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Sizing Vents & Exhaust Fans

Apr 8, 2008
Sizing a vent fan or heater for a bathroom or laundry room can be a frustrating and confusing experience without knowing the basic terminology. Here are some common terms and suggestions associated with determining the correct exhaust fan for your application:


Exhaust fans should be located in or near the shower or tub, and in an enclosed water closet. Keep exhaust points opposite the supply air source to ensure that the fresh air is drawn through the room. Bathroom doors should not be sealed too tightly at the bottom in order to allow "makeup air" to enter the room when the door is closed. The Home Ventilation Institute recommends that a fan should be left on for 20 minutes or more after a shower to clear humidity adequately and to ensure moisture and condensation in the fan body or ducting is minimized.


A Sone is a measurement of sound in terms of comfortable hearing level for an average listener. The lower the sone value, the more comfortable the listening environment. Sones are not decibels or volume, but rather how sound is "sensed." One sone is the equivalent of a quiet refrigerator.


CFM is the acronym for Cubic Feet per Minute -- the measure of air volume moved by the fan blower. Choose a fan with a CFM rating appropriate for your room size to ensure adequate ventilation.

Static Pressure

Static pressure is the measurement of airflow resistance as it is pushed through ductwork which reduces the effectiveness of the fan. Learn more on the Home Ventilation Institute web site www.hvi.org.

Duct Types

It is recommended, where possible, to use rigid duct. It has less resistance to air flow and allows the fan to operate much more efficiently. If flexible duct is used, be sure the duct is as straight as possible. Larger diameter ducts will result in improved performance. Smaller diameter ducts will cause the fan to run harder, greatly reducing the CFM performance of the fan and create excess noise. A smooth surface duct allows for optimum airflow. For best results, use galvanized sheet metal or PVC. Flexible aluminum duct is durable, easy to install and often used. However, the ridges in aluminum flexible duct increase static pressure and can reduce air flow and fan performance. This results in lower CFMs, higher noise levels and greater energy consumption. The degree to which performance is affected depends on the length of duct, number and degree of elbows. Sagging or weaving a fan duct will also increase static pressure and reduce a fans performance. When using a flexible aluminum duct, support the entire length of the duct with braces or hangers to keep it as straight as possible for the entire run. If the duct lies across the attic, do not allow it to sag between each joist. Also, avoid weaving serpentine through trusses. Using a flexible duct made of nylon or vinyl is not recommended due to high static pressure caused by its ridges and curvature. Remember that a duct-free exhaust fan is not a ventilating device. It does not remove air from the room.

Static Pressure

Static pressure is the measurement of airflow resistance as it is pushed through ductwork which reduces the effectiveness of the fan. Learn more on the Home Ventilation Institute web site www.hvi.org.

Intermittent or Continuous Service

Continuous ventilation provides today's airtight homes with a relatively constant and controlled lower level of ventilation. Intermittent ventilation is used to exhaust sources of moisture and odors, while continuous ventilation is used to remove accumulated indoor air pollutants. Ventilating fans should be located near the source of moisture and indoor air pollutants in bathrooms, laundry rooms, kitchens, hobby rooms and smoking rooms.

So now that we have some of the terms and installation suggestions out of the way, what size fan is right for you? Are you considering intermittent or continuous duty fan?

Intermittent (Spot) Ventilation:

The Home Ventilating Institute (HVI) recommends the following Air Changes per Hour (ACH).

Bathrooms - 8 ACH
Kitchens - 15 ACH
Other Rooms - 6 ACH

Continuous (Whole House) Ventilation:

Many building codes have adopted the American Society of Heating Refrigerating and Air Conditioning Engineers (ASHRAE) Standard 62, as shown below.

House or apartment - 0.35 ACH

Once you decide on the duty type, use this simple formula for selecting the right exhaust fan for your application:

Calculate the cubic feet of your bathroom (length x width x height)
Divide by 60 (the number of minutes in an hour)
Multiply the result by 8 (the number of recommended air changes per hour)
Select a fan which has the CFM rating that you calculated.
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