Clearing the Air: 
Filters for Residential Forced-Air Systems

Dennis Creech is executive director, Mike Barcik is a research engineer, and Steve Byers is technical administrator at Southface Energy Institute in Atlanta, Georgia.

A typical residential forced-air system has a blower located in the air handler cabinet of the furnace or heat pump. The blower draws air from the house through the return ducts. The air is either heated or cooled at the air handler, then blown through supply ducts back into the rooms. The amount of air circulated is generally between 600 and 1,200 cubic feet per minute (CFM).

As this large volume of air circulates through the house, it picks up dust, pollen, and other particles and carries them to the air handler. These particles can stick to surfaces inside the air handler and ductwork. A dirty blower or air conditioning coil inside an air handler can increase operating costs by restricting air flow and heat transfer and can also cause premature motor wear (see "An Ounce of Prevention: Residential Cooling Repairs," HE May/June '91, p. 23). In addition, restricted air flow can lead to reduced heat transfer at the furnace heat exchanger.
 
 

Filters and Pressure Drop

In general, the more efficient the filter is at removing particles, the greater its resistance to air flow, and the greater the pressure drop it creates (for filters of the same cross-sectional area). If the increase in pressure drop is too great, it can reduce energy efficiency, damage the equipment, and increase duct leakage at unsealed seams. Therefore, the filter's function of removing contaminants must be balanced against the additional pressure drop it creates. One way to get good filtration without a lot of pressure drop is to increase the filter area.

The pressure drop that a filter or other type of air cleaner creates in a forced-air system changes over time. As particles accumulate, they reduce air flow, which increases the pressure drop. Following recommended maintenance schedules is critical to keeping the pressure drop at an acceptable level.

For typical residential forced-air systems, the increase in pressure drop across the filter, from the time when it is first installed to the time when it should be cleaned or replaced, should be less than 0.5 inches of water gauge (about 125 Pascals). If a filter will create a greater change in pressure drop than this, then a stronger blower and special ductwork design may be required.
 
 

Testing Filter Efficiency

Figure 1. Particle sizes of air pollutants. Larger pollutants are easier to remove from the air than smaller pollutants.

Upgrading the efficiency of the filter to remove more particles can help maintain the equipment as well as human lungs. However, all filters are not created equal. Some are much more effective than others at removing airborne contaminants, especially the smaller particles, which pose the biggest threat to health (see Figure 1).

ASHRAE Standard 52-76 describes three tests for determining a filter's efficiency at capturing particles. None of these tests measures a filter's ability to remove gaseous pollutants.
 
 

Weight Arrestance Test

The results of this test are the ones most likely to be used in marketing claims that a given filter has 80% or greater efficiency. Removing 80% of the large particles is relatively easy but does little to protect human health or equipment life. To demonstrate how ineffective such a test is, try pouring table salt through a standard panel filter with a weight arrestance rating of 80% or greater. Be prepared to clean the floor, since the salt will pass freely through the filter.
 
 

Atmospheric-Dust-Spot Test

Since ASHRAE Standard 52-76 sets guidelines for both the weight arrestance and the atmospheric-dust-spot tests, consumers can be easily confused by marketing claims. If a filter claims to be "80% efficient by ASHRAE Standard 52-76," be sure to look for the words "atmospheric-dust-spot" or "arrestance" to determine whether it is an efficient filter for small as well as large dust particles. Many products available at consumer retail outlets show weight arrestance and not dust-spot efficiency ratings.
 
 

DOP-Smoke Penetration Test

ASHRAE Standard 52

Which Product to Choose?

Panel Filters

Panel filters are inexpensive to buy-they cost between 50 and $5-but they have dust-spot efficiencies of less than 5%. Their filtering capability actually increases as they get dirty, but this is accomplished at the expense of restricting air flow and increasing pressure drop. To avoid restricting air flow, they should be changed every one to three months. They do a poor job of protecting the forced-air system and offer human lungs almost no protection from particles.
 
 

Electrostatic Panel Filters

Electrostatic filters are typically 1 to 2 inches thick and have low air flow resistance so that they can easily be substituted for a standard panel filter. However, they "face load," meaning that dirt accumulates primarily on the surface facing the direction of air flow. Face loading can significantly increase the pressure drop and reduce efficiency. The time between cleanings or before replacement can vary from one month to an entire heating or cooling season.

Electrostatic panel filters use static electricity to attract charged particles in the air stream onto the filter surface. While not as effective as electrostatic precipitators, they do not require electricity, and are less expensive. However, they often "face-load" on the surface facing the direction of the airflow, which can increase pressure drop and reduce filter efficiency.

Electrostatic air filters typically have a dust-spot efficiency of 10%-15%. They are marginally effective at capturing small particles (1 µ or less) but more effective than a panel filter on larger particles (greater than 10 µ), such as mold spores and pollen.

Prices for electrostatic filters vary widely. Some products are sold for under $10; others are priced at over $125. These filters usually last longer than standard panel filters.
 
 

Extended-Surface Filters

More efficient extended-surface filters have dust-spot efficiencies of 25%-45%. These filters are several inches thick and cannot fit in a standard 1-inch filter slot. They must be installed in a special housing in the return ductwork. The filter and installation costs can be several hundred dollars, with replacement filters costing around $30-$50.
 
