Sizing Fans & Exhaust Fans

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Sizing Vent Exhaust Fans

• 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

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• Location
• 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.

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• Sone
• 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
• 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. 

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• 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.

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• 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 todays 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.

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• 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 ACHKitchens - 15 ACHOther Rooms -
  6 ACH

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• 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.
• If you need help selecting a bathroom exhaust
  fan, heater fan, ceiling fan, combination
  fan/heater or ventilation system just give the
  professionals at Residential Landscape Lighting
  Design a call toll free at 800-239-2939.