • Window mount: Though not as popular as other types, a window-mounted bathroom fan can be inserted just like a window-mounted air conditioner.

Standard Fan Sizing

Bathroom vent fans are rated for the amount of air they can move, measured in cubic feet per minute, or CFM. Standard fan sizing applies to bathrooms that are 100 square feet or less. The rule of thumb is that you need at least 1 CFM per square foot of room area.

To determine the square footage of your bathroom, multiply the length times the width. For example, if your bathroom is six feet wide and nine feet long, its square footage is 54. Therefore, it should have a fan rated for at least 54 ​CFM. But before you start shopping, there are a few things to consider.

  • First, it's a good idea to oversize the fan slightly. In our sample 54-square-foot, for example, it's a good idea to install a 60 CFM fan for good measure.
  • Second, if your bathroom has a jetted tub or separate rooms or alcoves, you might want more than one fan.
  • The minimum allowed fan size is 50 CFM, so if your bathroom is, for example, only 42 square feet, you still need a 50 CFM fan. 

When sizing a vent fan, a factor to consider is duct size and length. Most 50 CFM fans will run well with a 4-inch round duct. But as you get up into the higher CFM fans, duct size will have to be increased to a 5- or 6-inch round duct. The length of the run, as well as the number of fittings and elbows will also affect the amount of air your duct can effectively carry.

There are somewhat complicated formulas for calculating proper duct size for different length runs and configurations, but the instructions that come with the fan will outline the requirements. Just make sure there is sufficient space for the ductwork in the area where you're installing the fan. Trying to force too much air through an undersized duct will make the fan work too hard and will provide insufficient venting.

by Zoe Lin


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