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Air Cleaning Systems

For tables and graphs see below this article.

Are air filter ratings misleading?

Your customer, concerned with the latest news on indoor air quality (IAQ), asks you about air filters. But which type should you recommend? Based on what?

Every mechanical product needs some measurement of performance to describe its level of usefulness. The unit for electric motors is horsepower, for furnaces, BTY. Air filters have an efficiency rating, measuring how much airborne dirt they can be expected to remove.

Filtration efficiency should be a straightforward, uncomplicated point of reference, but it is not because our industry allows three different kinds of dirt removal measurement.

That’s right – you can take any filter and give it three different ratings, and they are all technically correct.

Manufacturers can choose to rate the filter by the weight of dirt it removes (arrestance); by how much it can slow down the staining around diffusers (dust spot), or by its ability to remove specific sizes of airborne dirt particles (particle size efficiency).

No matter what kind of job the filter really does, the manufacturer can choose a test method that makes it sound good. That may seem like a dream come true for low and medium efficiency filter manufacturers. And it is.

It should also explain why almost all air filters are advertised as high efficiency, regardless of their true filtering capabilities.

This may seem as illogical as allowing electric motor manufacturers to choose the performance of either a pony, an old mare, or a healthy Clydesdale to describe the work output of one horsepower. It could allow a manufacturer of a 1/5 hp motor to compare its work output with that of a pony, and therefore call it a 1-hp motor because it equaled the work output of one small horse.

A firm can truthfully claim that a filter has high arrestance efficiency, for example, then imply that it is an extremely high efficiency air filter. The firm is not obliged to also state that the products dust spot efficiency or DOP particle size efficiency are relatively low.

But even if manufacturers did report all three ratings, how many consumers or contractors know what they mean? Very few. In fact, some air filter salespeople don’t know the difference.

Let’s think like a homeowner for a minute. After all, the consumer reasons, how could any filter legitimately advertised as having high arrestance actually be a low efficiency air filter? Contractors should have discovered the reality after a couple of years, after finding motors and fan wheels caked with dust.

Many duct-cleaning firms that promote filters as part of the solution to IAQ problems are themselves probably deluded about their true efficiency. Even allergists who recommend filtration to patients are confused about its true efficiency.

The abuses in the air filter industry because of poorly understood or confusing standards are not limited to the residential consumer.

Purchasing agents and maintenance personnel who buy replacement filters for commercial and industrial applications are confronted with exaggerated efficiency claims for an entirely different reason.

When the ASHRAE committee developed the Standard 52-76 Dust Spot Efficiency Test, they knew the procedure had a higher error rate, especially on low to medium efficiency filters. This means that if you ran repeated tests on the same product, you may get an occasional efficiency number much higher than on the other tests.

At the time, some filter manufacturers made an attempt to have the Air Conditioning and Refrigeration Institute develop a certification program to prevent potential abuse of an occasional unexplained high test. So the authors of 52-76 added a disclaimer:

“This standard is visualized as forming the test basis for product standards and classifications for performance certification programs.”

However, the ARI certification effort failed to pass, the manufacturers did not adopt it, and thereby opened the door to allow the unrealistic test reports we have today.

Some manufacturers who produce filters that really have a 15% to 20% dust spot rating can advertise them as having 30% to 40% ASHRAE dust spot efficiency, because they have a single independent test that gave an abnormally high reading.

If there are a number of manufacturers who produ8ce similar filters and one advertises efficiency rating significantly higher than the others through the use of an “accident” test, it gives the organization an unfair competitive advantage and can mislead filter users.

We believe the majority of air filter manufacturers and distributors would welcome a public airing of these abuses.

There is a growing awareness on the effects of indoor air pollution and a huge potential market for effective products. The competition is intense. For this to occur in an industry that has confusing rating standards and no independent certification authority, is an open invitation for abuse.

We suggest three actions to “clean up” the air cleaning industry.

1. A single uniform test standard must be developed that a diligent person can understand, using a rating system that presents a realistic picture of an air filter’s effectiveness and its capacity to remove pollutants from the air.

2. An industry wide certification program must be initiated to stop deceptive advertising of exaggerated claims.

3. Public education by unbiased individuals or an industry trade association should be initiated through widespread media coverage.

