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Sizing Fans & Exhaust Fans

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Title: Sizing Fans & Exhaust Fans


1
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

2
  • 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.

3
  • 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. 

4
  • 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.

5
  • 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.

6
  • 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

7
  • 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.
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