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Exhaust Fan Ventilation System Guide InspectAPedia®  -     Guide to using exhaust fans for removing or keeping out indoor contaminants Home exhaust fan ventilation strategies Best methods for cleaning & filtering indoor air

Here we discuss using exhaust fan ventilation systems to improve indoor air quality in homes. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. See ENVIRONMENTAL HAZARDS - INSPECT, TEST, REMEDY for our full list of environmental hazard identification and remedy related to buildings

© Copyright 2009 Daniel Friedman, Steve Bliss, Wiley & Sons, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Exhaust-Only Ventilation for Improving Indoor Air Quality

As detailed in Best Practices Guide to Residential Construction:

Exhaust-only ventilation is the most common approach, due to its simplicity and use of familiar components such as bathroom fans. However, unless houses are built very tight, there is little control over where fresh air enters the building. Also, depressurization can be a problem, particularly with high-capacity fans. In addition to the increased potential for backdrafting, a depressurized house tends to draw more soil gases, including radon if it is present. And in hot, humid climates, moist air infiltrating through exterior walls can condense on interior finishes such as the back face of vinyl wallpaper that is chilled by air conditioning.

Below we discuss three exaust-only ventilation approaches, followed by a discussion of Supply Only Ventilation and then Balanced Ventilation.

1. Single Port Exhaust Ventilation
2. Multiport Exhaust Ventilation
3. Ventilating Heat-Pump Water Heater

Single-Port Exhaust House Venting Systems

The simplest and least expensive central ventilation system consists of an automatic timer wired to one centrally located bathroom or laundry fan so it cycles on and off for a portion of every hour or for the 8 to 12 hours per day when most people are home, typically mornings and evenings. See the figure at left (click the image for details) about a single port exhaust house ventilation system. The simplest ventilation system uses a single, centrally located exhaust fan that runs on a timer or continuously at a low speed. The fan may also serve as a bathroom or laundry fan, but a dedicated fan is optimal. Passive air inlets are sometimes installed but will only work properly in very tight homes. Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above. Exhaust fan control switch: Since the house ventilation exhaust fan is doing double duty as a bath or laundry fan, it must have a manual override switch for intermittent use. In larger homes, two fans at separate locations can be used. Another upgrade is to use a dedicated fan in a central location, such as a hallway ceiling, which will provide better distribution of both exhaust and supply air. Exhaust fan noise: For the house ventilation system to work well, it is important to use a quiet exhaust fan of one sone or less and choose a central location. Also, the door to the bathroom with the exhaust fan must be undercut by 3/4 to 1inch, along with doors to all 4 bedrooms and other rooms that require ventilation. An alternative is to connect the rooms with through-the-wall transfer grilles.

The biggest drawback to exhaust-only ventilation is that there is little control over distribution of the incoming air. Makeup air will come via the path of least resistance. In a leaky house, this might be a window or drop ceiling in the bathroom with the exhaust fan, leaving the rest of the house un served by the ventilation system. For this reason, single-port exhaust-only ventilation works well only in relatively small, tight houses.

• Passive air inlets. Some contractors install passive air inlets in an effort to direct makeup air into bedrooms and main living areas. For these to work properly, however, the house must be extremely tight and doors must be left open or be cut at least an inch above the carpet. If a house is too leaky or rooms are cut off from household airflows, the inlets will function like other random holes in the building shell, leaking air inward or outward, depending on the wind, stack effect, and imbalances in the HVAC system.
  
  The inlets typically require at least 10 Pascals of negative pressure to operate. They do not eliminate depressurization as sometimes thought. In fact, they require it to work properly.

Multiport Exhaust House Venting Systems

This type of system uses a more powerful exhaust fan that is remotely mounted, typically in the attic or basement. See the figure at left for details of a multi-port whole house exhaust fan vent system). A multiport exhaust system improves air distribution by picking up air from bathrooms and main living areas. These are often used in conjunction with passive air inlets. Exhaust-only systems are best used in homes with electric heating or sealed-combustion appliances where backdrafting is not a concern. Illustration Source: Recommended Ventilation Strategies for Energy-Efficient Production Homes, 1998, by Judy A. Roberson, et al., Lawrence Berkeley National Laboratory, appearing in the text cited above. The multiport house exhaust fan system is ducted to exhaust grilles in bathrooms, laundries, and other wet areas, and sometimes to a centrally located pickup point in the main living space. A room with no outside walls would also benefit from a pickup point. Systems typically run on a low background speed with timer switches in bathrooms for higher-powered spot ventilation. If installed correctly, these systems are very quiet and provide good distribution of ventilation. Multiport exhaust systems may incorporate passive air inlets (see description above) that install either in windows or through the wall, providing some control over supply air. The inlets, typically three or four for a small house, go in bedrooms, main living areas, and other occupied rooms, such as dens or home offices. Inlets should be placed high on the wall away from beds, chairs, or other places where drafts might cause discomfort. Placement near a window is preferred. Because these systems use more powerful fans that depressurize the house, they should not be used in houses with fireplaces or atmospherically vented combustion appliances. They are also not recommended in hot climates, since hot, moist exterior air may be drawn into walls and condense behind interior surfaces chilled from air conditioning.