Some progress is underway. Several years ago, ASHRAE recognized the need for a new test standard based on particle size efficiency. The Standards Revision Committee responsible for Standard 52 tried to develop an acceptable standard, on a volunteer basis, without success.

Subsequently, funds from ASHRAE research were granted to have the standard developed by an outside contractor. A contract was awarded to Research Triangle Institute in April, 1991, for this purpose, and work is underway. But don’t look for results for at least three years. It may even take five years to complete the public review process and get the standard published.

Efforts to initiate a certification program should be undertaken by the National Air Filtration Association. The development and acceptance of a legitimate certification program will require leadership, resolve, strength in numbers, and a great deal of cooperation between distributors and manufacturers.

In essence, everyone in the industry needs to work together on this critical project.

In addition, NAFA needs to expand its horizon and grow in strength by adding new members. A meeting of the Certification Committee will be held Oct. 26 at NAFA headquarters in Washington, to develop guidelines for establishing product and company certification programs.

The Cost of Clean Air

A study published in the July 1992 edition of the ASHRAE Journal shows that the actual cost to clean household air with central air filters and cleaners ranges from $.30 to almost $7.00 per cfm, depending on the type of filter used.

The study also showed some surprising comparisons of various types of air filters and cleaners. For example, even though an electronic air cleaner may cost ten times more to buy and install than an electrostatic air filter, the cost per cfm of clean air delivered to the house is ten times less with the electronic cleaner.

Conducted by Francis Offerman, Steven Loiselle, and Richard Sextro, the study looked at the measured performance of four types of air cleaners commonly used in homes:

Conventional furnace filters. These are the garden variety filters that are installed primarily to protect the furnace blower.

Elecrostatic filters. These plastic mesh nonelectric filters, sold under Newtron, Permatron, Dust Free, Honeywell, and other brand names, remove dust by electrostatic attraction. Some performance claims for these filters, particularly those made for the Newtron brand, haven hotly contested within the air filter industry.

Electronic cleaners. Properly referred to as “two-stage electrostatic precipitators”, these electrified cleaners work by charging airborne dust particles as they pass through the cleaner, and then collecting the charged particles on the oppositely charged downstream plates. Common brand names are Honeywell, Trion, Electro-Air, and most of the major HVAC manufacturers.

High Efficiency Particulate Arrestor (HEPA) filters. Initially developed to protect workers in atomic bomb factories against radioactive dust. HEPA filters are the most effective filters for removing very small (0.3 micron) particles from air. They have only recently been introduced for residential applications.

The experiments were conducted in the Indoor Air Quality Research House at Lawrence Berkeley Laboratory – a tightly built, 600 square foot test building with a forced air heating system.

To test the effectiveness of each type of filter, researchers filled the house with cigarette smoke, allowed it to mix thoroughly, and then started the central furnace blower with the air cleaner installed and operating. As the system operated, they continuously measured the concentration of cigarette smoke and compared the removal rate with the “natural” decay rate without the filter operating.

The calculated effectiveness, called the “effective cleaning rate” (ECR), is defined as the amount of clean air that would be required, in cfm, to produce the same cleaning effect provided by the air cleaner. In other words, operating an air cleaner with an ECR of 100 cfm would be equivalent to bringing 100cfm of perfectly clean air into the house.

The cost to remove particulates and to deliver clean air to a house is calculated as the total cost to install and operate the air cleaner divided by the cubic feet of clean air delivered (ECR). Total operating costs include costs for installation and maintenance, replacement, blower power consumption for continuous operation, and, for some filters, power consumption of the cleaner itself.

Table 1 lists the total costs, ECR, and clean air costs for the four filters examined. The cheapest way to clean household air, according to this study, is with an electronic air cleaner. Even with a $780 installed costs (probably a high estimate), the cost per cfm is only $0.30.

Table 1
Heating fuel
and system
Annual Ventilation Cost Per Person (heating season only - 15 cfm continuous ventilation)
Boston, MA
5874 F degree days
Washington, D.C.
5004 F degree days
Chicago, IL
6238 F degree days
Vancouver, BC
5412 F degree days
Montreal, PQ
8047 F degree days
Fairbanks, AK
14,409 F degree
Natural Gas
80% eff.
Electric Resistance
100% eff.
Fuel Oil
80% eff.

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