Frequently Asked Questions

Yes! As your air filters get dirty, they begin to restrict the air flowing through the unit. This causes the HVAC unit to work harder to maintain a sufficient air flow. Because your unit is working harder and longer, it is using more electricity, which over time can dramatically increase your utility costs. In addition, dirt acts as an insulator disrupting the heat transfer of the air flowing through the unit.

This also causes your unit to work harder, which uses more electricity. There is an optimum time to change any air filter; which is when it has reached its maximum efficiency with out becoming restrictive. By changing your filters on a regular schedule, you really can save money on your utility costs, and help promote a cleaner environment.

Not regularly changing your air filters can also lead to costly mechanical breakdowns. Keeping your air filters clean is truly the most cost effective way to maintain your expensive HVAC equipment, and a cleaner environment for your family, friends, and co-workers.

The required change-out frequency varies with filter type, unit location, and other environmental factors. For residential applications, standard disposable polyester or pleated air filters should at least be checked every 3 months. Often after that length of time, a filter will require a change, however environmental factors such as pet hair, heavy foot traffic in your home, and tobacco smoke can decrease the life of your filter; requiring a more frequent change.

For higher efficiency filters such as a spaceguard or other type of media air cleaner, the life can range from 6 months to over 1 year. Commercial applications have an even broader spectrum of change-out frequencies. Air filters in restaurants are commonly changed every 4 to 8 weeks depending on the customer traffic and filter type.. Retail chains, strip malls, and other small businesses vary, but usually are in need of a change every 12 weeks.

Industrial facilities, depending on the nature of their business, often require a filter change-out every week. Plants with heavy smoke, soot, and other airborne contaminants require a massive filtration system, and with all the pollutants in the air, the filters can quickly become saturated. For a free frequency recommendation, please contact Commercial Filter Service, Inc.

(1) The ability of a device to remove particulate or gaseous material from an airstream by measuring the concentration of the material upstream and downstream of the device.

(2) In the ASHRAE 52.1 Standard test method, it is a measure of the ability of a filter to remove the staining portion of atmospheric dust from the test air. This is officially termed Atmospheric Dust Spot Efficiency.

For air filters, efficiency is determined by the relative size of the filter fibers, the density of the fibers in the filter, the adhesive, if applied, and the velocity at which the air is moving through the filter. Generally, large filter fibers are effective on large particles at high velocities and small filter fibers work best on small particles at low velocities. Proper filter design, including choice of fiber diameter, density of media, and the proper adhesive, are necessary for a filter to function properly. With proper design it is possible for a filter to function well beyond its nominal theoretical efficiency range. For example, a properly designed synthetic media filter, with the proper adhesive, will function as well or better in the medium efficiency range than a dry fine fibered filter.

If the filter fibers are designed to remove large particles, they will not be effective on smaller particles regardless of the depth of the filter. For this reason, properly designed filters must be used to stop fine dust particles. Also, many filter fibers will initially stop dust particles, but will release them after unless the proper adhesive is employed to permanently trap the particles.

An “overall” reporting value of a 52.2-evaluated air filter is the expression of the Minimum Efficiency Reporting Value (MERV). The MERV is a single number that is used along with the air velocity at which the test was performed; to simplify the extensive data generated by the method of testing. MERV is expressed on a 16 point scale and is derived from the particle size ranges.

The ASHRAE Standard 52.2

ASHRAE Standard 52.2 features many improvements over the 52.1 standard. Data such as average efficiency, arrestance and dust holding capacity which are provided by 52.1 will remain important performance characteristics (see Table 1 for Application Guidelines for the two standards).

Some of the improvements found in the ASHRAE 52.2 standard include:

  • The use of mandatory (code) language, which enables the standard to be referenced by other codes that are developed.
  • Where 52.1 expressed efficiency as an overall percentage, 52.2 expresses efficiency as a function of specific particle sizes.
  • The 52.2 method of test will create results that are reliable and verifiable.
  • Seventy-two (72) data points are reduced into a single curve that typifies the minimum efficiency of a filter.

MERV Std 52.2

Average ASHRAE Dust Spot Efficiency Std 52.1

Average ASHRAE Arrestance Std 52.1

Particle Size Ranges

Typical Applications

Typical Filter Type

1-4< 20%60 to 80%> 10.0 µmResidential / Minimum Light / Commercial Minimum / Equipment ProtectionPermanent / Self Charging (passive) Washable / Metal, Foam / Synthetics Disposable Panels Fiberglass / Synthetics
5-8< 20 to 35%80 to 95%3.0 – 10.0 µmIndustrial Workplaces Commercial Better / Residential Paint Booth / FinishingPleated Filters Extended Surface Filters Media Panel Filters
9-1240 to 75%> 95 to 98%1.0 – 3.0 µmSuperior / Residential Better / Industrial Workplaces Better / Commercial BuildingsNon-Supported / Bag Rigid Box Rigid Cell / Cartridge
13-1680 to 95% +> 98 to 99%0.30 – 1.0 µmSmoke Removal General Surgery Hospitals & Health Care Superior / Commercial BuildingsRigid Cell / Cartridge Rigid Box Non-Supported / Bag
N/A£ 0.30 µmClean Rooms High Risk Surgery Hazardous MaterialsHEPA ULPA