 

High-Efficiency Particulate Air Filters

A HEPA filter should be designed and installed by knowledgeable professionals and can cost several thousand dollars. Replacement filters cost approximately $150-$200 and can last a year or more.

Figure 2. Electronic air cleaners place a charge on particulate pollutants, which then cling to oppositely charged collector plates. If the collector plates aren't cleaned occasionally, the charged pollutants pass right through the filter, bonding instead to walls and curtains.

Electronic Air Cleaners

To maintain their efficiency and avoid contamination, electronic air cleaners must be cleaned regularly. Be sure that there is easy access at the air handler to remove the metal plates for cleaning and to clean the housing.

Electronic air cleaners require electricity and must be fabricated to fit in the return ductwork. They can cost from $600 to $1,200, including installation. The power demands of these units vary from 20 W to 50 W, so the operating cost of some units will be more than double that of others. Depending on the unit, the price of electricity, and the amount of time the system operates, users can expect to pay between $3 and $30 per year in operating costs.

In producing the high voltage necessary to charge particles, electronic air cleaners also produce small amounts of ozone, a highly reactive gas composed of three oxygen atoms. Ozone can be an irritant to lungs, eyes, skin, and respiratory membranes. Most health experts feel that the small amount of ozone produced by a properly installed and maintained system poses little threat to healthy individuals. Nevertheless, extremely sensitive individuals may react to it. Some electronic air cleaners rely on a special adsorption filter (see below) to reduce levels of ozone entering the airstream; however, the long-term effectiveness of these adsorption filters is unknown.
 
 

What About Gases?

An adsorption filter that removed a large percentage of gases (by weight) from the air would have to be several inches thick and would create too much resistance for a standard residential air handler. Most residential adsorption filters are about 1 inch thick and can remove some gaseous pollutants. However, a product's ability to remove specific gases, such as formaldehyde or other VOCs, varies. There is also the possibility of long-term desorption or release of gases, which can occur when the filter becomes saturated.

The pleated accordion shape of this filter is typical of many "extended surface" filters. These filters achieve higher efficiencies by increasing filter surface area, and can replace a standard panel filter without significantly restricting airflow.

Ozone and Negative Ion Generators

According to an article published by the American Industrial Hygienist Association Journal, despite "long-term and widespread use of these devices, there is a lack of evidence in the scientific literature that would support ozone as effective at low concentrations to remove organic contaminants from indoor air. Rather, scientific evidence exists that implies that low levels of ozone will not effectively remove most indoor air contaminants. Subjective claims of improved air quality may instead be explained by evidence indicating that ozone may act only to mask odors or to convert some odorous compounds to less odorous but potentially more toxic compounds."¹

The Federal Trade Commission has signed agreements with manufacturers requiring them to cease unsubstantiated advertising claims or to provide "competent and reliable scientific evidence" to support marketing claims that ozone generators eliminate or clear specified chemicals, gases, mold, mildew, bacteria, or dust from the user's environment; the product does not create harmful by-products; and the product prevents or provides relief from allergies, asthma, and other specified conditions.²

Negative ion generators operate by releasing negatively charged ions into the airstream. These ions attach to dust particles, giving them a negative charge. The charged particles are then attracted to other surfaces in the home, such as walls, ceilings, and furniture, which have a more positive charge.

Although negative ion generators remove particles from the airstream, the particles are collected on room surfaces and can give them a dirty appearance. Some models include a filter to capture the charged particles before they can attach to room surfaces. Over time, the particles can lose their charge and be released back into the air. There is little scientific evidence to support the effectiveness of negative ion generators in removing air contaminants.

HEPA (high-efficiency particulate air) filters can achieve maximum efficiencies. Despite their tendency to restrict air flow, they are used when "clean rooms" are needed for either extremely sensitive individuals or in commercial settings. The whole-house filter shown here contains both a HEPA filter and a bank of adsorption filters, and can be used either with a forced-air heating/cooling system, or as a stand-alone unit.

Houseplants

Protect the Clean Air

Along with sealing the ductwork and building envelope, efforts should be made to reduce the sources of indoor pollutants. Paint, solvents, adhesives, and household cleaning solutions all contain potential indoor air pollutants. Reduction or removal of the sources is the sensible first step in cleaning household air.

Ironically, putting in a good quality air filter can actually reduce indoor air quality. Sloppy installation in the return can add a postfilter hole for air to enter the HVAC system, often from a pollutant-heavy area such as a basement or attic. Here, an inspector uses a smoke test to demonstrate the air leakage around an electronic air cleaner.

Notes

2. Decision and Order for complaint by "United States of America Before Federal Trade Commission, in the matter of Alpine Industries, Inc., and Living Air Corp., corporations, and William J. Converse, individually and as an officer of Alpine Industries Inc. and Living Air Corp." Docket #C-3614, Federal Trade Commission, Sept 22, 1995.

Resources

Bower, John. Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems. Bloomington, Indiana: The Healthy House Institute, 1995.

Liddament, Martin W. A Guide to Energy Efficient Ventilation, Coventry, Great Britain, Air Infiltraton and Ventilation Centre, 1996.
 

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