Education

Tuesday, November 11th, 2014

Basics of Bath Fans

The Basics of Bath Fans

In the old days, if your bathroom was smelly or humid, you opened a window to air it out. Doing so in the dead of winter or the dog days of summer, however, came with a significant energy and comfort penalty.

Surprisingly, modern building codes still support the use of windows for bath ventilation. According to the 2009 and 2012 International Residential Code(sections R303.3 and M1507.3), bathrooms with an operable window don’t need a fan.

Bath fans are great at exhausting humid or smelly air, so (in spite of the code loophole) every bathroom should have one.

How much ventilation do I need?

According to code, a bathroom without a window must have an exhaust fan with a ventilation rate of 50 cfm for intermittent operation or 20 cfm for continuous operation. In the past, many builders and code officials interpreted this to mean that the fan should be rated at 50 cfm. Yet once a 50-cfm fan is connected to ductwork. It also can be equipped with a time-delay feature, humidistats, and occupancy sensors for more-precise ventilation control.
In the old days, if your bathroom was smelly or humid, you opened a window to air it out. Doing so in the dead of winter or the dog days of summer, however, came with a significant energy and comfort penalty.

Surprisingly, modern building codes still support the use of windows for bath ventilation. According to the 2009 and 2012 International Residential Code(sections R303.3 and M1507.3), bathrooms with an operable window don’t need a fan.

Bath fans are great at exhausting humid or smelly air, so (in spite of the code loophole) every bathroom should have one.

How much ventilation do I need?

According to code, a bathroom without a window must have an exhaust fan with a ventilation rate of 50 cfm for intermittent operation or 20 cfm for continuous operation. In the past, many builders and code officials interpreted this to mean that the fan should be rated at 50 cfm. Yet once a 50-cfm fan is connected to ductwork, it may move only 25 cfm because of the duct’s static pressure.
 
Where does the makeup air come from?
If the door is closed and the bathroom fan is exhausting 50 cfm, then an equivalent volume of makeup air is coming into the bathroom.

If the bathroom has an exterior wall, some of the makeup air is coming from the exterior—through leaks around the window, for example. Many bath fans also pull some makeup air from the crack between the fan housing and the ceiling drywall. You don’t really want unconditioned air to be entering the bathroom through this route, so you should seal that crack when installing a fan. You should also make sure that the crack between the bottom of the bathroom door and the bathroom floor is wide enough to allow makeup air into the bathroom. When makeup air comes from under the door, an equivalent amount of exterior air is entering the house through holes in the home’s building envelope. The exhausting of conditioned interior air and the replacement of it with unconditioned air from the outside is an energy penalty associated with running a bath fan, so you don’t want to use it more than necessary.

Let’s say your fan is only pulling 35 cfm—not enough to satisfy code or green-building program requirements. You could swap it for a more powerful model, or you could fix the ductwork by increasing its diameter, by using smooth-walled pipe instead of flex duct, or by reducing the number of elbows. Any of these approaches will work, but the last one will result in quieter operation, likely will be less expensive, and will use less energy.
 
Read the entire article here.