Note: This table is intended to be a general guide to filter use and does not address specific applications or individual filter performance in a given application. Refer to manufacturer test results for additional information.
(1) Reserved for future classifications
(2) DOP Efficiency


How Data Is Obtained

An air filter’s performance is determined by measuring the particle counts upstream and downstream of the air-cleaning device being tested.

Particle counts are taken over the range of particle sizes six times, beginning with a clean filter and then after the addition of standard synthetic ASHRAE dust loadings for five additional measurement cycles.

A laboratory aerosol generator, which operates much like a paint sprayer, is used to create a challenge aerosol of known particle size in the air stream. This will generate particles covering the 12 required particle size ranges for the test (See Table 2).

The challenge aerosol is injected into the test duct and particle counts are taken for each of the size data points.

The filter’s performance, on each of the twelve particle sizes, during the six test cycles (a total of 72 measurements) is determined. For each measurement, the filtration efficiency is stated as a ratio of the downstream-to-upstream particle count. The lowest values over the six test cycles are then used to determine the Composite Minimum Efficiency Curve (Note: in many cases, this will be the initial reading before the five dust loads). Using the lowest measured efficiency avoids the fiction of averaging and provides a “worst case” experience over the entire test.





0.30 to 0.40



0.40 to 0.55


0.55 to 0.70


0.70 to 1.00


1.00 to 1.30



1.30 to 1.60


1.60 to 2.20


2.20 to 3.00


3.00 to 4.00



4.00 to 5.50


5.50 to 7.00


7.00 to 10.00


The twelve size ranges are placed in three larger groups according to the following schedule: ranges 1-4 (or E1, which is 0.3 to 1.0 ìm), ranges 5-8 (or E2, which is 1.0 to 3.0 ìm), and ranges 9-12 (or E3, which is 3.0 to 10.0 ìm). Averaging the Composite Minimum Efficiency for each of these groups will calculate the average Particle Size Efficiency (PSE), and the resulting three percentages (E1, E2, E3) are then used to determine the MERV.

Standard Test Airflow Rates

The Minimum Efficiency Reporting Value (MERV) must be stated with the air velocity at which the filter was tested. For example, if the filter was tested with an air velocity of 492 FPM and was found to be MERV 10, the filter’s Minimum Efficiency Reporting Value would be MERV 10 @ 492 FPM. ASHRAE Standard 52.2 tests are to be conducted at one of seven airflow rates:

118 FPM (0.60 m/s)
246 FPM (1.25 m/s)
295 FPM (1.50 m/s)
374 FPM (1.90 m/s)
492 FPM (2.50 m/s)
630 FPM (3.20 m/s)
748 FPM (3.80 m/s)

Minimum Final Resistance

Final resistance must be at least twice the initial resistance at the test airflow rate, or the values in Table 3, whichever is greater.

Average Arrestance By Standard 52.1

Filters with an efficiency of less than 20% in E3 (MERV 1 through MERV 4) must be tested per the arrestance test of ASHRAE Standard 52.1

A measure of the ability of an air filtration device to remove a synthetic dust from the air.

ASHRAE arrestance is a measure of the ability of a device to remove ASHRAE dust from test air.

The Choices in the 1 inch variety are normally:

  1. Standard Fiberglass Throwaway Filter – these are filters from the 1950’s that are designed to remove only the larger particles from the air and, in industry-designed testing, do not do that very well. They are only 3-5% efficient at removing 10 micron particles and larger from the air. These filters do have a very low resistance to the flow of air and, for this reason, are the filters most often sold for home units.
  2. Polyester Throwaway Filter – these filters are 10-15% efficient, and are a higher quality filter than the standard fiberglass throwaways. These can be used when better filtration is required or desired.
  3. Pleated Filters – these filters achieve more filter surface area by folding the media into a 1 inch frame and can use a higher efficiency media without adversely effecting the resistance to flow. – to a point.
  4. Others – any filter that does not fall into the above two categories (metals, plastics, cellulose, electronics, etc.).

In today’s marketplace, higher efficiency filters are available at most retail home stores and homeowners need to be aware that some of these filters (usually in the MERV 11 and higher category) may create low airflow problems in their system. In most cases polyester disposable and pleated filters work the best in homes.