Guide to Balanced Fresh Air Ventilation Systems for Indoor Air Quality Improvement

How to use balanced fresh air ventilation systems for indoor air quality
Using balanced air ventilation systems for removing or keeping out indoor contaminants
How to select & use heat recovery ventilator systems
How to select & use energy recovery ventilation systems
Best methods for cleaning & filtering indoor air
Questions & Answers about how to design, install, and use balanced fresh air ventilation systems in buildings
References

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Balanced fresh air ventilation design, installation, & troubleshooting in buildings: this building ventilation design article explains the use of balanced fresh air ventilation systems, heat recovery ventilators, and energy recovery ventilators to improve indoor air quality in homes. Lots of people consider dust, pollen, mold, animal allergens, pet hair as the main culprits in indoor air quality issues, allergies, and asthma aggravators. But many indoor contaminants are simply too small to see, or are not particles at all but rather gases or chemicals. Balanced ventilation systems combine fresh air input with stale or contaminated air exhaust to improve indoor air quality while saving on building energy cost.

Balanced Ventilation Air for Improving Indoor Air Quality

Balanced air ventilation system (C) J Wiley, Steven Bliss

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Also see BALANCED VENTILATION, HEAT COST SAVINGS. As reported in Best Practices Guide to Residential Construction:

Balanced ventilation uses both a supply and exhaust fan to provide fresh air while keeping house pressures neutral. Linking a multiport supply system (described above) with a bathroom exhaust fan on the same switch is a form of balanced ventilation.

Well suited to extreme climates, balanced ventilation provides optimal distribution of incoming fresh air and tempers it for comfort.

HRFs reclaim heat from the exhaust air while ERFs, recommended for hot humid climates, dehumidify and cool incoming air. For good performance, systems must be properly installed, balanced, and maintained.

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.

Most balanced ventilation systems, however, use a heat-exchanger to transfer heat and, with energy recovery ventilators (ERVs), humidity between the two air streams. These systems, sometimes called air-to air heat exchangers, are the most expensive option for whole-house ventilation; but, if installed properly and well-maintained, provide optimal comfort and ventilation. Depending on the type of heat exchanger, balanced ventilators are referred to as either heat-recovery ventilators (HRVs) or ERVs.

Balanced Ventilation System Controls

One of our clients became so frustrated with problems balancing fresh air in her home that she simply cut out a window pane. This article discusses a better approach.

HRVs and ERVs are typically run continuously, but they also may be set to run 8 to 12 hours per day when people are at home.

In addition, most have a high-speed mode that can be manually activated for spot ventilation of kitchens and bathroom.

Some balanced ventilation systems also use dehumidistats to automatically turn on or increase ventilation when the air reaches a preset humidity level.

Balanced Ventilation System Ducting

While a dedicated ductwork system is the best approach for HRVs and ERVs, to save money they are often piggybacked onto the home’s HVAC ductwork. In one approach, the HRV or ERV draws exhaust air from the return ductwork and feeds fresh air into the furnace’s return plenum. In a slightly better arrangement, the HVAC ductwork is used only for supply, while the exhaust side picks up stale air in bathrooms, laundry, and kitchen. Neither approach distributes fresh air as well as a dedicated duct system.

Also, since HRV/ERV fans operate at 100 to 200 cfm while air-handler fans are often sized at over 600 cfm, trying to integrate the controls, balance airflows, and provide the correct amount of ventilation air is challenging and rarely works well.

The most common approach is to run the ventilation system only when the thermostat calls for heating or air-conditioning, providing too little ventilation. Heat-recovery efficiencies are also compromised, typically due to unbalanced airflows.

Balanced Ventilation System Maintenance

One drawback of HRVs and ERVs is that they require more maintenance than other ventilation systems. Numerous studies have found that many of these systems significantly under perform in the field due to both installation errors and poor maintenance.

In addition to cleaning or changing intake, outtake, and internal filters, the homeowner or service person needs to clean the core once a year or more to prevent mold and bacteria growth. It is important to follow closely the manufacturer’s recommendations.

Unless the homeowner enjoys the responsibility of HVAC maintenance, the work is best handled by a professional service company.

Balanced Ventilation System Cost Effectiveness

For HRVs and ERVs to work properly and achieve the rated efficiencies, they must be installed correctly and balanced well, and the house must be very tight.

