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Whole house ventilation systems:
This article explains how to design, buy, and use a whole house ventilation system to improve indoor air quality in homes.
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Whole House Ventilation Strategies for Improving Indoor Air Quality
There are a number of strategies for providing whole-house
ventilation, which vary in cost, complexity, and effectiveness.
All strategies, however, can be categorized as either exhaust-only, supply-only, or balanced.
ASHRAE Standard 62-1989 recommends a minimum ventilation
rate in houses of 15 cfm per person, or .35 air changes per hour (ACH), whichever is greater. Based on
the ACH method, a three-bedroom house of 1,500 sq ft
with 8 ft ceilings would require:
(1,500 x 8) x .85 x .35 / 60 = 60 cfm
Multiplying the volume by .85 accounts for partitions and
exterior wall thickness.
Using the per person method and assuming two people
in the master bedroom and one in each other bedroom,
the rate is also 60 cfm.
The revised ASHRAE standard 62.2, released in 2003,
uses the formula of 7.5 cfm per person (based on the number
of bedrooms plus one) plus an factor of .01 cfm for each square foot of house area. For example, based on
the new ASHRAE standard, the same three-bedroom,
1,500-square-foot house would require:
(7.5 x 4) / (1,500 x .01) = 45 cfm.
As these calculations show, a low ventilation rate is
adequate if run on a continuous basis. A higher continuous
rate would be advisable for a home with higher-than-average
moisture levels or pollutant sources such as smoking.
Intermittent ventilation can also work as long as the total
daily ventilation rate is equivalent, but is most effective
when the system is timed to operate when people are home
breathing air and generating pollutants.
A two-speed or
variable-speed fan provides flexibility, allowing the ventilation
rate to be raised when needed, for example when
painting a room or during a party. More important than
the precise number of cubic feet per minute, however, is
a well-designed system that is quiet, reliable, and low-
maintenance, ensuring it will actually be used.
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TABLE 7-1 Whole House Ventilation Strategies
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Installation Tips for Whole House Ventilation Systems
Whole-house ventilation systems should be installed by
people familiar with the equipment. Since they normally
operate at 100 to 200 cfm rather than the much larger fans
found in air handlers, they are less forgiving of errors.
Numerous field studies have found heat-recovery ventilators
performing poorly due to installation errors and poor
For good performance with whole-house
ventilation systems, follow these general guidelines:
Size the whole house ventilation system correctly. Oversizing will increase
heating and cooling costs.
Choose quiet, efficient fans in the house ventilation equipment
Keep HVAC or ventilation duct runs as short and straight as possible.
Locate fresh air intakes away from pollution sources
such as cars, pesticides, and outlets from HVAC equipment
or exhaust fans.
Seal all ducts and insulate where required. Examples:
Insulate intake ducts that run though a hot attic or exhaust
ducts that pass through a cold, unheated space.
Integrate spot ventilation in bathrooms or provide
Use separate spot ventilation in kitchens due to grease.
Place supply registers high on walls and away from
beds, sofas, chairs, and other places likely to cause
Keep controls as simple and automatic as possible.
Educate homeowners about the system and
Watch out: many indoor contaminants are simply too small to see, or are not particles at all but rather gases or chemicals. See ENVIRONMENTAL HAZARDS - INSPECT, TEST, REMEDY for our full list of environmental hazard identification and remedy related to buildings.
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: email@example.com
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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