Packaged multiport house exhaust venting systems are available from American Aldes, Fantech, and a few other few manufacturers (see Resources, page 297 in Best Practices Guide to Residential Construction)

Ventilating Heat-Pump Water Heater

This variation on exhaust-only ventilation passes the exhaust air through a heat-pump water heater, reclaiming heat from the outgoing air stream. Some systems can be reversed in summer, functioning as a supply ventilation system while cooling and dehumidifying the incoming air. A packaged heat-pump ventilating system is available from Therma-Stor.

-- Adapted with permission from Best Practices Guide to Residential Construction.

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• 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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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

ENVIRONMENTAL HAZARDS GUIDE
AIR POLLUTANTS, COMMON INDOOR
ALLERGEN TESTS for BUILDINGS
ALLERGENS in BUILDINGS, RECOGNIZING
ALLERGY TESTS for PEOPLE
ALLERGY TEST ACCURACY
FLOODS IN BUILDINGS-priorities
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
MOLD: A COMPLETE GUIDE TO MOLD
ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
  Health Effects of Air Pollutants
  Common Indoor Air Pollutants
  Key Strategies for Improving Indoor Air Quality
  Whole House Ventilation Strategies
  Whole House Ventilation Table
  Exhaust-Only Ventilation
  Single-Port Exhaust Venting
  Multiport Exhaust Venting
    Ventilating Heat-Pump Systems
  Supply-Only Ventilation
    Forced-Air Supply Ventilation
    Multiport Supply Ventilation
  Balanced Ventilation
    Heat Recovery Ventilation
    Energy Recovery Ventilators
  Air Filtering Strategies
  Particles in Indoor Air - Chart
  Quick Guide to Gases
  Air-Cleaner Types
    Particulate Air Cleaner Table
  Air Filter Effectiveness
  Real-World Effectiveness of Air Cleaners
  Finding & Reducing Air Pollutants
  Radon Hazards
  Formaldehyde Hazards
  Biological Pollutants
  Volatile Organic Compounds VOCs
  Pesticide Exposure Hazards
  Lead Exposure Hazards
  Asbestos Exposure Hazards
  Carpeting and Indoor Air Quality
  Combustion Appliance Contaminants
  Backdrafting Appliances
  Fireplace & Woodstove Contaminants
  INDOOR AIR HAZARDS TABLE
  INDOOR COMBUSTION PRODUCTS & IAQ
INDOOR AIR QUALITY METHODS COMPARED
LEAD POISONING HAZARDS GUIDE
MOISTURE CONTROL in BUILDINGS
MOLD ACTION GUIDE - WHAT TO DO ABOUT MOLD
MOLD ATLAS & PARTICLES INDEX
MOLD BY MICROSCOPE
MOLD CLASSES, HAZARD LEVELS
MOLD CLEANUP GUIDE- HOW TO GET RID OF MOLD
MOLD DETECTION & INSPECTION GUIDE
ODORS, Smells, Gases in Buildings-Diagnosis & Cure
RENTERS & TENANTS GUIDE TO MOLD
SEWAGE BACKUP TEST & CLEANUP
STAIN DIAGNOSIS
TECHNICAL & LAB PROCEDURES
THERMAL TRACKING

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
• Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Wiley.com and also at Amazon.com. See our book review of this publication.
• Basement Moisture Control, U.S. Department of Energy
• Crawl Space Moisture Control, U.S. Department of Energy
• Energy Recover Ventilation Systems for Buildings, U.S. Department of Energy
• Energy Savings Methods: Whole House Systems Approach, U.S. Department of Energy
• Log Homes: MInimizing Air Leakage in Log Homes, U.S. Department of Energy
• Log Homes: Controlling Moisture in Log Homes, U.S. Department of Energy
• Log Homes: Log Home Design, U.S. Department of Energy
• Moisture Control in Buildings, U.S. Department of Energy
• Moisture Control in Walls, U.S. Department of Energy
• Natural Ventilation for Buildings, U.S. Department of Energy
• R-Value of Wood, U.S. Department of Energy
• Spot Ventilation for houses, U.S. Department of Energy
• Slab on Grade Foundation Moisture and Air Leakage, U.S. Department of Energy
• Straw Bale Home Design, U.S. Department of Energy
• Vapor Barriers or Vapor Diffusion Retarders, U.S. Department of Energy
• Ventilation for energy efficient buildings, Purpose, Strategies, etc.,
• Whole House Ventilation Systems, U.S. Department of Energy
• Whole-House Balanced Ventilation Systems, U.S. Department of Energy
• Whole-House Exhaust Ventilation Systems, U.S. Department of Energy
• Whole-House Supply Ventilation Systems, U.S. Department of Energy
• ...

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