Even so, the added cost over a basic ventilation system will be recouped only in the most extreme United States climates with the highest energy costs. However, in very cold or hot climates where mechanical ventilation is needed during most of the year, the added cost may be justified by the comfort of tempered, filtered ventilation air, the effectiveness of the distribution, and the lack of pressure-related problems.

Heat Recovery Ventilation Systems - HRVs

Heat Recovery Ventilation System (C) J Wiley, Steven Bliss

Used primarily in cold climates, HRVs have two air streams that pass over one another in a plastic or aluminum heat exchanger.At left we show a photograph of a heat recovery ventilator.

Used primarily in cold climates, heat-recovery ventilators (HRVs) pass two air streams by one another within a plastic or aluminum heat exchanger, recapturing 60 to 75% of the heat from the outgoing air stream. This unit, installed in an attic, is hung from chains to reduce noise and vibrations.

Recovery of heat from the exhaust air typically ranges from 60 to 75%, if properly installed and balanced. During the summer, if air-conditioning is used, the heat transfer reverses, cooling the incoming hot air. Systems generally have exhaust ports in rooms that generate moisture or pollutants, including bathrooms, laundry, and kitchen and supply ports in bedrooms, living rooms, and other main living spaces.

Because they have both supply and return ducting, HRVs provide the best distribution, exhausting air from bathrooms and other wet areas and providing fresh air to primary living space. The kitchen typically has its own range hood, so grease does not get into the HRV system.

Original Best Construction Practices Guide photo source: courtesy of David Hanson, memphremagog Heat Exchangers, appearing in the text cited above.

Defrost cycle for heat recovery ventilators

In cold climate applications, a defrost cycle is required. It usually switches on at about 20°F to keep frost from building up in the core as condensation from the exhaust stream begins to freeze. Systems either recirculate indoor air or preheat incoming air to prevent freeze-ups.

Energy Recovery Ventilators - ERVs for Indoor Air Ventilation

Energy-recovery ventilators are primarily used in air-conditioned homes in hot, humid climates. They are generally recommended for climates where the cooling load exceeds the heating load and where sustained freezing temperatures are rare. Sustained temperatures below 10°F can damage the permeable core material used in many ERVs.

ERVs either use a dessicant-coated plastic wheel or a special “enthalpic” core material to move moisture (latent heat), as well as sensible heat, between the two air streams. In summer, incoming air is cooled and dehumidified. Since dehumidification is the biggest component of air-conditioning costs in humid climates, it is important to find a unit with a high TRE (total recovery efficiency) rating, indicating that it can transfer large amounts of moisture. To achieve the rated efficiencies, the units must be run at the recommended airflows.

In cold weather, an ERV will tend to humidify the incoming air, since the moisture transfer is always toward the less humid air stream. This is rarely a problem, however, since the cold incoming air holds so little moisture to begin with that the net effect of the air exchange is to remove humidity from the house.

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

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

Air-to-Air Heat Exchangers for Healthier Energy-Efficient Homes, Kenenth Hellevang, PH.D., P.E., Extension Engineer, and Carl Pederson, Energy Educator, North Dakota State University, AE-1393, NDSU Extension Service, North Dakota State University, Fargo ND 58108, March 2009. Web search 02/04/2011, original source: http://www.ag.ndsu.edu/pubs/ageng/structu/ae1393.pdf, reproduced for non-commercial use, with permission.

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

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

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Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
Historic Preservation Technology: A Primer, Robert A. Young, Wiley (March 21, 2008) ISBN-10: 0471788368 ISBN-13: 978-0471788362

Air Leaks, Moisture Problems, Vapor Barriers & Ventilation in buildings - References, Products

Basement Moisture Control, U.S. Department of Energy
Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
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
Quality Standards for the Professional Remodeling Industry, National Association of Home Builders Remodelers Council, NAHB Research Foundation, 1987.
Quality Standards for the Professional Remodeler, N.U. Ahmed, # Home Builder Pr (February 1991), ISBN-10: 0867183594, ISBN-13: 978-0867183597
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. DOE: how vapor barriers work, types of vapor diffusion barriers, installing vapor barrier
Ventilation for energy efficient buildings, Purpose, Strategies, etc.,
Weather-Resistive Barriers, how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
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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